Topics in..
.
Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079 These articles are intended to serve the readers of T H JOURNAL ~ by calliny attention to new developments i n the theory, design, or availabilily of chemical laboratory inslrumentation, or by presenting useful insighh and ezplanations of topics that are of practical importance lo those who use, or teach the use of, modern instrumenlalion and instrumental technioues. The editor inviles c&espondence from prospective eonlribu1ol.s.
XLV. Pressure Transducers-Part
One
DAVID J. CURRAN Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01002 Pressure is a seelw quantity expressed in terms of force normal to a unit surface ares. Since we live subject to a finite pressure, this vmiable affects our nNnirs in ways too numerous la mention. It is not surprising however t,hat the meamrement of presswe has received considerable attention. The result is an array of instruments, rather spectacular in terms of its number and variety, for pressure measurements. According to Behar, instruments wailable cover a pressure range of lot5 (1) and i t is likely that another order or two of magnitude has been added to this in the last decade. No single instrument will adequately cover the entire range but some types of pressure transducer system are capable of meaningful prssure measurements over a range of about loD. As a consequence of the wide variety of disciplines concerned with pressure measurement, a large number of units of pressure are employed. Table I presents a short listing of conversion factors far those units most familiar to chemists. I n addition to the unit of measurement, the reference point against which the measurement is made must be distinguished and several possibilities arise here. The terms commonly used are absolute,
gauge, and differential. The first refers to a. reference point of zero pressure, the second to a. reference point of atmospheric pressure, and the third to an arbitrary reference point, but one which may he either of the first two, or one which need not even be constant. Although standard conditions for normal atmospheric pressure have been agreed npon, prevailing atmospheric pressure is seldom equal to normal atmospheric pressure and is not necessarily constant. Either consideration may be of no consequence to the measurement being made. A vacuum is a negative gauge pressure and is also commonly referred to as a reduced pressure (i.e., a pressure less than atmospheric). An earlier seriei of articles on pressure measurements written by S. T. Zenchelsky has appeared in these columns (2). Commercially available devices for the measurement of reduced pressures were eonsidered. The purpose of the present series is to expand the discussion to include ahsolute, gauge, and differential measurements but to limit the discussion to include only representatives of that clsss of pressure transdncers which reqrdre some kind of mechanical input and which produce an electrical output. Soch transducers have been classified by Lion as in-
Table 1. conversion Factors for Pressure Units (Rounded t o 4 significant figures)
Pa Bar Atm Torr psi in. HzO
Pa
Bar
1 1.000 ( + 6 ) 1 013 (+5) 1.333ii-z) 6.895(+3) 2.378 (+2)
1.000(-5) 1 1 013 1.333(-3) 6.895(-2) 2.378 (-3)
Atm
Torr
9.870(-6) 7.502(-3) 9.870(-1) 7.502(+2) 1 7 600 1+21 .. . 1.3~-3) 1 6.805(-2) 5.172(+1) 2.458 (-3) 1.878
psi
in. KO
1.451(-4) 1 . 4 11 1.470 ( + I ) 1.934i-zj 1 3.613 (-2)
4.205(-3) 4.205(+2) 4 068 (+2) 5.3n-ij 2.767(+1) 1
1 pascal (Pa), the internationally agreed upon unit of pressure in the SI system, = 1 newton of force per square meter. 1 bar = 10"ynes/cms. 1 tom = 1 mm Hg a t O0C. psi: poonds (weight) per square inch. Inches of water, specified a t 4% Interpretation of tabular data: 1 atm = 1.013 X 10' Pa.
