Topics in..
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Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079 soriesfor weighingin remote or inacressihlr locations. The beam and hxla~~cing mechanism can be mounted separate f r o m the power supply and controls. For example, weighing in a. vacuum system ol. controlled-atmosphere chamber is arcomplished with only the wires snpplyiug w r rent to the eoil passing through the walls of the container. Tho hnlnnces arc
These articles, most of which are to be contributed by guest by calling a t t a t i o n authors, are intended to seme the readers of this JOURNAL lo new deyelopmenk i n the theory, design, or availability of chemical laboratmy i m h . u m a t u t i a , OT by presentilzg usefnl insights and explanations of topics that are of practical importance to those who use, or teach the use of, modern instrumatation and instrumental technipes.
(Conlmmrl on page A8)
XXXV. Modern Laboratory Balances (concluded) Part One-Lever-arm
Balances (cont'd)
World's First Electrobalance?
ROLAND F. HIRSCH, Department of Chemistry, Seton Hall University, South Orange, New Jersey 07079 ELECTROBALANCES
A number of lever-arm balances are available in which an eleotromagnetic counterforce is used to restore the beam to the null candit,ion. The principle of operation of themoresensibive instruments of this elms is illust,rated in Figure 10. The sample is loaded on a. pan hung from loop a or loop b (lower sensitivity). A counterpoise may he placed on the pan below loop e. The beam deflects downward on the left. to the beam, and a stationary permanent. magnet is mounted with its poles above and below the coil. When current passes through the eoil a. torqw is applied to the beam proportional to the current. A potentiometer ( R I )is adjusted to provide suffioient current to the wi1.e coil to bring the beam to the horizontal, as indioated by the alignment of the reference line and the beam pointer, or by an a~itomaticnull detector. Then, potent,iometer R2 is adjusted to measure the voltage across RI, with the aid of the galvanometer G. The variahle resistance ndjusts R1 to zero with the empty pan or container hung from loop a
or b. A calibrsbing weight hung from the same loop sets the 100'7, position of R2. The position of R1 a t null halance is thus equivalent to t,he fraction of t,he calibrating mass represented by thesample mass. I n this type of balance the crilical factors are the stability of the batt,ery or rectified AC power snpply, the sensitivity of the galvanometer, a d the prerision of R2. ActudIy, the laaler is the most significant component, since the current stability is high and regularly cheeked in calibrating R*, and galvanometers of more than dequat,e sensitivity are available. The limit of precision of the electromagnetic mechanism is t,herefore that of the best potentiametea available for the p u r p o s e a b o u t i O . O l $ i . This i. not, however, the limit of precision of weighing, since part of the sample may be tared by a weight, in loop c. With Class M weights, the taring of up to 500 mg is possible wit,h a precision of better than 1 5 fig. A 50 mg electromagnetic range will not affect overall precision, if a high quality potentiometer is used. If the weights are properly calibrated, some elect,rohnlances can be operated with precision of 1 0 . 1 r g for milligram-sized samples. The Cahtl Instr\lment, Co. offers awido range of elect,robalanees, as well as acces-
IR%li:
"The parallel plates were m Electromognelis Lever-Arm Balance.
