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 ENGINEERING EXPERIMENT STATION 
 
 STRUCTURAL RESEARCH SERIES S-15 
 
 ENGINEERING USRARY 
 UNIVERSITY OF ILLINOIS 
 
 NOTES ON THE APPLICATION OF SR-4 STRAIN GAGES 
 
 By 
 
 H. C. ROBERTS 
 
 UNIVERSITY OF ILLINOIS 
 URBANA, ILLINOIS 
 
TECHNICAL REPORT 
 
 Project N6onr»71j Task Order V 
 Project Designation No, NR 031-lfi2 
 
 NOTES ON Tlffi APPLICATION OF SR-^ STRAIN GAGES 
 
 by 
 
 Howard C, Roberts 
 
 Research Associate Professor 
 
 of Civil Engineering 
 
 An informal discussion of some of the experimental 
 difficulties which may be encountered in the application 
 of wire-type resistance strain gages with the commercial- 
 ly available instruments for their use,, 
 
 A technical repov t 
 
 ±n coC'pe^i.-.-.^-.. ^-. ..... 
 
 THE UNIVERSITY OF ILLINOIS, DEPARTMENT OF CIVIL ENGINEERING 
 
 and 
 THE OFFICE OF NAVAL RESEARCH 
 
 Ur : ^iDois 
 20 July 1949 
 
 ' Revised s 10 February 1951 
 
I 
 
 I 
 
 i\ 
 
^ :2a I (I? Br,5^f- 
 
 Co p. Z2 
 
 NOTES ON THE APPLICATION OF SR-4 STRAIN GAGES. 
 
 CONTENTS 
 
 Page 
 
 The General Problem o.^ooooooooooooooooo 1 
 The Available Instruments „ o » o » » „ » . o » « <> » „ » o 3 
 
 S^L 0(w-dL1|JLX Ooooo i;;oeoooo oooooooo o«ooco XXii 
 
 LIST OF FIGURES 
 
 Fig, No, Page 
 
 lo D-Co Bridge Circuit oo<.ooo^oooo..o<.oool8 
 2o A-C„ Bridge Circuit » , o o o » o o <. » , o » » .. o » , 18 
 3. Photographs of Typical Instrxments o » » » o o » » » >, 19 
 4.0 Diagram of Carrier System o » <. o <> o » « » » . o » « « 20 
 5o Photographs of Switch Units ooo<,ooooo.<,oo<,20 
 
i 
 
 J 
 
 ij 
 
NOTES ON THE APPLICATION OF SR=4 GAGES 
 and the associated instrumen'ts . 
 
 This is a revision of a report of the same title, first issued in 
 Julys, 194.9o No material has been deletedo The new material includes some 
 
 not available at that time; and some already^^fvailable material requested 
 by users of the first edition of this report. 
 
 The General Problems 
 
 The vdde application of the wire-type electric resistance strain 
 gage is due largely to its low cost and its relatively good electrical 
 stability when properly applied,, Unless used with suitable precautions, 
 however, these gages way give quite misleading and inaccurate results tinder 
 certain circumstances. During operation, the change in ohmic resistance of 
 such a gage is quite small, so that a very sensitive instrument is required 
 as a reading auxiliaryo Instruments of such high sensitivity often have 
 other characteristics interfering with their convenient use. Many workers 
 have encountered experimental difficulties, but comprehensive discussions 
 of these dif f icultles ^ and of the probable troubles to be encountered with 
 commercially-available instruments, have been absent frcm the literatureo 
 
 This di.scus3ion cannot be rcmplet- ^ '- -^ible 
 
 to anticipate here all the factors whj. ■:':.. m.-g. ^_ ... .. u_- -z test. 
 