Dr. Curran received the BS degree in chemistry from the University of Massrtchusetk. After service with the USAF he enrolled at Boston College where he wrote a. MS thesis in the area of autornabic instrumentation for caulometric titrations. The Phl) degree wa5 awarded by the University of Illinois in 1961 upon completion of a thesis involving the palarography and chronapotentiometry of aromatic nitro compounds a5 a mean? of analysis for nibrate ion. From 1961 to 1963 he was assistant professor of analytical chemistry a t Seton Hall University. Fallowing a summer research associateship at M.I.T., he joined the chemistry department s t the University of Massachusett,~. His research and teaching interests are in ihe area of chemical i~lstromentation and eleetraanslytiesl chemistry. His current research pmgrams involve the use of pressure transducera in chemical analysis and the use of ~.efractorymaterials as eiectrades in electroanalytical chemistry. put transducers (3). In a sense, t,he word "transducer" is not useful by itself since i t means a device which transfers. Thw, a. conveyor belt is a transducer. More commonly, a transducer is thought of in terms of energy conversion and Neubert (4) has made the useful distinction between transducers which are energy converters and those which are energy controllers. The latter require some auxiliary source of energy while the former do not. He further points out that transducers useful for measurement purposes are properly (Continued on page A402)
Volume 46,Number 6,June 1969
/
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Chemicul Instrumentation referred to as instrument transducers since the use of this word as a modifier alters the meaning to include t,he important concept of a known and reproducible relationship, which can be attained with a known limit of accuracy, between inpnt and output. An mambiguous further breakdown of pressure transducers into some kind of classification is a little difficult because the useful pressure ranges of the various t,ypes overlap in many cases and the choice exists of considering the nature either of the input or of the output of the device. However, practice in the industry seems to be to classify them according to t,he nature of the electrical ontput mechanism and this expedient will be adopted here. Wibh the exception of a few miscell~neous t,ypes, the pressure transducers to be considered fall within the electrical output charact,eristics listed in Table 11. An arbitrary but representative selection of examples of commercially available devices will he presented. Detailed discussions of these types of pressitre transdoeers, and others, are found in the hooks by Neuhert (4) and by Lion (3) and in references ( 1 ) and (5). Table II. Classification of Electrical Output Characteristics of Pressure Transducers 1. Variable resistance 2. Variable inductance 3. Variable capacitance 4. Piesoeleetrie
A number of types of pressure transducers are classified as energy controllers. The input pressure is applied to some mechanical pressure-sensitive device which in turn controls the electrical output,. Clearly the mechanical characteristics of the input device are very important to the overall performance of the transducer. Such input devices include: diaphragms of varioos tgpes; capsules; bellows; C-shaped, twisted and helical Bourdon tubes; and several other types of tubes. In general, the pressure range increases in the order listed. Diaphragm deflections on the order of Angstroms have yielded meaningful pressure measurements while Bomdan tube deflections may be frsctions of an inch. Details and literature citations may he found in references (5) and (4). I t is necessary to distingoish between transducers and transducer systems. The latter are complete instruments including all components from the pressure input to the electrical readout. They therefore include the transducer as well as power supplies and any other circuitry and auxiliary equipment necessary. A large nunher of companies in the United States are engaged in the manufacture of pressure t,ransdr~cersand systems, and useful listings of them are found in a number of sonrees including the Buyers Guide Issoes of Instruments and Control Systems ( 6 ) and Electronics (7) and the Laboratory Guide of Analytical Chemistry (8).
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Journal of Chemical Education
POTENTIOMETRIC PRESSURE TRANSDUCERS The potentiometric transducers are so named because the electrical output is taken between the wiper arm and one end of s. variable resistor (potentiometer). The position of the wiper arm is controlled by a. mechanical linkage which responds to the displacement of the pressure input device which is used to sum the force applied to it. Compact units weighing as litt,le as a few ounces are available from a number of manufacturers. Industrial models with explosion proof housings weigh only 8. few pounds. The National Pipe Thread (NPT) type of firling is common a t the pressure input but nearly any fitting can be provided bath a t the pressure inpnt and at the electrical input/outpot connect,ors. Stainless steels and Ni-Span-C are msterials commonly exposed to the pressure media. Other materials are available and oil filled units can be purchased for very severe chemical environmental conditions. Pressure ranges from 0-1.5 to 0-20,000 psi are available in gauge and absolute configurstians as standard units and ranges outside of these limit* can be obtained on npecial order. Differential unit8 are arranged in either zero-center (bidirectional) or zero-end (unidirectional) configurations with pressure ranges from a few tenths to several tbhoosand psi. Resistances are typically in the range of a few hundred ohms to 1.0 kilohms with a power dissipation of 0.5 to 2 W within the operating temperature limits of the transducers. Of special importance to the user is the nstme of the inpnt/output relationship of the transducer. Since both mechanical and electrical considerations are involved, the