Volume 45, Number 1, January 7 968
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Chemical lnstrirmentatien portnl,le and ran be used immediately after movmg them. Two manna1 models are %,Id by Cahn, along wit,h several reoordillg balances. In the Cahn .\I-10 ($820) the beam is snspeuded by a. pivot,, which limits sensitivi1.y 11, & I pg and rapacity to 175 mg. I1 is hxtt.erv . ooerated. The Cahn GRARI halanre has a torsion hand snsoension. semit,ivil,yof 1 0 . 1 rg, w d capacity of up t ~ 1.5 , g. I t is shown in Figure 10. I t can be uhtained in battery (8973) or lineoperated ($1095) versions. All Cahn bnlanres come wilh x sel of precision t.esistor.s whirh are placed in the electrical circuit t,o select. the weighing range. The smallest I.ange is 0-1 mg (full scale on Rz). Interchange belween ranges is pussihle dtrring a weighing operxt,ian. Precision of the electnmic system is 10.' times the muge, and pl.ecisiou of weighing a tared sample is xboul. I0 ppm. The SchulL~ Model 30 microbalance has a t,wu-pan, equal-arm, design. The beam ;tnd pan holders are suspended by plnt,inum alluy hands. The null position is det,ennined with the aid of a photot r m s i s t , ~imbrrlnnce ~ detector (to he described in a later section). The balance meehnrrism may he mounted separately from t.he controls. The capacity is 1 g, and four electronic ranges can be chosen. P1.eeisiov uf the electmmagnetio system is +0.01:;,, :andlargeweights can be-measured to ~0.0110.5~, precisiou (5 ppm) if partially tared. The instrument is line-operated. The Beckman EMB-1 microbalance ($1035) is also s, two-pan instrument. The balance mechanism and control unit are aepilrate from each other, faoilit,ating remote iust,allatian of the mechanism. Phototransistom provide a signal which is amplified lo produce automatically the current necessary to balanoe the beam. The oapacily is 2.5 g on either pan and the weighing ranges are from 0-1 to 0-200 mg. Sensit,ivity is 0.1 pg, and precision ranges from ;t0.05 to =t0.001% (10 ppm) if taring is used. A socket is provided for cormect,ion of t,he unit. to a 10 or 100-mv potentiometric recorder. The EMB-1 may be operated interchangeably from batteries or AC lines Some 1,alances are partial electrobalanees in that s. small portion of the total capacity is supplied by an electromagnetic force coil. The Minsco Model 2D05 ($284) is much less sensitive than the micro balances described above. I t is a two-pan halanee with a parallelogram suspension which u s e a reed pivot for the fulcrum. After the sample has been tared t,o t,he nearest 500 mg, a servo system deterts the residual imbalance and produces a ourrent bhrough a coil placed in ,a permanent magnet gap. The current is nutomat,irally adjusted nnt,il the null balance is reached. The Model 2D05 has a capacity of 50 g and an ac~ecuraoyof +0.5 mg on its 500 mg electramrtgnetio range. Other models are available, with different capacitiev and weighing rang%?. The
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~
~
Ill-g
Figure 11. Quartz Uitramicrobolonce, from R. Belcher, "Submicro Copyright 1966. Used by Permission of Elsevier Publishing Company. ing force (and taring force) is provided by a permanent magnet and a current carrying wire coil. Balancing i? done manudly and is more precise than in the Minsco.
Torsion Balances In this section we will discuss lever arm balmoes in which the counterforce is supplied by twisting a wire or fiber which acts as the support for the beam at its fulcn~m. Suppose a wire of length L and radius r is suspended a t both ends so that it is taut. At the center a beam is attached perpendicular to the wire, and in a horizontal pasit,ion. If a weight is now placed a t one end of t,he beam, a torque r is applied to the wire at the fulcrum. The wire will twist in the direction of the torqne. The angle B a t which the strain on the wire just balances the torque is given by
in which K is a constant chnracterist,ie of the material from which the wire is made. I n most torsion balances, a. graduated dial is attached to one end of the wire or fiber, and this d i d is rotated manually until the beam is returned to the horizontal null position. If the dial has been calibrated with a known weight, observing the number of graduations it mnst be rotated to balance the unknown makes possible calculation of the weight. There are two groups of balances sold which use the torsion principle. These are the highly precise ultramicro balances, usually constructed of quartz, and the less precise torsion balances, which employ metal beams and suspensions.
Ultramicrobalances These t,wo balances are electrobalances in a sense, ill that the sub-gram connterhalanc-
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SECTION OW E N D CYLINDER
J
Methods of
Organic Anolyrir".
