Topics in..
.
I
Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
1hese articles are inlended lo serve the readem O ~ T I I MJOURNAL bv callino attention to new develwmenls in the them, design, or availabilily of w by p~esent&g useful insighla andexc h e m i c a ~ l a b ~ m instrumenla&m, to~ planations of topics that are of practical importance to those who use, or teach the use of, modem instrumentation and inetrumatol techniques. The editor invites correspondence from prospective contn'butws.
XLV. Pressure Transducers-Part
Figure 8. Dotametricrlnc. Type 1014 Barocel Electronic Manometer.
Two
David J. Curran, Department of Chemistry, University of Massachusefts, Amhersf, Massachusetfs, 01 002 CAPACITIVE PRESSURE TRANSDUCERS The capacitance of any capacitor is directly proportional to the dielectric constant of the medium relative to air, r, and to a geometry factor, 6. The proportionality constant (permittivity constant) has a numerical value of 8.85 X lo-" coulomhP/newton-metera. Thus, all capacitive type transducers depend on either a variation of the dielectric constant with constant geometry or a variation of the geometry with constant dielectrio constant. For a parallel-plate capacitor, the geometry factor is given by the ratio of plate area, A, to plate separation distance, d, so the additional option exists of varying either A or d while holding the other constant. In useful working units, the capacitance of the parallel-plate configurstion is given by:
This equation is valid provided d is small compared with the plate dimensions so that fringe effects a t the edges of the plates may be ignored. The number of design configurations possible for capwitive transducers is large, since: (a) various geometries can he used with single or multiple plates per electrode in either single or differential units; (b) a choice of linear or rotary displrtcement is possible; (e) the variable can be plate area, plate separation, or dielectric constant. Design formulas for a number of cases are given by Neubert (4) and by Foldvari and Lion (10). The most popular arrangement for pressure trmsducers is the parallel-plate capacitor with variable plate separation. Frequently, the design is differential, consisting of a flexible plate (diaphragm) placed between twc fixed plates. Two different types of diaphragms are distinguished: membrane diaphragms where the radially-prestressed metal is usually welded to a support, and clamped d i e phragms which exhibit stiflnessin bending. The nature of the mechmical deformation
of each of these types of diaphragms is different, one from the other, but the fractional change in cspacitance of s. single
membrane diq;hraLm is particularly useful far the measurement of small pressures. The overall characteristics of a capacitive pressure tlansdocer system depend not only on the design of the transducer itself, hilt also an the electrical configuration in which the transdwer eepacit,or is placed and on the method used to convert t,he change in cspecitanee to an electrical output signal. Foldvrtri and Lion (10) list the following approaches to the latter: 1. Methods based on the measurement of charge and discharge times 2. Methods based on division of oharge 3. Capacitive ac-voltage dividers, ac bridges, and differential transformer methods 4. Resonance methods 5 . Frequency modulation 6 . Diode twin-T networks
The first two methods me not suitable for continuous readout applications. Examples of commercially available instruments are presented below. Dafamefrics Incorporated, Waftham, Mass. Pressure transducer systems manufaetured by Datametrics are multirange instruments designed for the measurement of pressures in the low to medium (several atm) range. Figure 8 illustrates the Type 1014 Barocel Electronic Manometer. The pressure transducer shown in the foreground is the Barocel Type 511 which is available in standard ranges of 0-1, 0-10, (t-100, and 0-1000 torr and 0-1, 0-10, and 0-100 psi, gauge, differential, absolute, or sealed-to-absolute. The Barocel has a differential parallel-plate capacitor configuration with a membrane type diaphragm. Electrically nonconducting liquids and gases may be accepted a t the pressure
