THERMODYNAMICS LABORATORY

(1)

(2)

PUBLISHING COUNCIL:

Donat Mierzejewski (przewodniczący), Joanna Kryza (sekretarz), Jan Polcyn, Paweł Dahlke, Bazyli Czyżewski, Przemysław Frąckowiak, Jarosław Kołodziej, Małgorzata Lesińska-Sawicka, Sylwester Sieradzki

REVIEWER:

Prof. dr hab. Andrzej J. Wojtowicz

TECHNICAL EDITING AND COVER DESIGN Damian Leszczyński

One hundred and ninety-seventh publication Stanislaw Staszic University of Applied Sciences in Piła

Piła 2020

e-ISBN 978-83-62617-93-7

(3)

Agnieszka Hołubowska, Bartosz Szałański, Jarosław Robert Mikołajczyk

THERMODYNAMICS LABORATORY

This publication is the translation of the notes in the Polish language with the original title: „Laboratorium Termodynamiki”

Piła 2020

(4)

We want to thank very much prof. dr hab. Andrzej J. Wojtowicz

for his constructive critical comments to the notes without which the value of this publication would be much smaller.

Authors

(5)

TABLE OF CONTENTS

1. INTRODUCTION ... 2

2. H&S FOR LABORATORY CLASSES IN TECHNICAL THERMODYNAMICS ... 3

3. PLAN OF CONDUCTING LABORATORY CLASSES IN THE SUBJECT “TECHNICAL THERMODYNAMICS” 4 4. TECHNICAL THERMODYNAMICS LABORATORY REGULATIONS ... 5

5. PRACTICAL IMPLEMENTATION OF THE ISOCHORIC PROCESS ... 7

6. CONNECTING TANKS ... 15

7. DETERMINATION OF EQUILIBRIUM INTERMEDIATE STATES DURING THE DECOMPRESSION OF A TANK WITH REGARD TO THE ADIABATIC CHANGE ... 60

8. MEASUREMENT OF COMPRESSION PRESSURE OF A COMBUSTION ENGINE ... 64

9. DETERMINING THE THERMAL CONDUCTIVITY COEFFICIENT FOR DIFFERENT MATERIALS ... 81

10. ESTIMATION OF MEASUREMENT ERRORS ... 90

11. ABSOLUTE ERROR OF A SIMPLE PHYSICAL QUANTITY MEASUREMENT ... 92

12. ABSOLUTE ERROR OF A COMPLEX PHYSICAL QUANTITY MEASUREMENT ... 125

13. DETERMINATION OF THE MINIMUM REQUIRED NUMBER OF MEASUREMENTS ... 132

14. TABLES ... 144

15. A WORD OF SUMMARY ... 153

16. LITERATURE ... 154

(6)

1. INTRODUCTION

The international standards describing measurement uncertainties were defined in 1995 from the initiative of the International Committee for Measures (CIPM).

The International Organization for Standardization issued a document „Guide to the Expression of Uncertainty in Measurement” which sets out standards for the calculation and recording of measurement uncertainty. The Polish version of the ISO standard was published in 1999 by the Central Office of Measures under the title ‘Wyrażanie niepewności pomiaru. Przewodnik’ [93] (‘Expression of measurement uncertainty’.

A guide [93]). The above-mentioned standard has been in force in Poland since 1999.

It introduces, among others, the distinction between the terms "errors" and "measurement uncertainty" as the term is popularly understood, it adopts a uniform terminology and methods for determining measurement uncertainty. Therefore, students are required to familiarize themselves with the above applicable standard for the measurement of physical quantities, calculating and recording of measurement uncertainty, and the methods of using this standard in laboratory practice when performing and processing measurement results during classes in the Thermodynamics Laboratory.

(7)

2. H&S FOR LABORATORY CLASSES IN TECHNICAL THERMODYNAMICS

2.1 You can carry out all projects after having been first granted approval from your tutor.

2.2 Perform only these actions which are provided in the instructions to a given exercise.

2.3 Do not overburden pressure tanks above values given in the instructions.

2.4 Use the compressor and compressed air only to increase the pressure in tanks. it is forbidden to use the compressor for any purpose other than educational purposes.

2.5 A special precaution should be taken in exercises with elevated temperature ("Practical implementation of isochoric process" and "Determination of the thermal conductivity coefficient") because of the possibility of burns.

2.6 Compression pressure should be measured under the supervision of the tutor.

(8)

3. PLAN OF CONDUCTING LABORATORY CLASSES IN THE SUBJECT

“TECHNICAL THERMODYNAMICS”

3.1 Practical implementation of the isochoric process.

3.2 Connecting tanks of the same volume.

3.3 Connecting tanks of different volumes.

3.4 Determination of equilibrium intermediate states during the decompression of a tank with regard to the adiabatic change.

3.5 Measurement of compression pressure of a combustion engine.

3.6 Determination of the thermal conductivity coefficient.

(9)

4. TECHNICAL THERMODYNAMICS LABORATORY REGULATIONS 4.1 Classes are conducted in fixed groups. A later change of the group may take place

only in particularly justified cases with the consent of the head of the laboratory.

4.2 Attendance at all classes is obligatory.

4.3 Students who take additional tools, electronic components and devices are financially responsible for them and should account for them after completing the exercise.

4.4 The systems assembled during lab classes should be presented to the tutor for checking before connecting to the power supply.

4.5 The results of measurements obtained are presented to the tutor for their signature.

The sheet with measurements, signed by the tutor, must be attached to the report.

4.6 Each exercise should be performed on time, according to the plan. In the event of an excused absence or justified delay in performing the exercise the tutor may agree for catching up in an additional period.

4.7 The report should be submitted for the following next classes as shown in the schedule, counting from the day of completing the exercise, under the penalty of lowering the grade and not allowing the next exercise.

4.8 The final grade for the exercise is a sum grade resultant from the report and theoretical knowledge related to the performed exercise. If one of these grades is a failing grade, then the exercise is failed.

4.9 The final semester grade is a resultant of the test grades and exercise reports.

4.10 The grade may be entered into the electronic student’s book after the student has accounted for the borrowed materials, tools and devices.

4.11 During the performance of an exercise students should strictly follow the instructions and orders of the tutor and follow the rules of health and safety at work.

4.12 In the event of an observed hazard or accident during the exercise, students should immediately notify the lab tutor.

4.13 The report on the exercise should contain a title page with the subject of the exercise, date and group composition.

(10)

PAŃSTWOWA UCZELNIA STANISŁAWA STASZICA W PILE

[STANISŁAW STASZIC UNIVERSITY OF APPLIED SCIENCES IN PIŁA]

DEPARTMENT OF MECHANICAL ENGINEERING

TECHNICAL THERMODYNAMICS LABORATORY

SUBJECT OF THE EXERCISE:

………...