Belcher (Q), and sold by Oertling. The torsion fibers (K and I,) are att,ached to a. transverse rod (F) which is suspended from above by two fibers (H).This is sometimes referred to as a funicular suspension. The tension in t,he horizontal fibers is kept constant by a quartz spring (N). The left end of the fiber K is fitted to the gradmted dials (not shown). The beam is composed of two quartz rods ( A ) s pointer ( B ) and two t,ie bars (C and D). The pan supports ( J ) are fixed to the end cylinden ( E and 6, shaded in the inset. diagrams in Figure 11). Weighing is very sensitive to small temperature fl~~ctoations (even two Celsius degrees can be significant in work of highest precision) and to air currents. The balance is therefore housed in an insulating cabinet. The sample pans are hung in wells which have doors that can be opened without exposing the entire mechanism to drafts or temperature changes. The pointer is observed through a small transparent window a t the base of the upper balance cabinet. The size and location of this opening minimizes the amount of light which can reach the mechanism and cause a temperature change. After the sample (and a tare weight or counterpoise) has been loaded and the beam is released, the deflection of the ~ o i n t e r is observed. The torsion fiber system is then twisted hy rotating a set of control knobs, until the pointer has returned to the center of the screen. The degree to which the fiber hes been twisted to accomplish this is given by a digital indicator. The Oertling Q01 balance, described above, bas a capacity of 250 mg in each pan. The range of the torsion fiber is slightly more then 1 mg, with each division on the indicator equal to about 0.1 fig. Normally, reproducibility is better than 1 0 . 0 8 pg. I t is worth noting that the (Continued on page A1O) Circle No. 118
01
Readers' Service Card
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Chemical Instrumentation
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beam can be removed and replaced quickly and easily, since it is not fused to the transverse suspension rod.
Chemical Instrumentation The Rodder Model E ultramicrobalance is very similar in design to the Oertling Q . A close-up view of the beam and suspension is in Figure 12. The two microscope ohjectives are used to observe the positions of horizontal index fihers attached to each arm of (.he beam. The torsion fiber is rotated until null balance is reaohed, as indicated by coincidence of t,he index fibers. The wells holding the sample pans are at bhe bottom of the case show, in t,he picture. The standard version ol this balance has a sensitivity of 0.05 ~ ga, 200 mg capacity, and a G mg tnrsiou fiber range. Balances with sensitivity t,o 1 nanogram are made by Radder on special order. Several other ultramicro lnrsion halawes have beer1 described 19, l o ) , but are not commercially available. Two ultramicrobalances which do not use the torsion principle will now be considered, before other, less sensitive torsion balances are discussed The Mettler Uh17 balance ($2430) has n sapphire knife edge suspension, and an optical scale similar to that found in this oompany's larger balances. The sample is loaded on tap in a shelbered compartment. The weighing range of the Uh17 is 2 mg, and reproducibility is ~ k 0 . pg. 1 A 1mg variable tare is built in. The total capacity of the balance is 103 mg, of which 100 represenls the weight of a special weighing boat furnished with the instnlment. Ainsworth's Type 22 qlmfz nltramicrobalance is similar to those described earlier, except that it does not have a. torsiou fiber to balance small weights. I t can be mounted in any enclosure which provides satisfactory pruteetian against air currents and temperature gradient-. The user constroets his own readmlt system.
Precision Torsion Balances A number of companies offer torsion balances less sensitive t,han the nlt,ramicrobalances. These instruments are useful for rapid weighing of small samples, and for application such as determination of surface tension, where continoons variation of the torque on the beam is neces8ar.y. The Roller-Smit,h halances come in a large variety of ranges-thirty in all. Most have hooks at both ends of the heam, 40 thst a tare or co~mberpoiiecan be used to extend the capacity of the instrument to three t,imes the weighing range. The most sensitive halance atfered by this company (LG00031IG, 3330) has a 3-mg range and f2 r g sensitivity. Other balances are similarly priced and have ranges up to 50 g (LGOO;iOGhf, $385, ~ t 0 . 2g sensitivity). Special balances for surface tension, density, and yarn-number measnrements are also available. The Bethlehem torsion balances are quite similar to t,he Rollor-Smith models.