situation is best expressed in terms of a calibration curve or plot of input pressme versus electrical output obtained under static conditions. Expressing both of bhese quantities on a relative basis in terms of percent, the ideal calibration curve would be s. straight line between the points with coordinates (0% 0%) and (loo%, 100%). The term static error band is used to express the maximum deviation of the output from this straight line, but the term is usually only applicable between 2y0 and 98% of the input range. One of the chief reasons for this range of applicability is that i t has not been possible to physically arrange the end windings of the potentiometer so that a truly linear response can be obtained in these regions. The static error hand includes the effects of the following sources of error: nonlinearity, friction, hysteresis, finite resolution, and repeatability. A typical static error value for off-the-shelf transducers is +I% of full scale output. The dynomie error band includes all of the effects listed for the ststio error band except friction which is eliminated by mechanically vibrating (tapping, for example) the transducer during testing. Other sources of error include: temperature, shock, vibration, and acceleration. The latter three are probably of minor importance to chemical applications of pressure transducers hot specifications for errors arising from these sources are
frequently given by manufacturers. The effect of temperature changes is expressed in the same fashion as the static error band as a percentage error referred to full scale output which covers that temperature range specified for the transducer. For example, a transducer may be rated for operation between -10O0F and +300°F with a temperature error band of &2Yo. I n short, t,he temperature error band is the static error bend determined over the specified temperature limits. The temperature range given in this example is fairly typical far standard units and temperature error bands are usually in the range of one to a few percent. It is noteworthy that the linearity error is smaller than the static error hand. This means that for those applications where only relative changes in pressure are important, somewhat better performance can be achieved than would he indicated by the static error band. In this regard, specifications for resolution are usually provided and are frequently a few tenths of R. percent of full scale outputs. Dynamic response is of interest in many applirat.ions. Potenbiometric pressore tra,nsducers are fairly slow in this regard with response times in the millisecond range. Specifications are usually given in terms of response to a step fonction pressure inpnt a5 measured over one time cans t a n h t h a t is, the time required to respond to 63y0 of the full pressure input signal. A block diagram of a potentiometric transducer system is shown in Figure 1 where the dotted line encloses all units which may be physicdy housed within the transdocer. The pressure sensitive devices which have been used are diaphragms, capsules, and Bourdon tuhes. The output may be fed directly to a readout device or through a signal conditioner unit (such as voltage to frequency conversion) to the readout. Extremely simple systems may be constructed or purchased with potentiometric transducers since the power supply need be nothing more than a. Hg battery and a few resistors and the readout can he a panel meter, although care should be taken not to load the potentiometer with the meter. Inexpensive (around $225) line operated systems are available from a. nomber of manufacturers including: Robinson-Hdpern, West Conshohocken, Pa.; C-I3 Electronics, Glenside, Pa.; International Resistance Co., Control Components Division, Philadelphia, Pa.; snd Thomas A. Edison Industries, Instrument Division, West Orange, N. J. These systems consist of two units: the
Figure 1. Block diagram of o typical potontiometris pressure transducer system. The dashed line indiceter thore units whth may be homed within the transducer.
(Catinued a page A404)
Chemical Instrumentation pressure transdocer which houses circoit,ry for xc to do conversion, and a panel meter for readout. System accuracy is +2% relative. Some loss in accuracy is suffered by u i n g the panel meter since the t,ransdocer generally bas a better accuracy. Solid state voltage t o frequency conversion modules calibrated in pressure units are manufactured by Dynamic Precision Controls Corporation, Hagerstown, Ind. Special featmes are available with potentiometric trsnsdueers whieh reflect, the versatility of potentiometers. Tapped slide wire and dual potentiometric outpnts can be obtained from companies such as Bourns, Inc., Trimpot Division, Riverside, Calif.; Conrac Corp., Avionics Group, 330 Madison Ave., N. Y., N. Y.; and Sparton Southwest Inc., Albuquerque, N. I f . Aero Mechanisms, Inc., Van Nuys, Calif., offers their Model 9207 differential presvure lransducer whieh has a n output. proportional to the square root of the differential input. The transd~teersmentioned thus far all operate open loop. Farce balance, servo, or null type potentiometric pressure transducom are also available. Since t h e transdneer operates via a mechanical input and an electrical output, the feed back system is of the electromechanical tvve (note the analogy t o the patentiornet& strip chart recorder hut with inpnt-output features reversed). All of t,he u s d advantages of feedback systems apply here such as improved stability and accuracy, and less noise and distorbion. The principal disadvantage of electromechanical feedback systems is their relatively slow response times. A differential model operating a t h 2 0 0 in. of water (approximately *0.5 atm) is made by Aero Mechanisms, Inc. Dual output potentiometers with s, turns ratio of are provided. Conrac Corporation's Model .%TI8 Null Balance Transducer ha5 a n abvalute accuracy of hO.l t o 5 0 . 2 % of full scale output for several ahsalute or differential ranges between 0 and 2 stm.