FORENAME AND SURNAME:

……….

………

DATE OF PERFORMANCE:

………..

(11)

5. PRACTICAL IMPLEMENTATION OF THE ISOCHORIC PROCESS 5.1 Introduction

The phenomenon of changing the state of a thermodynamic system, i.e. changing at least one of its parameters:

 pressure p;

 temperature T:

 volume V

is called a thermodynamic change. The change in the state of thermodynamic system is most often a result of supplying or removing the heat from or to the system, or performing work on the system. A thermodynamic change is the transition from the initial state of the system to the final state of the system through many intermediate states.

Depending on the method of heat supply and work, an infinite number of changes can be realized, only some of which have a wider use in the technology. One of the characteristic thermodynamic changes is the isochoric process; its characteristic feature is that it occurs at a constant volume V.

Since we consider a closed system in which the amount of thermodynamic factor (m = const) and the type of factor (R = const) do not change, then the equation of state of gases for the initial and final equilibrium state can be determined using the following equations:

Dividing the above systems of equations by sides, we get the following relationship:

During the isochoric change the pressure of the thermodynamic medium changes proportionally to the temperature changes.

Because:

as a result, the work done by the thermodynamic medium subjected to isochoric change is zero.

(12)

Therefore, the equation of the first law of thermodynamics takes the form:

The total amount of heat Q1-2 supplied to or removed from the thermodynamic medium causes only a change in its internal energy ΔU.

Fig. 1. The installation diagram for testing the isochoric process during heating of a thermodynamic medium

1 – valve for filling the tank; 2 - valve for emptying the tank; 3 - tank; 4 - pressure gauge;

5 - thermometer T1; Q1-2 – heat supplied

(13)

Fig. 2. The installation diagram for testing the isochoric process during cooling down of a thermodynamic medium

1 – valve for filling the tank; 2 - valve for emptying the tank; 3 - tank; 4 - pressure gauge;

5 - thermometer T1; Q1-2 – heat received

Fig. 3. The diagram of the isochoric change in the pressure p - volume V system 1 - beginning of isochoric change; 2 - end of isochoric change; p1 - initial pressure of isochoric change of a thermodynamic medium; p2 - final pressure of isochoric change of a thermodynamic medium; V1 - initial volume of isochoric change; V2 - final volume of isochoric change

(14)

Fig. 4. The diagram of the isochoric change in the temperature T - entropy s system 1 - beginning of isochoric change; 2 - end of isochoric change; T1 - initial temperature of isochoric change of a thermodynamic medium; T2 - end temperature of isochoric change of the thermodynamic medium; s1 - initial entropy of isochoric change; s2 - final entropy of isochoric change

5.2 Purpose of the exercise

The aim of the exercise is to prove that during the change taking place at a constant volume, the pressure of the thermodynamic medium changes proportionally to the temperature changes.

5.3 Description of the exercise

For this exercise we will use cylinders with an installed thermometer to measure the air temperature inside the cylinder and a manometer measuring the pressure inside the cylinder.

By gradually increasing the pressure inside the cylinder in the range from 1 to 5 [bar], we record the values of air temperatures inside the cylinder.

By successively reducing the pressure by releasing air, we record the temperature values at decompression.

Table 1. Pressure measurement results during air compression and decompression Pressure

at compression [bar]

Comments

Air temperature [°C]

Pressure at

decompression [bar]

Air temperature [°C]

Note: For safety reasons, the pressure value should not exceed 5 [bar].

(15)

Put the second (small) cylinder in a container with hot water and the record subsequent changes in pressure and air temperature in the cylinder (Fig. Y). Note the results in table 2.

Fig. 5. Stand for testing the influence of temperature on pressure in the isochoric process 1 - tank jacket; 2 - cylinder; 3 - hot water; 4 - split cover; 5 - cylinder filling valve; 6 - T1

thermometer measuring the temperature inside the cylinder; 7 - pressure gauge; 8 - valve for emptying the cylinder; 9 - T2 thermometer for water temperature measurement

Table 2. Pressure measurement results during heating Air temperature [°C]

Comments

Air pressure [bar]

Note: As the cylinder is immersed in hot water you must be careful because if careless you may suffer burns.

5.4 Working out the results

Based on the obtained measurement results draw graphs of temperature and pressure relationship. Plot the results from Table 1 onto one graph.

5.5 Review questions

1. What is the temperature and pressure relationship during the isochoric process?

2. How do you explain the fact that the change of pressure and temperature is not proceeding in the same way for compression and decompression?

(16)

5.6 Notes for the exercise

Fig. No. 1 and 2 show installations that would actually make it possible to test the pressure-temperature relationship for the isochoric process while maintaining a constant mass of the thermodynamic medium and through heat exchange between the system (gas in a tank or cylinder) and the environment (heater or cooler). However, this exercise is done in such a way that the increase of pressure in the cylinder is conducted not by heating but by using a compressor or other cylinder with a reducer. On the other hand, the pressure reduction is realized not by cooling (receiving heat), but by "releasing air". In this method the internal energy of the gas in the cylinder during compression is increasing but not thanks to the supply of heat but due to the fact that in the energy balance the air flowing into the cylinder from the outside brings in energy equal to its enthalpy - not its internal energy. The excess of enthalpy over internal energy for the air flowing into the cylinder can be treated (is equivalent) as heat supplied from the outside to the gas (air) cylinder constituting the tested system. The problem is that the mass of the thermodynamic medium inside the cylinder is growing simultaneously, so the ideal gas equation for a constant mass cannot be used. Consequently, the increase of temperature and pressure will to some extent be similar to a constant mass system to which we supply heat (equal to the difference between enthalpy and specific energy times the mass of the outgoing air), but this happens in a variable mass (increasing - when compressing, decreasing - when decompressing). Solving such a problem is not easy because it is not a closed system, neither stationary, nor even partially stationary.

When we release air from the cylinder (decompression) the temperature and pressure in the cylinder will decrease, which is intuitively obvious as the air in the cylinder does the work of emptying the pV with the air "pushed" out of the cylinder. This can be described quantitatively using the internal energy and enthalpy of air remaining in and flowing out of the cylinder.

For the system shown in Fig. 5 it is possible (with the use of a heater) to implement the isochoric process in a closed system with a constant mass. When heated to a higher temperature the cooling experiment requires only the patience of the experimenter.

Below presented is a simulation of air compression in a variable mass system in order to assess the quality and acceptability of the approximation through the isochoric process in a constant mass system. A support program was used for calculations in technical thermodynamics for the ideal gas model (so as not to introduce additional complications at the comparison). In addition, for the non-stationary conversion used was the so-called a partially stationary conversion approximation which is valid when the change in mass of gas in the tank is relatively small.