 At the risk of seeming to "talk dovn* . coo ...tsr of the instruments, the 
 discussion has been made as non-techni c»le. It is felt that 
 
 perhaps many of those in need of this ... are not specifically 
 trained in alternating-current bridge 
 
 For most field work the resistance .. i'r gage ii . _.— e^ted 
 in a bridge circxiit. (For some applications , v ^^ynamic effects are 
 ©f imterestj, a simpler circuit is used, i 
 
 here,) With such a bridge circuit, direc . .... c... -.■ . .. — _ _ 
 
 sensitive galvanometer can be used| but because of the many ej.per imental 
 difficulties encountered in using sensitive i,, it 
 
 has become almost standard practice to ec^'' ._ .., :ridge 
 
 with an amplifier and a more rugged indl. 
 
 The frequency of the alternatirf ■— -t;. . v.; — ; -^--Ite these 
 strain-gage bridges is almost invariably than the oO-cycle 
 power ciirrent. There are two principal reasobo for thiss fixstj the earcit- 
 ing current is usually generated in the equl rr--'^ -■••'■■ >v ^ ••-■.tji-- ^-i,.-..,,r -:• /i-^ 
 oscillator, which may occupy little spat e * i 
 high| andp second, in order to avoid con^ 
 
 currents (which are everywhere) it is dee >u- 
 
 mote from the 60"Cyw^le current or its nr.- ^ 
 
 tamination of the bridge-circuit out p. mmon 
 
 practice to introduce filtering or dit .■ .>. - ^c.^. ^.- ihls 
 
 trouble. For dynamic records, with ec . . arrler^type 
 
Digitized by the Internet Archive 
 
 in 2011 with funding from 
 
 University of Illinois Urbana-Champaign 
 
 » 
 
 http://www.archive.org/details/notesonapplicati15robe 
 
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2o 
 
 amplifiers, it is necessary to use carrier frequencies trom four to ten 
 times as high as the maximum strain frequency to be measuredo Instrxunents 
 intended for static tests only, such as those described herep employ similar 
 amplifiers for convenience. 
 
 The installation and the balancing of a direct»current bridge is 
 a straightforward and relatively simple matterp but this is not true of 
 alternating-current bridges » In a d-=c bridge only steady currents flow, 
 and their flow is controlled only by tne resistances of the various parts 
 of the circuit o In an a-c . bridge the current flow is affected also by in- 
 ductance and by capacitanceo Consequently;, a bridge circuit which is per= 
 fectly balanced for direct current may be hopelessly off balance for 1,000 
 cycle alternating currento Gage bridges excited by alternating current re<= 
 quire balancing for resistance^, for capacitance, and for inductance^ and 
 the several balances must be established simultanecusly, 
 
 Thfere is often some inconsistency, even in the published litera- 
 ture, in the statements of exact balance conditi©ns for a^c •, bridge circuits. 
 In a direct=current bridge, like the circuit of Figo 1, the bridge is bal- 
 anced when the resistances of the arms of the bridge are such that no 
 potential difference appears between the points A and B^ across the indicate- 
 ing galvanometero In an alternating-current bridge^, the bridge is balanced 
 when no potential difference appears across the indicating instrument at any 
 point in the alternating curren t cjeleo The average current through the in- 
 dicating instrument may be zero without a true balance having been establish- 
 edo 
 
 A d=c bridge is usually balanced by observing the deflection of 
 its galvanometer 5 these deflections „ by their direction, indicate the 
 polarity of the unbalance ^ and thus the direction in which adjustment should 
 be madeo In an a-c orldge there is an analogous change in •polarity* of 
 the output of the bridge j on one side of the balance condition the output 
 potential is in phase with the exciting potential, while if the bridge is un- 
 balanced in the other direction the output potential is 180 dego 2^ of phase 
 with the excitation. If there is a reactive unbalance (that is, one involv- 
 ing capacitance or Inductance) there will be some "phase-shift* in addition 
 to the 180 dego "phase-reversal* found at the balance point. With bcth d -c 
 and a-c bridges, this change in "polarity" at the time of passing through 
 the balance point is often employed as the criterion for balance,, because of 
 its convenience o 
 
