A sensitive, rugged pressure gauge is described that was developed to determine the density (composition) and the viscous pressure loss in flowing streams of UO2-NaK slurries. The differential pressure sensors are tvO metal bellows that are rigidly connected in opj'osition, the net displacement of which is detected by a linearvariable differential transformer. The sensitivity of the unit is 0;05 mm Hg pressure and the range is 100 mm Hg with an accuracy of ±0.4-. By simply inserting various calibrated springs in the instrument it ispossible to increase or decrease the range and maintain the sensitivity at 0.05 of the range.
INTRODUCTION
THERE
rugged pressure gauge is required for measuring dif-are numerous applications where a ensitive, ferential or absolute pressures in systems that contain liquids or corrosive fluids, but the available commercial instruments eithe.r lack the sensitivity or are too expensive to use in quantity. Recently Sancier and Richeso& have described a unit, which employs a linear variable differ-ential transformer (LVDT) to translate the mechanical motion of a bellows into an electrical signal. Their unit is rugged and has high sen sitivity, ±0.G2 mm Hg; it employs two bellows, one for temperature compensation and the other for pressure measurements. As the unit is designed, a corrosive or electrically conducthg fluid can be placed in contact with only one side of the measuring bellows.The instrument to be described was designed to measure the differential pressure in a flowing stream of sodium-potasszm alloy (NaK) or in suspensions of. UO2 in NaK with sufficient sensitivity to determine the density (com-position) as well as the viscous pressure loss. It possesses, however, sufficient general utility so that details of con-struction and a description of performance are being presented.
Two measuring bellows were used so that the test fluid is in contact with one side of each bellows. The pressure sensing unit combined with the indicator had a range of 100 mm Hg pressure, could detect a 0.05 mm Hg pressure change, and was found to have a calibration factor con-stant to ± 0.4% over the range 6 to 60 mm Hg.. The unit was so designed that the differential range could be in-creased or dein-creased simply by inserting various calibrated springs in the instrument; since the sensitivity is deter-mined by the LVDT and ancillary equipment, it remains constant at 0.05% of the range.
APPARATUS
A cross section of the gauge is shown in Fig. 1. The two metal bellows (A) are silver soldered or welded to
cylin-* Based on work performed under the auspices of the U. S. Atomic Energy Commission.
I K. M. Sander and W. Richeson, Rev. Sd. Instr. 27, 134 (1956).
869
Ftc. 1. Schematic diagram of the pressure gauge.
Technische Hogeschool
DellI
drial metal housings (B), which are rigidly fastened, to one-half of the case (C) with dowel pins and machine screws. The other half of the case is removable so that the unit can be assembled. The electrical leads andthe stator (Dof the L'tDT are fastened to the fixed part of the case. A threaded r.od (E), whichalso carries the core (F) of the LVD'F connects the two bellows. This rod is of such, a
length that the bellows are compressed approximately in. when the unit is assembled, and sufficient clearance is allowed between the end plate of the bellows and the housing to give the necessary traverse and still have a positive stop if an excess pressure is applied to the system. Phosphor bronzecompression springs (G) bear against th'e end of each bellows and are retained by the nut (H) that screws into the bellows housing. The annular space between the bellows and the housing is filled with the test fluid through one of the openings in the housing; two openings (I) are provided to facilitate cleaning of the unit. All metal parts are made of the same material as the bellows, so that drifts due to differential expansion or contraction are eliminated. Also, the two bellows connected in opposition act as additional temperature compensators.
Since the spring constant of a bellows is not constant with extension and bellows are subject to hysteresis, the spring constant of the unit is determined, as far as possible, by the compression springs. This results in a more con-stant sensitivity and, as' pointed out above, makes the unit more versatile; it is necessary to change only the springs to change the range. As the units were made, the
7,. t
ARCHIEF
OF SCIENFIFIC IX.STRLMENTS VOLIME 20. NIMELR 10 OCTOBER. 1958
Differential Pressure Gauge for Use with Liquids and Corrosive
Fluid?