is clearly an advantage in having the highest possible velilc of S for t,he wire wed in the strain gauge. Temperature vmiriations are the most important source of error affecting strain ganges. Two effects may he distinguished: a change in the wire resistance as reflected in a change in bhe zero point with no strain applied externally, and a change in the strain on the wire, which shifts the calibration cmve. Eibher the gauge must be ape1,ated s t constant temperatwe or some form of temperature compensation must he supplied. Two distinct classes of strsin gauges have been developed. The first of these is the nnhonded hype which in principle is nothing more than a wire with one end fixed and the alher end free to be suhjocted t o a displacement by a force somming device. The second ehss is the bonded strain g a ~ ~ g eHere, . the gauge is acboally bonded to t,he member under strain. For example, if the force summing device were a diaphragm, then an nubonded gauge wonld involve direct branslation of the diaphragm displxcement to the movable end of the gauge wire while in the honded case, the strain gsuge in its entirety could be banded by some sort of adhesive to the diaphragm. However, sometimes a strain sensing element is interposed between the force somming device and the strain gauge. The important difference between the two classes rests in the fact t h a t s t equilibrium the force applied t o the diaphragm is balanced primarily by the wire of the gauge itself in the unbonded case and primarily by the diaphragm in the bonded case. Given the same force acting on the diaphragm in both cases, the nnhanded gauge is the more efficient of the two. For this reason, the unbonded gange is usually chosen for low force situations hut either may he used a1 high range*. High temperature apemtian uf t,he bonded stl.ain gauge is ~wtmllylimited by t,he temperature characteristics of the adhesive used in the honding process. The nubonded gange
STRAIN GAUGE PRESSURE TRANSDUCERS The strain gauge has been developed to a high state of perfection as a n electramechanical transducer. The literature on the subject is vast and only the barest essentials can be provided here. If a uniform stress within the elastic limit is applied along the longitudinal axis of a wire, the wire resistance, R, will change because of the increase in the length of the wire (AT,), the decrease in the wire diameter, and the change in the resistivity of the metal. Thra, pressure transducers incorporating strsin gauges fall within the elassificat~ion of variable resistance pressure transducers. The stmin sensitivity, S, is expressed as a p w e number by the equation, S = (AR/R)/(AI,/L). Talues vf S for wires of most metals and alloys cat, be pwil.ive or negative and are typically less than seven and freqnently close trr two. Othel. faet,ow being e q d , there
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Journal of Chemical Educafion
Figure 2. BLH Electronicr, Strain Gouger.