 It imst be recognized that in ©very a=^ bridge there are capaci- 
 tances and inductances, not shown on the diagram,' These are strays—due to 
 imperfections in the components composing the bridge, unwanted coupling be- 
 tween wires, etc. Perfect balance for the bridge circuit can be reached 
 only when all of the circuit parameters— resistance,, capacitance, and in- 
 ductance— have magnitudes in the proper ratios. The drawing and formula 
 of Figo 2 Illustrates this, A minimum (b-ut uct lero) bridge output may be 
 secured by adjusting any one of these three parameters; this minimum 
 should not be confused with the true balance. 
 
 In Fig, 2, all of the resistances ir,. each arm of the bridge are 
 considered as "lumped" and occurring in the positicns shown;; the same is 
 true of inductances and capacitances, Ot '.j this is net true, nor is 
 
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 j 
 
 i 
 
it true that any circuit parameter can be altered with no change in any 
 other parameter.. Because of thiSp the most stringent requirements as to 
 balance conditions must be imposedo 
 
 The user of resistance=type strain g&ges wishes his data to 
 indicate only the strains occurring in the specimen. Any resistance 
 change from other causes will give erroneous datao Likewisep any changes 
 in bridge-balance caused by capacitance or iadttetance can produce errors 
 unless means are provided to eliminate themo For entirely satisfactory 
 results, the gages should experience change in resistaace only ftrcm strains, 
 and the bridge circuits should give indications proportional to changes 
 in resistance only— not to either inductance or capacitance changes o 
 
 Tests continued over a long period of time will inevitably be 
 subjected to distrubing factors which might not appear at all in a shorts- 
 time test. More stringent requirements must be imposed on both equipment 
 
 and techniques if long=time stability is required. 
 
 The A vailable Instruments s 
 
 Portable instruments used with wire^type resistance strain gages 
 are usually alternating-cwrrent bridges p equippe<i with siii table amplifiers 
 and indicating instruments » Direct-<!urrent bridges can be used? there is 
 one especially designed for this applieatioQo Direct^ctsrrent bridges are 
 nearly alviays less convenient than a<=c bridges p howoTer., because the out- 
 put potential of a strain-gage bridge is only about OoOOOl volt for a 
 stress of 600 pounds per square inch, in steelo Battery= powered instru- 
 mentsn housing in one case the bridge circuit,, exciting oscillator , ampli- 
 fier j, and indicating instruments, are most widely usedo Many of these are 
 fitted with a slide=id.re assembly in the bridge circuity graduated directly 
 in increments of strain. The photographs of Figo 3 show four of the most 
 popular of these portable instruments, (In the case of the Baldwin- 
 Southwark instrumentp two models are showms the basic instniment in both 
 is similar,) In this discussionj, only those jjustrunents described as 
 "portable* are included^ there are others, intended ©nly for laboratory 
 usep and not here listed, Alsop many laboratories have constructed their 
 own instruments! obviously these cannot be listed here. 
 
 The first of these instruments to attain wide use was the SR-4 
 Portable Strain Indicator, made by Foxboro and -iistributed by Baldwin- 
 Southwark, This instniment is built around the electrical circuit drawn 
 in Fig, 4., in block=diagram form. It contains a slide-wire unit,, gradu- 
 ated in micro-inches, and fitted with a sh^tl.ng d&vice by which correc- 
 tions for gage-factor can be made. The slide='Wire length is limited to 
 2^,000 micro- inches 5, but when the end of its travel is reached resistance 
 cen be added in series with the slide-wire so that meaeiireaent can be coni- 
 tinued. In this instruaent there is a tett-polnt switch by which ten in- 
 crements of 1,000 micrO^inches (approximately) can be introduced in suc- 
 cession as neededj thus its total range is 12,000 nicro-incheso 
 
I 
 
 » 
 