ports. System stability is suoh that the detection limit of the 0-1 tom transducer is 1 X 10-Vorr. Converting the upper limit of the highest range psi Barocel to torr, i t is seen that the system will measure pressures from 1 X lo-$ to better than 5 X lo3 torr. The two capacitors formed by the transducer are arranged as an ac-voltage divider and placed in an LC-bridge network. The bridge output voltage is determined by the r h o of the two capacitances. The ac carrier signal (10 kHz) applied to the bridge is amplitude modulated in response to the input pressure. Because capacitive transducers have high output impedance, and because bridge circuits present grannding problems, some elect,ronie circuitry is physically located within the housing of the Barocel. This includes the transformer for coupling the carrier signal with the inductive arms of the bridge, a unity gain solid state impedance isolation amplifier, ac t,o dc conversion circuits to Dower the amolifier. and an out. ducer can he operated remotely from the rest of the transducer system. Cable lengths of 15 i t are standard but 500 ft can he supplied. The cabinet of the Type 1014 houses a regulated dc power supply, an amplitodestabilized 10 kHz oscillator, a high precision calibrated voltage attenuator (range switch), a signal amplifier, a synchronous phase-sensitive detector, a panel meter for readout, and dc output jacks. Solid state circuits are used throughout. The sequence of events for an input signal from the Barocel is the following: attenuation according to the range switch setting ( X I , X0.3, XO.l, X0.03, XO.O1, X0.003, X0.001), signal amplification, phase sensitive ae to dc conversion, and readout. The result is a &5 v dc full-scale signal for all positions of the range switch down to the XO.OO1 setting. Polarity of the dc output is determined by which pressure port (PCor P*) receives the highest pressure. System accuracy is + I % of full scale when panel meter readout is used. The aecuraoy of the dc output voltage is f0.20% of reading plus 0.0370 of full scde (Continued on page A466)
Volume 46, Number 7, July 1969
/
A465
Chemical Instrumentation on the X I and X0.3 settings m d 1 0 . 1 % of reading phrs 0.03% of full scale on the remaining settings. Linearity for the two highest range switch settings is *0.1'% of full scale and better than +0.05% of full scale on all positions of the range switch from X 0.1 through XO.OO1. Hysteresis depends on the range of the 511 Barocel: the worst case is i 0 . 1 ' % of pressure cycle for the 0-100 psi model and decreases to O.OOlO/Ofar the 0-1 torr unit. Another Baroeel series, the 538, is designed to provide better accuracy far the higher pressure ranges. Hysteresis error has been rednced to +0.04% of presswe cycle for the 0-100 psi transducer. Other system specifications include: an internal volume of 0.3 cubic in., standard, and less than 0.1 cubic in. on special order; a typical response t,ime a t one atm line pressure i s 3 msec; the transducer operating temperature range is 0-15O0F. Temperatme errors are less than 5 X 10-'%/F0 of sensor full range for zero shift and less than 1 X lo-'% of reading/Fa for sensitivity shift.
Figure 9. Dofarnetricr Inc. Modular Electronic Manometer System.
Othel transducer systems available from lhtametrics are the Model 1023 and Model 1018 Barocel Electronic Msnometers and a modular system (Figure 9) consisting of the Type 511 Barocel transducer, the Type 1015 Signal Conditioner and Type 700 Power Supply. The Model 1018, shown in Figure 10, is a. modification of the Type 1014 to incorporate automatic digital readout. I t is likely that this feature is achieved by analog to digital eonversion of the analog signal of the system. Binary-coded-decimal (BCD) readout is also provided.
Table 4. MKS Series 77 Baratron Electronic Pressure Meter System Characteristics
Transducer ranges: fI, i 3 , 1 1 0 , &30, 1 1 0 0 , *300 zt1000 torr, standard Resolution: ~nfihi$e(voltage output) Hysteresis: Less than 0.00390 of pressure cycle applied for 30 tom range, 0.015% for 100 torr, 0.04% for 300 ton, and 0.08% for 1000 torr ranees Accur&: A . Meter readout. One to three percent of full scale depending on range selected. Individual scale accuracy can be improved bv front an el add..""...".."
Figure 11. Dotometri