Inc.,
Etched Foil
is therefore inherently more useful under extreme temperalure conditions. The 1.eInlionship between the change in resistance
and the applied slrain is linear for both types of gauges to a t least 1% relative and accuracies of 1 0 . 1 % relative can be achieved with either arrangement. Tho rhange in resistance of strain gauges earresponding to a maximum strain inpnt is small being on the order of 1y0of the total resistance. Multiple windings are nsed to increase the total resistance of both the honded and unbonded types and the flat grid is a papular arrangement for the former. I n addition to wire filaments, several other resistive elements have heen used in bonded strain gauges. Metal fails are ,wed by a, number of manufacturers. The foil is usually in the form of a. grid, and is plwdnced by an etching process. Figure 2 shows a. ohotoeraoh of etched foil atrain on a backing whieh may serve to elecbrically insolate the gange. Additional insulation may be obtained from the bonding adhesive. When backing is nsed, both i t and the adhesive are integral parts of the strain gauge and great care must be exercised in their use since t,he stress applied t o t h e gauge is transferred through them. T h e bonding material is also sometimes used t o coat t h e gauge itself to protect i t fram homidity a t ambient temperature. Another resistive element in use is the semiconductor. Development of strain gauges utilizing semiconductor properties has been an outgrowth of solid state technology. The gauge (or gauges) may be produced directly on a semiconductor material, usually silicon, by inbegrated circuit diffusion techniques. Controlled addition of n- or p-type impurities produces the desired linearity and temperature characteristics. The gauge factor, G, for a bonded strain gsuge is the slope of the linear ontpnt,/input relationship and is determined fram a plot of the fractional resistance change, ARIR, versus applied strain as measnred with the strain gauge cemented to a standard. Semiconductor gauges have high gauge fact,ors, typically 2 ~ 5 0t o 1 2 0 0 according to Perina (8). The combination of semicondltct,or technology and high gauge factor ha5 permitted the prodoction of extremely small pressure transdueet.~. Several examples will he cited below. Vacuum depositiau techniqnes have l~oerr adapted to the production of thin film metal and metal alloy strain gauges ( 8 ) . A ceramic film is deposited on a. metal srrbstrate and the strain gauge is then deposited un the ceramic inst~lator. A principal advantage of this method of preparation is the control it affords over the properties of bath the insolator and the gauge. Geometl.ies can be made rinifarm in terms of shape, area, and thieknew. This permits optimization of characteristics such as gauge resistance, gauge heat dissipation, and temperlzture campensation. Gauge factors comparable with those of silicon semi con due to^^ gauges have been achieved. The Wheatstone bridge appears to be universally wed as the electrical configot~ation for strail, gauges. B1.idge excitation is freqnently 10 vdc but ae excitation is also possible. Higher excitation voltages (Continued on page A406)
Chemical Instrumentation can be nsed with the higher resistance gauges since t,he main limiting iactur on t,he excitation voltage is the pl.ohlem of heat dissipation in the gange resistance. The o s u d output impedance of the bridge is 3Xl ohms but several thousand ohms can be obtained in thin film and semicondnetor gauges. Several different arrangements of the strain gauges in the hridge arms are possible. Most common pmctiee for static measurements is to w e active strain gauges in all four arms. One pair of opposite arms is tensed and the remaining pair is compressed. The symmetry of bhis push-pull operation provides good temperature compensation and the highest possible outprlt. Bridge dfbalance is measnred a t the output terminals. T h e magnitude of t h e signnl is most often n. few millivolts per volt of excilatian for full scale output. However, some bridges with semiconductor gauges have out,pnts ten times this figure. Other bridge arrangements are one or two active arm circuits which may include dummy gauges far temperatwe compensation. Strain is applied to the active gauges but not t o the dummy gauges. Still other circuits use mtive gallges and passive resistance components. The latter circuits are mare commonly found with strain gauges designed for dynamic response where temperature compensation may not be quite so critical.
Figure 3. Cutaway view of Viatran Corpomtion Model PTB103 P r e m r e Transducer.
Pressore transducers based on unhanded
Figore 3 shows a cnt,zway view of Vintmn Ca~.poration(Buffalo, N.Y.) Hodel PTB103 pressure transducer which is nvnilnhle in twenty-six ranges from 0-5 throtlgb 0-25,000 psig or psin, and from (1-5 through 0-2000 psid (ouidirectioaal). The pressme connection shown a1 Ihe hotlum of the picture is s in. N P T female pipe
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Journal o f Chemical Education
connector. F h i d pressure is applied to a, stainless steel force-somming diaphragm which is mechanically coupled to the farce sensing element located above the diaphragm. A four active arm foil Wheatstone bridge circuit is actuated by the sensing element which is enclosed in a reference pressure can a5 shown. Resistors for circuit adjustment can be seen external to the pressure can. The electrical connector s t t,he top of the stainless steel case is a standard Cannon t,ype. The nnit stands about 3'19 in. high and is Z 1 / 9 in. in diamet,er a t the widest point. Easy access for cleaning and repair is provided by the holted eonsttuction. Ot,het specificat,ionsare listed in Table 111. Modifications available inclode: Teflon coating on all parts exposed t,o Lhe pressure fluid, shunt. calibrating resistor to simolate a. particular pressme input, special electrical or pressure connect,on, and higher proof preswwes. Other pressure transducers available from Viatrnn Corp. feature higher pressure ranges, better acm~racyand hidi~.eelionaldifferential operation. Complete transducer systems are also msnofnet,ltrcd in portable, rackmoon1,ed and cabinet forms.