Input data (initial state 1):

 p1=100.000 kPa;

 T1=293.150 K;

 V1=1.000 m³;

 v1=0.8413 m³/kg;

 rho1=1.189 kg/m³;

 h1= −5.017 kJ/kg;

(17)

 s1=6.870 kJ/kg·K;

 R1=0.2870 kJ/kg·K;

 cp1=1.003 kJ/kg·K;

 cv1=0.7165 kJ/kg·K;

 u1= −89.15 kJ/kg;

 m1=1.189 kg

The condition of the air in the cylinder after allowing the air from the compressor into the cylinder in the amount of 1% of the mass of air contained in the cylinder (state 2):

 p2 =101.4 kPa;

 T2=294.3 K;

 V2=1.000 m³;

 v2=0.8330 m³/kg;

 rho2=1.201 kg/m³;

 h2= −3.851 kJ/kg;

 s2= 6.870 kJ/kg·K;

 R2=0.2870 kJ/kg·K;

 cp2=1.003 kJ/kg·K;

 cv2=0.7165 kJ/kg·K

The cylinder air condition after the heat Q is supplied to the cylinder in an amount equal to 1% m1 (h1 − u1) (state 3):

 p3=100.4 kPa;

 T3=294.3 K;

 V3=1.000 m³;

 v3=0.8413 m³/kg;

 rho3=1.189 kg/m³;

 h3= −3.851 kJ/kg;

 s3=6.873 kJ/kg·K;

 R3=0.2870 kJ/kg·K;

 cp2=1.003 kJ/kg·K;

 cv2=0.7165 kJ/kg·K

The quotient of pressure to temperature for state 1 (initial state) is:

p1/T1=0.3411222923418046

(18)

The quotient of pressure to temperature for state 2 (final state after transformation realized in the experiment) is:

p2/T2=0.3445335152652226

The quotient of pressure to temperature for state 3 (final state after the isochoric process) is:

p3/T3=0.3411222923418046

The quotient of pressure to temperature for states 1 and 3 is identical (isochoric process in a closed system), while for state 2 it is different. For the same temperature rise (294.3 K for states 2 and 3), the pressure increase is slightly higher for state 2 than for state 3. This is due to the fact that for state 2 we introduced not only heat into the system but also additional mass by adding air to the cylinder. These differences are not big but as more air is allowed into the cylinder (further compression) they will accumulate and grow.

The largest tanks that university laboratory is equipped with have the volume of 0.02 m³ which is much less than in the above-mentioned simulation, thus the quality and obtained approximations of results are much better than those presented in the simulation.

Of course in industrial conditions commonly used are tanks / cisterns with a volume of the order of 8; 16; 20 m³ or more, then the obtained measurement errors would be significant. Nevertheless, students should remember that for isochoric process the characteristic feature is that apart from the fact that it takes place at a constant volume V (V = const), the amount of thermodynamic factor (m = const) and the type of this factor do not change (R = const).

The application of the isochoric process model for an ideal gas to describe the course of compression and decompression of air in a cylinder is an approximate approach but at least in part it reflects the trend observed for the real simulation illustrated by states 1 and 2.

(19)

6. CONNECTING TANKS 6.1 Introduction

In technical practice we often encounter the need to connect two or more tanks working in one system. Connected tanks may differ from each other in capacity or pressure of the medium that prevails in them. In case of a complex operating system the conduits connecting the tanks may have a capacity so large that it cannot be ignored in considerations (they are then, as it were, another tank in a system with a shape different than classic tanks).

Thermodynamically, each of the above-mentioned tanks constitute a separate thermodynamic system which is itself in a thermodynamic equilibrium. When these tanks are connected they interact with each other until the equilibrium occurs in the entire system.

Under conditions of equilibrium, for each of the connected tanks valid is the equation of state of gases:

In general, you can write:

The following equation is also valid for a system of connected tanks:

where:

i – number of connected tanks

6.2 Characteristics of air in terms of physical properties

The analysis of real physical processes in the world around us is characterized by a multitude of variables, a scarcity of information, and computational complexity. For this reason, for the description of physical phenomena we use simplifications called models. Models make the calculations easier but the omission of indirect factors often leads to some errors in their results.

In pneumatic systems the working medium is gas, most often air. The characteristics of gas include: elasticity, compressibility, fluidity and complete filling of the volume into which the gas is allowed. Moreover, unlike a solid body, but being similar to liquids, gases are characterized by fluidity which means they assume the shape of the tank in which they are located, hence the term ‘fluid’ is used for gases and liquids.

The fluid is determined by the following parameters:

 p -pressure expressed for gases in the absolute system, and for liquids in units of overpressure,

 q– density,

 T- absolute temperature expressed in Kelvin

(20)

The relationship between the individual parameters of fluid state is called the equation of state:

F( p , q , T )=0

A characteristic phenomenon for fluids is the thermal expansion of fluid, consisting in the increase in volume depending on the temperature rise (this property does not apply to water in the temperature range 0 ÷ 4 ° C). This variability is determined by the relationship:

ΔV =αVΔT ,

where:

 ΔV denotes the increase in volume,

 α - volumetric thermal expansion coefficient in [1/K],

 ΔT - temperature difference, and V- the initial volume.

For air, we assume the thermal expansion coefficient as for ideal gas, which is:

α= 1

273,15=0,00366.

Gay-Lussac's law indicates that at constant pressure compressed air changes its volume depending on the temperature change according to the following relationship:

V₁ V2

=T₁ T2

.

For most technical applications, air is replaced with an ideal gas model that obeys Clapeyron's law, Joule-Thomson's law and Avogadro's law.

Clapeyron's law defines an ideal gas equation of state as the relationship:

where:

 p stands for absolute pressure,

 v - specific volume,

 q - density, and R - individual gas constant which for air is R=287m²/(s²·K).

The Joule-Thomson law says that the internal energy of an ideal gas is directly proportional to the absolute temperature T:

where cv means a specific heat of gas at a constant volume.

Avogadro's law states that under normal physical conditions one mole of an ideal gas occupies a certain volume determined by the formula:

where:

 Vm is the molar volume,

 μ - the mass of gas molecule,

(21)

 v – specific volume.

Normal conditions are the conventional values of pressure and temperature. A distinction is made between normal physical conditions and normal technical conditions.