(Continued on p a p A14)
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Chemical lnstrumentatisn Weighing ranges go from 1.5 mg ($440) to 50 g ($412 double hook or $336 single hook). Sensitivities are about one part in two thousand, and aocuracy about f0.2% of full scale. Brinkmann Micro-Torque balances are all of single-pan design, with meehanie~l taring replacing the second pan. The optical readout ir f d l y digital, rather than combined digital and vernier. Otherwise the system is similar to the Roller-Smith design. Micro-Torque balances come in eight ranges from 1 mg (f 1 pg accuracy, 6480) to 10 g ( f 1 0 mg acouracy, $410). The precision is somewhat better thau that of the other balances of this type. Sauter precision balances use a torsion spring to provide the torque to balance the beam. They are quite similar to the balances described above. Eleven models are standard, with weighing ranges from 1 mg (f 1 r g readability, $325) to 2.6 g (+2.5 mg readability, $322).
Part Two-Strain
Balances
The gnmp of instruments heretofore discussed was designed around the balancing of force and counterforce on a lever arm. The following section of this article is devoted to balances in whioh the applied force or torque (due to the object being weighed) produces a. strain, which is in turn used as a measure of the force. Actually, the torsion balance could be placed in this section, but it has a, lever arm construction and shares many characteristics (null condit,ion, taring) with other bdances of that type.
Theory and Design of Strain Gauges ( 1 1-14) Strain gauges are devices for measuring linear deformations of objects. Strain can be defined as the linear deformation of a body, usually due to the application of an external force. Since a mass In a gravitational field produces a force, strain gauges can be used to measure masses. Before discussing commercial instruments, the underlying principles of strain gauges and their construction will be described. The simplest type of strain gauge i. a metd vire or bar. When one end of the wire is fastened to B support, and a pulling force is applied to its other end, the wire stretches. The change in length (A1/1 is related to the applied faroe (F) per unit srea. (A) through the Young's Modulns (Y) of the metal:
y = -( F I - 4 ) (AW
Representative values of Y are 2 X 101* (Continued on pare A16)
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Chemical instrumentation dynes/cm' for steel, 1.4 X 10'' for l e d , and 6.9 X 10" for aluminum (15). A closely related type of strain gauge is the helical or spiral coiled spring. Its properties are similar to t,he straight wire or bar: it will lengthen (shear) when coupled to a pulling force, and compress when subjected to a pushing force. The main difference is that the effecbive value of Y can he adjusted over a very n-ide range. Uswlly the elongation of a sprillg is expressed in terms of IIooke's Law:
Al = F / k , where the farce constant (k) is expressed in load per unit extension. Fused quarte, for ext~mple, can be used to construct springs with force constants ranging from 0.05 to 500 mg/mrn or more. T h e leaf or cantilever spring shows similar p r o p e r t , i e ~ t h edownward displacement under a load can be related to the force. A strain or force could also be determined by measuring the compression of a fluid to which it k applied. This technique is primarily suited to pressure measurements, and will not be discussed further. The types of strain gauge3 devcrihed so far have several d~sadvantaeeq. i\Ianual even if s. micrometer or interferometer is available. T h e systems respond slowly tachanges in the load; several seconds are usually reqnired for equiiibrium to be reached. Also, many metals show hysteresis effects, and strain galrgea oonstruct,ed from them may require frequent reealihration. I n an ideal system, the maximum load would strain the wire to such n small extent t,hat t,he linear displacement would be infinitesimal. I n such a situation, however, visual observation of t,he displacement would not be possible. A numher of elect,rical properties of materials have suggested t,hemselves as a bssis for strain measurement, for electrical quantibies are