Figure 4. MB Electronics Model 151 -EBA-1 Pressure Trmrducer.
Andher ioor adive arm foil Wheatstone Bridge st,rain gauge pressure transducer is MB Electronics (New Haven, Conn.) Model 151-EBA-1, shown in Figure 4 for tho 0-.i psig range. This in a highly nccwste unit with a combined error (linearity, hysteresis, and repeat* bilit,y) of f0.2% of f d l scale out.put, maximum. Ot,her ranges available in this model are 0-10, 0-1.5, and 0-20 psig, psia, and psis (pounds per sqnare inch sealed). The ~.efcrencepaint for the latter type is ~ ~ s u s l 14.7 l y psia but the reference pressure compartment is permanently sealed rather than vented to the atmosphere. Anot,her M B Elee1,ronics pressure transducer is shown in Figure 5. The Model 510 is pradnced in nine ranges from 0-50 through 0-10,000 psia. I t is particularly interesting because it represents an advance which overcomes one of the important shortcomings of strain gaoge pressure transdocers, namely low output,. Combined with t,he foil strain gallge circtlit (4 aetivc arms) is an integt.ated circuit m o d ~ ~ l e which provides x 5 v output a t full scale pressure input with 28 v dc applied. Another interoat,ing M B Electronics pres-
(Continued on page A408)
Chemical lnstrumenfation
temperatore range is from -4.52-F to t 2 5 0 " F and the camnensated temnerature range is from -1320'~ to +77"F. ~
~
.~~~
Table Ill. Specifications for Viatran Corporation's Model PTBI 03 Pressure Transducer Linearity (terminal) error Less than +0.7.570, full seslo output Less then +0.2570, full scale output Hysteresis Less than +O.lrJ,, full scale output Repeatability error Infinite Resolution Zero balance Within +2%, full scale output at 7 0 T Full scale ootnut 3 mv/v minimum Bridge resistance 120 or 350 ohm 120 ohm: 10 v maximum Excitation voltage 3.50 ohm: 1.5 v maximum -40 to 2;iO0F, 350°F optional Operating temperature Less than 2%, full scale autput,/100'F Thermal zero shift Less than 270, R111scale ootput/l0O0F Thermal span shift ,50070 or 10,000 psi, whichever is less far 5 through Burst pressure 5.000 nsi 20& 0;. 30,000 psi, whichever is less for 10,000 through 25,000 psi 150% or 25,000 psi, whichever is less Proof pressure 2 kHz (nominal) far 0-5 psi extending t o 40 kHz Natural freqnency (nominal) for 0-25,000 psi Sensitive only in the axial plane where sensibivity Acceleration error extends from 0.0870 FSO/G for 0-5 psi to 0.01% FSO/G for ranges over 0-100 psi Less than one millisecond Response time 0.040 cubic inch- nominal (does not include fitting) Cavity volume IX130 to $186 Prices sure transduoer is the Model 172, de-
signed for cryogenic work. Pressore ranges from 0-20 through 0-10,000 are standard in psig! psia, and psis and go as low as 0-10 in psrg and psia. The operable
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Journal o f Chemicol Educotian
Other manufacturers using integrated circuits for increased antput, with strain gaoge pressure transducers include: West Coast Research Corp., Los Angeles, Calif., (Continued on page A4lO)
Chemical Instrumentation and Coutant Electronics Limited, England (distributed by Ilentronics, Ine., Hackensack, N. J.).
Figure 5. MB Electronics Model 510 Pressure Trmsducer.
Figure 6 shows a Model STD-H2 pressme transducer made by BLII Electronics, Inc., Walthsm, hZasn. This is a. high temperature high pressure unit designed in two standard pressure ranges, 0-30,000 psig and 0-50,000 psig with continuous duty to 6 0 0 T and temperatitre compensation to 500'F. Another model, the STI), has operating pressure ranges as high a7 0-200,000 prig wiLh a compensated temperature range of 15 to 11S0F. High pressore range bonded transducers are
Figvie 6. BLH Electronics Model STD-HZ Pressure Tionrducer.