Normal physical conditions mean:

Normal technical conditions mean:

During work in a pneumatic system gas undergoes changes in pressure, temperature and volume. The laws of physics describe these transformations using models of thermodynamic transformations. A special case of such a transformation is the adiabatic transformation which assumes a complete lack of heat exchange with the environment. In technical applications, with very fast compression or decompression of gas, the heat exchange with the environment is many times smaller than the work performed on the gas, therefore such a rapid increase or decrease in gas volume can be treated as an adiabatic transformation. The course of the adiabatic transformation is described by Poisson's law:

where:

 - pressure,

 - volume,

 K – the adiabatic exponent for air 1.4 6.3 Selection of measuring devices

Analyzing the expected maximum values of relative pressure in individual tanks and assuming a reserve of pressure values for measuring equipment, the use of measuring devices with the following ranges was planned (Table 3):

Table 3. Pressure values for the selection of sensors

Tank number Maximum

expected pressure value[bar]

Value range for the pressure

gauge[bar]

Value range for the pressure transducer[bar]

1 5 0 ÷ 10 0 ÷ 10

2 1.89 0 ÷ 6 0 ÷ 6

3 1.07 0 ÷ 6 0 ÷ 2.5

(22)

The selection of instrumentation for pressure measurement was also determined by the availability of pressure gauges and pressure transducers on the market, taking into account the minimum margin of the measured pressure range, of at least 100% of the expected value.

The expected temperature range in individual tanks is from - 56.97 ºC to +123.63 ºC. In a real system there is a very high heat dissipation through uninsulated tanks, pneumatic lines and fasteners, and the process itself is somewhat stretched in time so that the recorded measurements allow visualization of the results in the form of a multi-point graph, therefore it was assumed that the maximum temperature changes during the process would reach no more than than 20% of the calculated value.

Table 4. Temperature values for the selection of sensors Tank

number

Minimum expected temperature [K]

Maximum expected temperature [K]

Maximum difference of

expected temperatures [K]

20% maximum difference of

expected temperatures [K]

1 216.18 293 76.19 15.24

2 293 396.78 103.78 20.76

3 293 360.70 67.70 13.54

On the basis of realized maximum expected temperature difference the expected values of the minimum and maximum temperatures were adjusted, taking always the normal technical conditions as the starting value:

Table 5. Corrected temperature calculation results for sensor selection.

Tank number

Minimum expected temperature after correction

[K]

Maximum expected temperature after

correction [K]

Minimum expected temperature after correction

[°C]

Maximum expected temperature after correction

[°C]

1 277.76 293 4,6 19.85

2 293 313.76 19.85 40.61

3 293 306.54 19.85 33.39

The expected anticipated temperature values made it possible to select a popular Pt100 resistance thermometer with a signal converter from resistance to voltage, preconfigured for measurement from 0 to 60ºC.

6.4 General requirements for the recording equipment 6.4.1. Sensitivity and noise

The sensitivity of a measuring device is the smallest change in the value of a measured physical quantity that a given measuring device can register. It is expressed in units of quantity measured directly or indirectly per unit of deviation or other unit of the

(23)

recording equipment. The required sensitivity of recorders depends on their purpose, it can be of the order of, for example, volts, milli- or microvolts per centimeter of deviation of the writing element (in the case of archiving data in the form of recording on a paper tape).

A distinction is made between the total sensitivity (voltage or current input of the amplifier per unit of deviation) and the sensitivity of the recorder itself as voltage or current at the head input per unit of deviation).

The maximum sensitivity of the recorder with the amplifier is limited by the noise level at the input. Electrical noise is any undesirable signal that interferes with useful signals which occurs at the output or in some part of an electronic system. It limits the minimum value of the useful signal that will be distinguishable at the circuit output, thus it limits the sensitivity at the circuit input. Noise arises inside the system or it may come from outside in the form of interference.

6.4.2. Fidelity of reproduction

The fidelity of reproduction of the recorded function is determined by means of static characteristics and dynamic characteristics, which include:

 transfer characteristic, i.e. the dependence of the output quantity on the input quantity,

 measurement errors characteristic, i.e. dependence of the output error on the input quantity,

 nonlinearity error characteristic, which gives the dependence of the error caused by the non-uniform transfer coefficient depending on the amplitude,

 dynamic characteristic, changing over time.

Defining these characteristics is tedious, requiring special automatic equipment.

Therefore, in measuring instruments, these are usually tested on the basis of a response to a unit signal or a square wave signal which does not give precise knowledge about the course of the frequency and phase characteristics but allows to make easy verification of the correct operation of the measuring instrument and the possibility of its correction.

6.4.3. Reliability of the instrument

According to the definition contained in the Encyclopedia of Management, reliability is the probability of an event which consists in that a product used in certain conditions will maintain the ability to meet the requirements set for a specified period of its use.

The level of reliability of devices for data archiving and visualization is influenced by factors such as:

 temperature increase due to normal work of the device,

 ambient temperature,

 humidity and other weather conditions,

 visible and ionizing radiation,

 a magnetic field,

 chemical factors,

 biological factors (e.g. molds, insects),

(24)

 vibrations and shocks.

The influence of human factors includes working conditions of the operating personnel, dimensions of the room, comfort and aesthetics of the device itself and the room.

The instrument reliability is a time function. It does not decrease linearly but according to the so-called decay rate which determines how quickly elements become unusable. In the first period, so-called infancy period of A, there is a rapid loss of elements, in the period of operation B there is a proper working period. The decay rate increases significantly in the third period C, known as the aging period, where the curve takes a characteristic shape of a bell.

Fig. 6. Reliability curve

In order to maintain a high reliability of the measuring equipment it is important to take into account several factors at the stage of its construction. First of all, the device should be planned in such a way as to limit the temperature rise during the operation of the device. For this purpose applied are appropriate elements that emit small amounts of heat, appropriate housing design and natural or artificial ventilation. In case of working in special conditions, e.g. in rooms exposed to various vapors, it is possible to use hermetic housing. Another important factor influencing reliability is the number of components that make up the entire device - reliability decreases exponentially with the number of components used.

When a given construction design solution is used on a larger scale it is important to keep statistical data on damage during the service life: the type of instrument failures, number of failures over a specified period of time and their characteristics. Statistical measurements will help to develop an approximate measure of reliability in the form of a parameter called failure intensity. Failure intensity is expressed as the ratio of the number of failed instruments to the number of undamaged ones at a certain time, and it is some kind of a reliability measure.

6.4.4. Data recording system

In this data recording workstand, to record data and control the layout of the entire didactic workstand, we use an Arduino microcontroller with an original author’s program, written in a program language supported by Arduino. A necessary condition for such a design solution was to support at least 6 analog inputs. Technical characteristics of the above-mentioned microcontroller is presented below.