H. E. FLOTO\V, B. \1. ABRAHAM. .'.xr R. P.
CARLSONLab.
V.cieepsbouwkun
Argonne .Vaional Laboratory. Le,n,:t. IllinoisV
TABLE I. Typical calibration data.FLOTOW. ABRAHAM, AN!) CARLSON
Average0. 1021
±0.0X.4
atmosphere serves as the, reference pressure for, each bel-lows. There is a slight radial distention of the bellows as the system pressure is changed, which causes a change in the effective area. This results in a zero shift for the instru-ment. Gauges have been constructed of stainless steel and of brass. Since the stainles-steel bellows are stiffer, there is practically no zero shift. By compressing the brass bel. lows during assembly this effect is reduced to tolerable limits. The zero shift can be eliminated entirely if the case is made gas tight and the case pressure adjusted to the mean of the pressure difference measured by the unit. The spring constant for the gauge described, two bellows plus two springs, was approximately 15 kg/cm deflection.
The LVDT (Automatic 'remperature Control Company, Philadelphia, Pa., type 6208A) had a linear range of ±0.15 in. and an output of 0.18 my per exciting volt per 0.001 in. armature motion. The indicator, obtained from the same company (multichannel indicator class 6103A), had a linear scale subdivided into 1000 divisions; each division was equivalent to approximately 0.1 mm Hg pressure. Three volts 60-cycle ac were used to excite the LVDT in the pressure gauge and the matching transducer in the indicator. A multirange vacuum tube voltmeter could have been used in place of the indicator at a considerable loss in predsion. The instrument can be used as an absolute gauge by connecting only one bellows to the system and connect-ing the other to a source of gas and a mercury manometer or other suitable pressure indicator. In this case the gauge will be brought to mechanical and electrical null, and a voltmeter would be adequate.
A set of typical calibration data are given in Table I. These were obtained by connecting the gauge to a differ-ent ial manometer filled with di-butylphthalate. Various pressures of helium were admitted to the system and the differential pressure read to ±0.02 mm with a cathetàm-eter. The density of the oil was determined by direct
comparison with mercury at the beginning or end of the calibration measurements. All pressures were then reduced to standard millimeters of mercury at 0°C. The data in Table I demonstrate the precision of the measurements over the calibration range. It was found that the calibra-tion may change the order of 1% over a period of a year due to a gradual relaxation of the springs.
The operational reliability of the pressure gauge was established by measuring the density of NaKat 54°, 265°, and 545°C and comparing the measured values with the corresponding values reported in the literature.2 The meas-ured and the literature values are given in Table II; each
TABLE II. cornparsonof the measured densities of eutectic NaK with literature values.
-Temperature (°C) 54 265 545 p(Measured) (g/cc) 0.861±0.003 0.813±0.004 0.747±0.004 p(Literature) -(g/cc) 0,858±0.003 0.807±0.003 0.740±0.003
measured value in the table is an average of three or more determinations. The agreement is considered excellent and demonstrates the satisfactory performance of the gauge.
In addition to sensitivity, ruggedness, and reliability, the gauge has several features worth emphasizing. First, the range can be changed by replacing springs without tampering with the test system, which may contain re-active or corrosive fluids. Second, the signal can be trans-mitted to a location remote from the apparatus and, finally, the gauge can be operated at an ambient temperature up to 200°C, the temperature limit of the LVDT.
2 C. B. Jackson, editor, Liquid MeaIs Handbook (1955), Na-NaK,
Supp.,TID-5277. Differential pressure (mm Hg) Instrument reading (scale divisions Sensitivity (mm Hg/scale division) 6.078 59.4 0.1023 11.087 109.2 0.1015 16.429 161.4 0.1018 35.626 347.5 0.1025 58.151 567.3 0.1025