(Conlinurd on page A/,lZ)
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Journol of Chemical Education
Chemical Instrumentation also available from Astw Corp., Willow Grove, Pa. (0-150,000 psig), Standard Controls Inc., Seaille, Wash. (0-1011,000 psig) and General Transducet. Co., Sauta Clara, Calif. (0-100,000 psig). The latter company olfers combination pressure and t,ernnerat,nre trensducers which w e a n iron-constrtnt,an t,hermocouplefor temperat,nre measurement. Provision is made for water or air coaling the unit. The force summing diaphragm can be exposed to temperstores as high as 7 X ° F and pressures to 15,000 psi can he mcasnred. These transducers have been especially designed far the pla~t,icsand rubher industry, particularly for extroder operation. Alnor Instrument Company, Chicago, Ill., also supplim transd\~cersfor this po&se. G&d Transducer Co. also makes pressure transdoeer syst,ems includine " doal indicator units t,o simultaneously read hot,h temperature and
- - ~ ~ ~ . ~ ~ ~ ~ ~ ~
""eq.al,r*
The BLH Eleetronies transdncer shown in Figwe 7 is the illadel IIIIF-1)s. A semie&~dnct,orstrain gauge bridge provides a foil scale out,p,it of one volt with 10 v se or dc bridge excitation. This onit can be wed as a replacement for wise and foil transducers. One of t,he great advantages of silicon semicondncbor gaoges is their compatibilit,y wilh very small siae requirement,^. When small size is combined with fltmh diaphragm configuration, very good dynamic response is possible.
Figure 7.
BLH Electronics Model DHF-DS Prer-
'we
Transducers of thin type are manufactured by Scientific Advances, Inc., Waltham, Mass., K d i t e Semiconductor Products, Inc., Ridgefield, N. J., and Whittaker Carp., Instrument Systems Division, Chatuworth, Calif. The natural frequency of the transducer depends on the pressure range. For pressnres to several t h o ~ ~ s a n d psi gauge or absolute, n a t o r d frequencies to as high as 500 kHz are available. The small siae is &king. Kalite's Model CPS-125-1000 has a p r c w u e sensitive
Chemical instrumentation diaphragm of 0.08.5 in. diameter and s. t o t d configuration diameter of 0.125 in. Sensotic's Model SA-SA 8J-GH has an identical configr~rationdiameter and Bytrex's smallest transducer is the model HFO with dimensions: 0.09 in. diameter X 0.15 in. long. Unbonded strain gauge pressure transduce~sand thin film bonded strain gauge transdacers are made by Sratham Instruments Inc., Los Angeles, Calif. A great variety of models is manufactured for gauge, absolute, and differential measurements. Complete pressure t,ransdncer systems are also available. Unbonded strain gaoges can also be purchased from CEC Transd~leerProduete Division of Bell and Howell, Monrovia, Calif., and Dynisco Division of American Brake Shoe Co., Cambridge, Mass. Of interest is Dyniaco's Model P T 14 differentialpress w e t,rmsdncer far applications as low as 0-1 psi full scsle.
(3) LION, K. S., "Instrumentaticn in Scient~fieIlesearch," RleGrav-Hill Book Co., New York, 1059. (4) NI:UHI:~T, H. K. P., "Instrument Transducers," Oxford University Preis, London, 1963. ( 5 ) AnoNsoN, M. H., ed., "Pressure Hand-
book," Instruments Publishing Company, Ino., Pitt~sburgh, Pa., 1963. (6) Buyers Guide Issue, Znstrzrrnents and Control Systems, Himhach Publioations Division of Chilton Co., Philadelphia, Pa., 1969. (7) Bnyers Guide, Electronics, 40, No. 22A (1967). (8) Laboratory Guide, Anal. Chem., 40,
No. 9 (1968).
REFERENCES (1) B c m n , M. F., "Handbook of Measurement and Control," The Instruments Publishing Co., Inc., Pittsburgh, Ps., 1959, Chapter 3. (2) ZINCHI:ISIW, S . T., J. Chem. Ed., 40, A611; A771; A849 (1963). -
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journal o f Chemicol Education
(9) P s n l ~ o P. , R., Instruments and Control Systems, 38, No. 12 (1963).
Pert Two o j "Pressure Transdmers" will appear in the July 1969 issue.