Arduino is a microcontroller in the form of a board equipped with connectors for connecting external electronic components, for instance motors, relays, diodes, or speakers, as well as a universal serial bus (USB) connector for communication with a computer. Devices connected to Arduino can be powered by electricity taken from the USB connector, from a 9 [V] battery or by an external power supply, and they can be

(25)

controlled by a computer or by Arduino itself, after a prior programming and disconnecting from the computer.

Arduino is an open project. Initially, it was established as an aid to serve students.

It has been developed as a commercial project since 2005, and the name ‘Arduino’ is protected. However, thanks to sharing microcontroller documentation on the basis of Creative Commons, there are many other cheaper equivalents of the original Italian board which are now being created.

Arduino software is relatively uncomplicated and simple to use, based on the C programming language, and it is available free of charge for Windows, Linux, and Mac platforms.

Arduino is a family of printed boards, differing in size, amount of memory, number of connectors or type of controller for communication with a computer.

Currently, the most popular of them, and the one which is applied in the constructed laboratory stand, is Arduino Uno.

Arduino Uno board equipment includes, among others:

 ATmega328 microcontroller - manufactured by Atmel is the heart of the Arduino board. It is a 28-pin chip located in the socket located in the central part of the board and has a built-in processor and memory circuit, as well as electronics responsible for handling input and output contacts.

 Input and output connectors - are used to connect with other electronic components.

They can receive a digital signal (short circuit and open circuit) as well as an analog signal (voltage at the junction) which allows to connect various sensors, for instance temperature, pressure or light meters. There are 6 connectors on the board, marked as analog, and 14 digital connectors that can act both as inputs and outputs.

These connectors can provide a maximum current of 40 [mA] at a voltage of 5 [V].

When using these connectors as current connectors, be careful not to overload them.

 Power supply – on the board there is installed a voltage regulator that generates a constant voltage of 5 [V], regardless of the voltage supplied to it in the range of 7 ÷ 12 [V]. The voltage regulator is necessary for a correct operation of the microcontroller which is sensitive to power problems. The relatively large dimensions of the controller facilitate the dissipation of heat generated during voltage regulation, especially when powering external devices.

 Power connectors - provide power as indicated: 3,3 [V]; 5 [V] and 9 [V]. The GND connector (so called ground) supplies a voltage of 0 [V]. In addition, there is also a connector marked as "Reset" which after connecting to the ground pin (connector) has the same function as the Reset button, i.e. it causes the microcontroller to start the program execution from the beginning.

6.5 The description of the construction of the stand for the registration, visualization and archiving of data of the process of connecting tanks of the same or different volume

This didactic stand consists of three cooperating systems:

 pneumatic;

 electro-electronic;

(26)

 programming layer, i.e. a script in a language supported by Arduino, which allows you to record measurements.

The individual systems are described in detail in the following subsections.

6.5.1. Pneumatic system

The basis for the construction of the workstand are compressed air tanks with the following capacities: 10L, 10L and 20L. Tanks are used for storing the right amount of compressed air. The tanks are made of carbon steel, powder coated to protect against corrosion. Air tanks intended for trucks were used due to an easy accessibility and appropriate number of threaded connections.

Fig. 7. Diagram of the pneumatic system.

This solution turned out to be a bit problematic at a later stage of the stand construction, because the purchased tanks have M22x1.5 internal thread connections, while almost all fittings for pneumatic installations use inch threads. Reductions threaded from the metric tank thread to the popular inch threads of pneumatic components turned out to be so difficult to access that considered was boring and threading brass plugs for tanks with an external thread M22x1.5. All elements were tested for pressure at least twice as high as used on the workstand. For the construction of the workstand used were also reduction nipples with M22x1.5 and 1/2'' external threads, and additionally reduction sockets with internal threads 1/2'' to 1/2'' and 1/2'' to 1/4''

(27)

6.5.2. Selection of temperature sensors

Temperature measurement consists in measuring another physical quantity which is easier to measure and which is dependent on temperature in a known manner.

Usually, a direct measurement of the temperature of the tested body is not very convenient, therefore what is most often measured is the temperature of the appropriate temperature sensor in direct contact with the tested body. Depending on the method of heat exchange between the temperature sensor and the tested body, the measurement methods are divided into contact methods, in which the temperature sensor is in contact with the tested body, or contactless, where the heat exchange takes place e.g. by radiation or signals from the sensor are transmitted contactlessly to the device.

In the constructed laboratory stand, the electric contact method was used, with the use of a Pt-100 platinum resistance sensor.

In the resistance thermometers used is the phenomenon of changes in the resistance of pure metals or semiconductors with the increase of temperature. From pure metals - platinum is the most commonly used, due to a relatively high variation of resistance with temperature and a constant increase in resistance. Platinum resistance thermometers are the most accurate thermometers in the range from -190ºC to 630ºC. For less precise measurements, the measuring range can be extended up to 1000ºC.

Due to a high accuracy and availability in the system, used was a Pt100 resistance thermometer made of platinum which shows resistance values of 100 Ω at a temperature of 0 C.

6.5.3. Selection of pressure sensors

Pressure measuring instruments can be divided according to different classifications:

 according to the principle of operation: hydrostatic, spring, piston, electric;

 by application: for measuring absolute pressure, for measuring overpressure, negative pressure or for measuring differential pressure.

In the built laboratory workstand used are three electrical sensors to measure overpressure, i.e. pressure related to atmospheric pressure. These are devices equipped with electrical relays of mechanical impulses. There are three basic types of electrical sensors depending on the applied solutions:

 based on the principle of relationship between the thermal conductivity of gases and the pressure change;

 operating on the basis of variation of the resistance of conductors subjected to pressure (resistance sensors);

 based on the piezoelectric phenomenon, i.e. the formation of an electric charge under pressure in certain crystalline bodies, such as, for example, quartz or tourmaline.

The selection of the sensor should be influenced by: reference pressure, range of pressure variation, required measurement accuracy and expected measurement conditions. When selecting pressure sensors for the construction of the laboratory stand, the key was:

 adjusting the sensors in terms of the measuring range to the results of workstand calculations and measurements;

 the ability to generate current or voltage signals in one of popular industrial standards, e.g. 0÷20 [mA]; 4÷20 [mA]; 0÷10 [V];

(28)

 equipping the pressure transducer with 1/2'' thread or other popular inch thread to avoid looking for unusual reductions.

Finally, industrial pressure transducers were used which allowed the power supply with 24 V voltage and generated a standard output current signal in the range of 4÷20 [mA].