easily monitored and recorded. Some gauges measure the change in capacitance when a strain a l t e ~ st,he geometry of a capacitor, others measure inductance changes caused by the strainproduced displacements of a magnetic coiL4 Piezoelectricity (16) occws ill crystals which have a. permanent internal dipole. When such crystals are stressed a voltage is produced. The efficiency of conversion of mechanical force to electrical power is 1% for quartz, 507, for many ceramics, and 80% for Rochelle salt. On the other hmd, quarte is much more rigid snd stable than other piezoelectric materials, so it is more popular for force and strain measuremenk An advantage of piezoelectric gauges over the resistive gauges discussed helow is that the piezoeleot,rios are self-generating. The signal is produced within the q m r t s ervstal, while resiative devices must he
'
Not,e that these devices can be used as imbalance detectors in lever arm balances, a-well as in sbrain gauges. ~
~
(Continued on page ,418)
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Chemical Instrumentation interrogated with s n externally generated signal. A disadvantage of most piemelectrics is their extremely high impedance (about lon3ohms for quarts), which means that an electrometer or charge amplifier inust be used to detect the signal. The resistance gauge is the most u s e ful type of electronic strain gage. Several farceta-resistance transducers have been developed (14), some based on helical springs, others on potentiometers and slide wires. More generally valuable me the wire s h i n garlgcs (11,14), made from fine wires or foiL9. The unbonded gauge (Figure 13)has the wires (A, B, C, D) cow neoted between a fixed support ( F ) and the object (M) to which the force is applied. The bonded gauge (Figure 14) is cemented
Figure 13. Unbonded Resirlance Wire Stroin Gauge 1141. From Lion, "Instrumentation in Scientific Research". Copyright 1959. Used b y permission of McGrax-Hill Book Company.
Figure 14. Bonded Stroin Gouges. A-Spiral diaphragm gage for p r e r w e measurement. 8-Single-plain rosette. C-Stocked rosette. B and C are used for determining direction or well osmmgnitude of strains. D-Standard wire gage. E-Stress and strain gage.
directly to the surface of the object, with only a layer of electrical ininsulation s e p
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Chemical Instrumentation amting the wire or foil and the object. The resistance ( R ) of the wire gauge changes with the applied strain (AS) according to AR/R = GAS, where G is the gauge factor.
The change
in resistance arises from changes in the resistivity of the wire or foil and in the geometry of the gauge due to stretching or compression of the object to which it is bonded. The gauge factor may be positive (carbon, Nichrome, platinum) or negative (pure nickel), and varies with temperature. In determining the strain or force, the gauge is often used a~ one arm of a. Wheatstone bridge. The bridge is balanced in the null position; then the output of the unbalanced bridge is a measure of the strain applied to the gauge. Two or more eauees mav be used in the hridae to oancel temperature effects or to increase the output. Unbonded wire s h i n gauges are produced by Statham Instruments. Current models use s. design called the ZeroLength strain gauge. In the oonventional gauge the wires must be stretched no that they will have the proper tension even when heated by the operating current. Also, the wires may break if the moving object travels too fss. The Zero-Length devices, shown in Figure 15, are also designed with one of the wire fixed (b) and the ather end movable (a), but the ends are placed next to each other, and the wire is passed through a loop ( e ) attached to a weight or a spring. The spring keeps the wire under tension and supplies a constant load. As point a is moved by /a force applied to it, the load of the spring is transferred from section ac to bc, or vice-versa. A disparity between Lengths ac and be will result. Electrical connections at a, c, and b are used to monitor the strain-induced change in resistance of the two sections of the wire.
--
B
C
(Courtesy Slnfbom InsLwmcnla, Ine.)
Figure 15. Gauge.
Zero-Length
Unbonded
Strain
The bonded gauge is very easy to apply. It consists of the wire or a metal foil, cemented or glued to a. paper or plestic film support. The gauge is cemented t o the test structure (the support insulates
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the gauge from the structure) and leads from the read-out device me then soldered to the gauge. The bonded gauge is inexpensive and occupies little space; in some applications dozens or hundreds of them may he attached to the object being tested. Many varieties of bonded strain gauges me available. BLH Electronics, Ino. offers about 300 standard S R 4 gages, with various sensitivities, temperature ooefioients, temperature ranges, and response times. The output of a gauge also reflects the direction in which the force is applied, and certain gauges can he used to determine the directional characteristics of s complex strain pattern (see Figure 14). The gauges are usually oonstmcted of nickel or platinum alloys. A platinum-tungsten alloy, sprayed onto a ceramic suppmt, is used by BLH for high temperature work. A gauge (#FABT) is available with a built-in thermocouple element. MicroStrain, Inc., makes a variety of standard and special purpose gauges, ineluding some which are compensated for thermal expansion of metals to which they might be attached at high or low ternperatures. This company offers gauges as small as 1 mmz, mounted on epoxy foils.
APPLICATIONS OF STRAIN GAUGES TO WEIGHING Spring Balances A variety of instruments rtle available in which the weight measured strains a. spring, stretching or compressing it until the system is balanced. The Worden Mass Sorption Spring Balance is intended for m e in controlled atmosphere systems. I t includes a thermostatting jacket, stopcocks for oonnection to the vaouum or gas line, and a microscope (with micrometer) for observing the extension of the spring. An electromagnetic system for null balance operation of the system is optional. The springs are made from fused quartz. They are available for loads of 10 mg to 100 g, with precision up ta one part in twenty-five thousand. Fused quartz spring^ w e also sold separately by Worden, with sensitivities of up to ahout 1W mm extension per milligrrtm load. Since a. micmmeter cen observe changes of ahout =!z10-' mm, a precision of +lo-' fig is attainable with the most sensitive spring 8yy~temS. Fused quartz springs are also available fmm the Thermal American Fused Quartz Ca. Both straight and tapered springs are sold, with various coil diameters. This company also supplies quartz tubing and fibers for mounting the springs. In general, quartz springs may he used up to about 500°C, where a phase transition occurs. In precise work, correction for the temperature dependence of the Haoke's law constant may he necessary. Many companies manufacture metal spring scales, and a comprehensive listing is beyond thescope of this article. ChatilIon offem spring scales with capacities (Continued on page A%?)
from 250 g to over 100 kg. Reproducibility is not as high as with quartz springs, but is adequate for certain applications. This company also makes aseries of spring testers, which permit preparation of extension/compression vs. load calibration cumffl. Springs are also components of a number of weighing devices which will be dis-
eilssed later in this article. The Daytronic force transducers are an example. The Dillon Model P trade scale is an example of the use of a spring to provide the counterforce in a lever a m bslance. This design, shown in Figure 16, features four unequal arm levers, arranged so that the spring need only provide a force equivalent to a twenty pound load, while the actual load can be up to ten thousand pounds. Thc accuracy af the weight as (Continued on page Ab4)
(Courlcsy IT. C. Dillion and C o . )
Figure 16.
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Lever-Arm Spring Balmce.
Chemical lnstrumentution indicated on the dial is one part per thousand of I he full scale range. A unique cantilever spring ultramicrobalance has been developed a t the Dudley Observatory (17). A thin metal coated fiber is clamped a t one end and the sample is fixed to the other end with a minute amount of adhesive. The assembly is placed between the plates of a capacitor. A DC potential is applied to the plates and an AC signal to the fiber. The fiber oscillates a t a frequency which depends on the dimensions and the mass of the fiber, including the load. The system is calibrated with samples of known mass. A fiber 1 cm long and 7 p i n diameter is sensitive to 110-J0g (one-tenth nanogram), and even greater sensitivity should be possible with smaller fibers.