6.5.4. Pneumatic system parts list

Table 6. Pneumatic system parts list - tanks and their mounting No. Name, description Quantity Diagram/Photo

1. Air tank 10 l

4 x terminal M22x1.5 internal thread,

diameter Ø200 mm

2 pcs

2. Air tank clamp, Ø206 mm 4 pcs

3. Air tank 20 l

4 x terminal M22x1.5 internal thread,

diameter Ø240 mm

1 pc

4. Air tank clamp, Ø246 mm 2 pcs

(29)

Table 7. Pneumatic system parts list – union pieces No. Name, description Quantity Diagram/Photo 1. P26 NW 7.2 pneumatic quick

coupler 1/4'' BSP plug male thread

1 pc

2. Straight plug coupler G1 / 4'' - 6mm,

o-ring NBR

6 pcs

3. Threaded cross WWWZ PN16 G 1/4"(female thread) - G 1/4"

(male thread), nickel-plated brass

4 pcs

4. WWZ threaded tee

G 1/4'' female thread - G 1/4'' male thread

1 pc

5. Nipple G1 / 4'' 3 pcs

6. Reduction

female thread 1/4’’ – male 1/8’’

2 pcs

(30)

7. Reduction

Male thread 1/4’’ – female 1/8’’

2 pcs

8. Throttle-check valve G1/4’’ - 6 mm

o-ring NBR

2 pcs

9. Brass elbow

Female thread 1/2’’

1 pc

10. Nipple

Male thread M22x1.5 - male thread 1/2'’

12 pcs

11. Muff

Female thread 1/2"

5 pcs

12. Reduction socket

Female thread 1/2'' - female thread 1/4’’

3 pcs

(31)

Table 8. Pneumatic system parts list - valves No. Name, description Quantity Diagram/Photo 1. Filter-reducer

Threads 1/4’’

Adjustment range 0÷8.5 bar Filtration 0.04 mm

1 pc

2. Safety valve 8.5 bar female thread G1/4’’

1 pc

3. Valve

Female thread 1/4’’

3 pcs

4. Valve

Male thread 1/4’’

1 pc

5. Valve

Female thread 1/4’’ and male 1/8’’

2 pcs

6. Drainage plug

Male thread M22 x 1.5

3 pcs

(32)

Table 9. Pneumatic system parts list - measuring devices No. Name, description Quantity Diagram/Photo 1. Manometer

0÷10 bar

Male thread 1/4'’

1 pc

2. Manometer 0÷6 bar

Male thread 1/4'’

2 pcs

3. Pressure transducer Aplisens PC-50 0÷10 bar Thread 1/2"

1 pc

4. Pressure transducer Elektroserv 02.1.1 0÷6 bar

Thread 1/2"

1 pc

5. Pressure transducer Elektroserv 02.1.1 0÷6 bar

Thread 1/2"

1 pc

6. Temperature sensor PT-100

Thread 1/2"

3 pcs

(33)

Table 10. List of pneumatic system parts - seals No. Name, description Quantity Diagram/Photo 1. Seal

M22 12 pcs

2. Seal

1/2’’ 8 pcs

3. Seal 1/4’’ 12 pcs

4. Seal 1/8’’ 4 pcs

Table 11. Pneumatic system parts list – other elements No. Name, description Quantity Diagram/Photo 1. Polyurethane pneumatic hose

6 x 4 mm

Working pressure max. 15 bar

2 r/m

2. Muffler 3 pcs

All elements of the system were mounted on the OSB frame on a vertical plate measuring 1100 x 800 mm, fixed with metal angles and screws to a horizontal plate measuring 1100 x 600 mm. The whole piece is set on four furniture legs with an adjustable height, which allows to level the stand.

(34)

6.5.5. Electronic and electrical system

The task of the electric-electronic system is to ensure the required supply voltages, process the measurement signals (for pressure and temperature measurements) and send the measurement results to a cooperating computer, and control the operation of the entire device. The electric-electronic system consists of the following blocks:

 power supply system,

 control system,

 pressure measurement system,

 temperature measurement system.

Fig. 8. Block diagram of the electric and electronic system

(35)

The entire electrical and electronic system was placed in a dedicated installation box. All blocks were placed on the mounting plate inside the box. A clear cabling color system is used in the box, as shown in Table 13.

Table 12. Cabling color marking system

POWER SUPPLY CONSTANT VOLTAGES

PE - yellow-green N – blue

L – black

+12V – brown +24V – red

GND/0V gray („mass”)

WE – white (connection of Arduino input pins ) WY – black (connection of Arduino output pins )

On the door of the box there are LED diodes indicating the presence of supply voltages, the operating status of the device and the selected operating system, START and STOP buttons and the operating system switch. On the bottom wall of the box there are openings for the power cable and for cables for temperature sensors and pressure transducers.

Fig. 9. General view of the mounting box front The main switch and USB socket are located on the side wall of the box.

(36)

Fig. 10. View of the inside of the mounting box.

6.5.5.1. Electronic and electrical system. Power block The task of this block is to provide the following supply voltages:

 + 12V = (direct current voltage) to supply the ARDUINO UNO controller;

 + 24V = (direct current voltage) for supplying measurement and executive systems

(37)

Fig. 11. Power block

The power block is supplied with 230V / 50Hz voltage and converts it into the required direct voltage + 12V and + 24V. Moreover, with the help of LED diodes it signals the presence of the mentioned DC voltages. The power block includes:

 main switch - turns off the power to the entire device;

 B1 fuse - secures power supply to the entire system;

 B2 fuse - protects + 24V direct voltage;

 24V power supply (A1) - supplies + 24V DC voltage;

 12V power supply (A2) - supplies + 12V DC voltage;

 D1 diode with R1 resistor - indicates the presence of + 24V;

 D2 diode with R2 resistor - indicates the presence of + 12V;

 R1 and R2 resistors cooperating with D1 and D2 LED diodes are used to limit the current of the mentioned LEDs .

6.5.5.2. Electronic and electrical system. Control block

The control block is the heart of the entire device. Its tasks include the controlling of temperature and pressure measurement process in individual tanks and sending the measurement results to the cooperating computer. The control block is built on the basis of a programmable microprocessor controller ARDUINO UNO. The way the controller

(38)

works is determined by a dedicated control program. The control block consists of the following elements:

 ARDUINO UNO controller;

 START button - starts the entire measuring process;

 STOP button - stops the measurement process;

 WORK SYSTEM switch - selects the working system of the compressed air tanks;

 D3 diodes with R3 resistor - signaling the operation of the measuring system;

 D4 diodes with R4 resistor - signals the interruption of the measurement process;

 D5 and D6 diodes with R5 and R6 resistors - signal the selected compressed air tank working system;

 USB sockets - for connecting an external computer.

The R3, R4, R5 and R6 resistors cooperating with D3, D4, D5 and D6 LEDs are used to limit the current of the mentioned LEDs.