Piezoelectric Load Cells Load cells with piezoelectric elements are usually made with qoarta crystals. They are not only rigid and stahle to temperature, radiation, and mechanical shocks, but also have very fhst response times, approaching ten micmseoonds. Kistler load cells come in capnoities from 500 g t,o 50 megagrams. They are suitable for dynamic or sratic weighing, and taring can be accomplished electronically. The best sensit,ivit,y at,tainable is of t,he order of 1 5 g, with the charge amplifier or electrometer being the limiting factor in mast cases.
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Resishnce-Wire Strain-Gauge Load Cells Load cells are simple in construction (see Fig. 17). The load is placed on a platform which sits on s metal column. The force compresses the column. Four matched strain gauges are attached to the metal column, two aligned with the direction of stress, and the other two perpendicular to this direction (these two serve to compensate for tempelature variations). The four gauges make up s. Wheatstone bridge, with t,he similarly aligned gauges on opposite sides of the bridge. The ontput of the bridge is proportional to the strain-producing load-for greater sensitivity, more than one set of gauges may be used and the output,^ of the bridges combined. The load cell requir~sa constant voltage sowee and a millivolt meter. The range of capacities of strain-gauge load cells remarkable. By careful choice of column materid and gauges, load cells have been constructed with full scale capacities from 10 g to over five billion grams (5 Gg). Another factor which may he of significance far same applications is bhat the load cell platform usually deflects no more than a quarter millimeter when the maximum load is placed on it. The cells are compact, light. in weight, aud can e a d y be protected against cmrasive atmospheres. The Baldwin-Lima-IIamilton Co. was the first to offer strain gages and manufactures the largest variety of load cells and force transducers. For example, a (Cathued on page Ad81
Chemical instrumentation n 3 G 1 cell with 10 pound capacity has +0.25% accuracy and 2~0.10% repeatability, and costs 5470. U-1 cells with capacities from 50 to 2000 lb cost $375 and have +0.25Yo accuracy and +0.05% repeatability. West Coast Research Corp. mannfactures several types of load cells. This company also has designed a series of balances incorporating strain-gauge load eells. The instruments m e similar in appearance to conventional singlepan m a lyticd balances, except for the absence of knobs for manipulation of weights. The readout is either digital or analog (meter), with outputs for recorden. Precision approaches f0.020/o, which is surpassed by the analytical and precision balances described in Part I of this article. On the other hand, the West Coast Research bdanoes are more rugged and convenient to use, especially if they must be operated by remote control. The Model 318 (meter readout) balances sell for $795 with 1 kg cappacity and $995 with 100 kg capscity. Gilmore Industries has developed a system for weighing liquid hydrogen whieb eonld be applied to some laboratory weighing problems as well. The instrument is shown in Figure 18. The lever arm is balanced with an empty tank using the counterweight; the load cells then measure the buoyant, force of Lhe liqnid hydrogen
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Figure 17. Strain Gouge Load Cell. A-Outer coring. 8-Bare. C-Lood-sensitive column, ,&ing on bore. D-Load button or beoring. E-Junction box. F-Strain gauge renritive to load H-Leodr from gauger to junction box. opplied to the column. G-Strain gauger insensitiveto load.
added to the tank. The calibration weights simulate buoyancies corresponding to,various fractions of the tank capaoity. Load cells are particularly vah~able here beoause of their iast response and their very small linear extension when the load is applied.
Statham Instruments uses unbonded fitrain gauges in their Universal Transducing Cell (%l50),whieb has a 60-g c%ptpacity. I t can be obtained with a micro-scale accessory ($40) (Fig. 19) for full scale (Continued on page ASO)
capacities of 6, 12, and 30 g, with ac*0.25% and precision better than &0.1%. A portable, battery-op-
of Chemical l f l ~ t r ~ m e f l t a t i ~ fcuraoy l
2S%,50%.7S%.IOO% CALIBRATION WEIGHTS
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PLUMMET
PNEUMATIC CYLINDERS TO
(Counsw Mlmore Induabica, In