Fig. 12. Control block

(39)

6.5.5.3. Electronic and electrical system. Temperature measurement block Pt100 resistance sensors were selected for temperature measurement in compressed air tanks. The ARDUINO controller accepts only the signal in the form of direct voltage in the range of 0 ÷ 5 [V] on the analog inputs. For this reason it is not possible to connect Pt100 sensors directly to the controller inputs. It was necessary to use Pt100 sensor resistance signal converters into a 0 ÷ 5 [V] voltage signal. The applied converters have been pre-configured for the temperature range from 0ºC to 60ºC.

Fig. 13. Temperature measurement block

6.5.5.4. Electronic and electrical system. Pressure measurement block

Popular industrial pressure transducers were selected for pressure measurement in compressed air tanks. These transducers deliver 4 ÷ 20 [mA] at their signal outputs. It is one of the very popular industry standards that enable the transmission of measurement signals over long distances. Such a signal cannot be connected directly to the analog inputs of the ARDUINO controller (the controller accepts a voltage signal of 0 ÷ 5 V).

Therefore, it was necessary to use signal converters 4 ÷ 20 [mA] to 0 ÷ 5 [V]. The 4 ÷ 20 [mA] signal is obtained in the current loop. The selected transducers have passive

(40)

outputs, so a current loop with its own power supply had to be created for each converter.

All current loops must be galvanically isolated from each other. In order to create 3 separated current loops, 3 separating transducers 24V / 24V with current efficiency up to 82 [mA] were used. This current efficiency with a margin will satisfy the needs of the measuring current loop. These 3 converters are placed on a universal printed-circuit board creating the A7 current loop supply module.

The described current loops are closed in the following circuits: separating converter - pressure converter - signal converter 4 ÷ 20 [mA] to 0 ÷ 5 [V]. 3 signal converters 4 ÷ 20 [mA] to 0 ÷ 5 [V] constitute one printed-circuit board and form the A6 module. Each converter has 2 adjustable potentiometers. One is used to set the 0 [V] level on the output for the selected minimum current level in the current loop. The second is used to set the maximum output voltage level at the maximum current level in the current loop. In this case, a level of 4.5 [V] was set for the current 20 [mA].

Practical tests showed that the 4 ÷ 20 [mA] to 0 ÷ 5 [V] converters do not have a perfect separation between the input and output, which resulted in the mutual interaction of their output signals after connecting all three signals to the analog inputs of the ARDUINO controller. To eliminate the need to purchase more expensive 4 ÷20 [mA] to 0÷5 [V] converters, a simple multiplexer was used - built on the Pk1 and Pk2 relays. This circuit is switching individual outputs from 4 ÷ 20 [mA] to 0 ÷ 5 [V] converters to one analog input of ARDUINO. This solved the problem of mutual interaction of signals from the converters

In order not to increase the load of the ARDUINO controller power supply and its outputs, relays with 24 [V] DC coils - powered from the 24 [V] power supply were used.

Such relays could not be activated directly from the ARDUINO outputs. The relays are switched on with keys with T1 and T2 transistors. The bases of these transistors are controlled from the ARDUINO digital outputs by resistors R7 and R8 limiting the base currents. Diodes D7 and D8 protect the transistors T1 and T2 against their damage by the self-induction voltage generated in the relay coils at the moment of their switching off.

Relays Pk1 and Pk2, transistors T1 and T2, diodes D7 and D8 as well as resistors R7 and R8 were placed on a universal printed-circuit board to form A5 module.

(41)

Fig. 14. Pressure measurement block

(42)

6.5.5.5. Electronic and electrical system. Measurement automation system

The Arduino microcontroller is controlling the execution and registration of measurements. For the needs of constructed station a software program has been prepared in a language supported by Arduino which - after selecting the operating system (10L / 10L or 10L / 20L) and pressing the "START" button - performs pressure and temperature measurements. The program record is presented below.

/ * assigning names to the pins on the Arduino board (pin name and number) * /

#define ledPRACA 4

#define ledKONIEC 5

#define przycSTART 8

#define przycSTOP 9

#define pk1 3

#define pk2iLED1020 7

#define wajchaUKLADpracy 10

#define uklad1010LED 6

/ * defining variables used in the program * /

int cisnOdczyt1 = 0;// read value on the analog input for pressure in tank 1 int cisnOdczyt2 = 0;// read value on the analog input for pressure in tank 2 int cisnOdczyt3 = 0;// read value on the analog input for pressure in tank 3 float cisn1 = 0;// value of pressure in tank 1 converted into bar

float cisn2 = 0;// value of pressure in tank 2 converted into bar float cisn3 = 0;// value of pressure in tank 3 converted into bar

int tempOdczyt1 = 0;// read value on analog input for temperature in tank 1 int tempOdczyt2 = 0;// read value on analog input for temperature in tank 2 int tempOdczyt3 = 0;// read value on analog input for temperature in tank 3 float temp1 = 0;// temperature in tank 1 converted to degrees C

float temp2 = 0;// temperature in tank 2 converted to degrees C float temp3 = 0;// temperature in tank 3 converted to degrees C / * declaration of auxiliary variables * /

int praca = 0;

int koniecpom = 0;

int drukinfo = 0;

int uklad1010 = 0;

int naglowek1010 = 0;

int naglowek1020 = 0;

void setup() {

/ * settings of the transfer rate and input (INPUT_PULLUP) or output (OUTPUT) for individual pins * /

Serial.begin(9600); // setting the transfer rate pinMode(ledPRACA, OUTPUT); //diode WORK pinMode(ledKONIEC, OUTPUT); //diode END pinMode(pk1, OUTPUT); // relay 1

pinMode(pk2iLED1020, OUTPUT); // relay 2 and diode LED of the system 10/20 pinMode(uklad1010LED, OUTPUT); //diode of work system 10/10

pinMode(przycSTART, INPUT_PULLUP); // button START pinMode(przycSTOP, INPUT_PULLUP); // button STOP

(43)

pinMode(wajchaUKLADpracy, INPUT_PULLUP); // work system switch 10/10 and 10/20 / * initial state setting for digital outputs * /

digitalWrite(ledPRACA, LOW); // turning off WORK diode digitalWrite(ledKONIEC, LOW); // turning of END diode

digitalWrite(pk1, LOW); // switching off relay 1 (pressure reading from tank 1 only)

digitalWrite(pk2iLED1020, LOW); // switching off relay 2 and the diode signaling the 10/20 work system

digitalWrite(uklad1010LED, LOW); // switching off the diode signaling the 10/10 work system }

void loop() {

/ * loop commands * /

if (digitalRead(przycSTART) == LOW) {

work = 1; //assigning the value 1 to the variable "work" [“praca”] when the START button was pressed (conditions the execution of further commands)

}

/ * condition for selecting the 10L / 10L or 10L / 20L work system and, consequently, switching on the signaling diode, selecting the measurement header, etc. * /

if (digitalRead (wajchaUKLADpracy) == HIGH) { uklad1010 = 1;

digitalWrite(uklad1010LED, HIGH); // Turns on the work system 10/10 LED

digitalWrite(pk2iLED1020, LOW); // Turns off the 10/20 LED and the tank 3 reading on pk2 / * the condition for providing the measurement header always after changing the work system

* /

if (naglowek1010 == 0 && praca == 1){

Serial.println("Measurement started for the system: tank No. 1 (10L) and tank No. 2 (10L),

");

Serial.println("Pressure in tank_1 [bar], Pressure in tank_2 [bar], temperature in tank_1 [ C], temperature in tank_2 [C]: ");

naglowek1010 = 1;

naglowek1020 = 0;

} } else {

uklad1010 = 0;

digitalWrite(uklad1010LED, LOW); // Turns off the work system 10/10 LED

digitalWrite (pk2iLED1020, HIGH); // Turns on the 10/20 LED system and reading for tank 3 on pk2

/ * the condition for providing the measurement header always after changing the work system

* /

if (naglowek1020 == 0 && praca == 1){

Serial.println("Measurement started for the system: tank No. 1 (10L) and tank No. 3 (20L), ");

Serial.println("Pressure in tank_1 [bar], Pressure in tank_3 [bar], temperature in tank_1 [ C], temperature in tank_3 [C]: ");

naglowek1010 = 0;

naglowek1020 = 1;

} }

/ * the condition ending the measurement when the STOP button was pressed * / if (digitalRead(przycSTOP) == LOW) {

work = 0;

(44)

koniecpom = 1;

drukinfo = 1;

}

delay(200); // delay for stabilizing the system operation / * work in 10L / 10L * system/

if (praca ==1 && uklad1010 ==1) {

digitalWrite(ledPRACA, HIGH); //switching on WORK diode digitalWrite(ledKONIEC, LOW); // switching off END diode delay(200); // delay for stabilizing the measurement / * pressure reading in tank 1 * /

cisnOdczyt1 = analogRead(A2);// reading from analog input for pressure in tank 1 cisn1 = (5 - cisnOdczyt1 * (5.0/1023.0)); // convert pressure in tank 1 to voltage cisn1 = (cisn1 * 3.0488 - 2.927); // convert pressure in tank 1 into bar

if (cisn1 < 0) { cisn1 = 0;

}

/ *pressure reading in tank 2* /

digitalWrite(pk1, HIGH); // switching on relay 1 (when relay 1 is on and relay 2 is off, the pressure is read from tank 2)

delay(200); // delay for stabilizing the measurement

cisnOdczyt2 = analogRead(A2);// reading from analog input for pressure in tank 2 cisn2 = cisnOdczyt2 * (5.0/1023.0); // convert pressure in tank 2 to voltage value cisn2 = (cisn2 * 1.897 - 1.517); // convert pressure in tank 2 into bar

if (cisn2 < 0) { cisn2 = 0;

}

digitalWrite(pk1, LOW); // switching off relay 1 / * temperature reading * /

tempOdczyt1 = analogRead(A3);

temp1 = tempOdczyt1 * (60.0/1023.0);

temp2 = tempOdczyt2 * (60.0/1023.0);

/ * printout of measurement results to the serial port monitor * / Serial.print(cisn1);

Serial.print(", ");

Serial.print(cisn2);

Serial.print(", ");

Serial.print(temp1);

Serial.print(", ");

Serial.println(", ");

delay(200);

}

(45)

delay(200); // delay for stabilizing the system operation / * work in 10L / 20L * system/

if (praca ==1 && uklad1010 ==0) {

digitalWrite(ledPRACA, HIGH); //switching on WORK diode digitalWrite(ledKONIEC, LOW); // switching off END diode delay(200); // delay for stabilizing the measurement / * pressure reading in tank 1 * /

cisnOdczyt1 = analogRead(A2); // reading from analog input for pressure in tank 1 cisn1 = (5 - cisnOdczyt1 * (5.0/1023.0)); // convert pressure in tank 1 to voltage cisn1 = (cisn1 * 3.0488 - 2.927); // convert pressure in tank 1 into bar

if (cisn1 < 0) { cisn1 = 0;

}

/ * pressure reading in tank 3 * /

digitalWrite(pk1, HIGH); // switching on relay 1 (when relay 1 is on and relay 2 is on, the pressure is read from tank 3)

delay(200); // delay for stabilizing the measurement

cisnOdczyt3 = analogRead(A2);// reading from analog input for pressure in tank 3 cisn3 = cisnOdczyt3 * (5.0/1023.0); // convert pressure in tank 3 to voltage value cisn3 = (cisn3 * 0.811 - 0.649); // convert pressure in tank 3 into bar

if (cisn3 < 0) { cisn3 = 0;

}

digitalWrite(pk1, LOW); // switching off relay 1 / * temperature reading * /

temp1 = tempOdczyt1 * (60.0/1023.0);

temp3 = tempOdczyt3 * (60.0/1023.0);

/ * printout of measurement results to the serial port monitor * / Serial.print(cisn1);

Serial.print(", ");

Serial.print(cisn3);

Serial.print(", ");

Serial.println(", ");

delay(200);

}

/ * command to end the measurement after pressing the STOP button * / if (praca == 0 && koniecpom ==1) {

digitalWrite(ledPRACA, LOW);

digitalWrite(ledKONIEC, HIGH);

if (drukinfo == 1) {

Serial.println("Koniec pomiarów"); //printout of the final message to the serial port monitor

(46)

Serial.println(" ");

}

drukinfo = 0;

delay(100);

} }

The program is uploaded to the memory of Arduino installed in the assembly box of the laboratory stand. To read the measurement results you should start the computer intended to operate the station, then run the "Arduino" software using the icon on the control panel shown in the figure below.

Fig. 15. Arduino software icon

Before starting the readings, select the "Serial port monitor" option from the "Tools"

menu, as shown in the figure below.

Fig. 16. Turning on the Arduino serial port monitor

Only at this moment you should start the measurement by pressing the START button. After the measurement is completed, press the END button, holding it for 1 second. In order to obtain data visualization and archiving you should proceed as described in chapter 5.

(47)

6.5.6. Photographic documentation

Fig. 17. General view - front of the station

Fig. 18. View of tank No. 1.

(48)

(49)

Fig. 21. Mounting box front view

(50)

Fig. 22. Side view of the mounting box

(51)

Fig. 23. View of the inside of the mounting box

(52)

Fig. 24. View of the connection of diodes on the door of the mounting box