The self-contained oscillator in this ixistnunent provides the 
 alternating current for energizing the bridge circuit j its frequency is 
 approximately 1,000 cycles per second and its magnitude is about U volts » 
 The waTe-form of this alternating current is quite good when only the 
 normal load is appliedo The exciting current is applied directly to the 
 strain-gage bridge, and also to the phase-sensitive output curcuit« 
 
 The amplifier is a resistance-^capE-city^coupled vacuum-tube amp- 
 lifier 5 the bridge output is fed into this amplifier through a step-up 
 transformero No gain control is provided, nor (since the instrument 
 operates as a null-system bridge) is any needed. 
 
 The output circuit of this instrument is one of the standard 
 forms, a piiase-discriminating circuit employing a small dry-disc recti- 
 fier unit in the "ring modulator" connectioao This circuit, in simpli- 
 fied form, is shown in Fig,, 4o Its saost important characteristic is that 
 it can provide a direct-current output whose i&agnitude is approximately 
 proportional to the magnitude of the bridge output, and whose direction 
 is controlled by the phase of the bridge output « The operation of the 
 complete instrument is mad© much easier by this proTision, since the de- 
 flection of the indicating instrument shows tbs direction the slide»wire 
 knob must be turned to complete the balance « The actual operation of 
 this type of circuit vdll be described latero 
 
 The Type »K«» Portable Strain Indicator differs from the older 
 models principally in the input bridge circuit. While the older models 
 contain two arms of the four-arm gage bridge circuit within the instru- 
 ment case, completing the bridge circuit with the active and the com- 
 pensating gages, and feeding the bridge output directly to the amplifier, 
 the Type "K* contains in addition another complete bridge circuit, and 
 the signal fed to the amplifier is the difference between the outputs of 
 the two bridges o The output of the gage bridge is proportional to the 
 percentage difference between the two gage arms of the bridge, even 
 though other than 120-ohm gages are used. The output of the internal 
 bridge is propc»rtional to the unbalance produced by the slide-wire, the 
 range-extender, and other adjustments. This instrument, therefore, can 
 be used with gages of varying resistances, and still be direct-reading. 
 
 The Type *L'" Portable Strain Indicator contains a few small 
 changes from, the Type "K", Its range has been doubled? the slide-wire is 
 narked off from to 2,000 micros-inches, but the steps on the range 
 switch are each equivalent to 2,000 micro-inches instead of 1,000, and 
 the auxiliary "range-exTtender" switch provides an additional 20,000 micro- 
 inches range in either direction, instead of 10,000 as in the Type "K", 
 There is no overlap between ranges. An arrangement is pro^-ided by 
 which a four-arm external bridge can be used, if desired j a few seconds 
 with a screwdriver is sufficient to make the change. This model has an 
 "oscilloscope" Jack added j it is intended to facilitate quasi-dynamic 
 measurements and to be used in establishing reactive balance. 
 
I 
 
 » 
 
 ( 
 
The SR-A Low Frequency Strain Amplifier, Type ■KG"j is a 
 slightly modified Type "If* instrument. It is intended to work into 
 sensitive magnetic oscillographs, or high-sensitivity chart- type record- 
 ers. 
 
 Another popular instrximent, now available, is the Young Static 
 Strain Indieator„ This instrument is quite similar in function to the 
 Type "K" Baldwin-=South«ark Indicator, although differing somewhat in 
 appearance and in operationo It employs a ten-tvirn helical resistance 
 unit instead of a single-turn slide-wire, and consequently the range- 
 switch is not provided, A "range=ext-ender* , however „ is provided, as in 
 the Baldwin-^Southwark instruments » A "gage" factor" control is also pro- 
 vldedo Operation of this instrument is very much like that of the 
 Baldwin-Southwark units, except that instead of making large adjustments 
 by turning the range-switch, the teii-turn sllde^wire unit must be ro- 
 tated through as many turns as ciecessary. An "oscilloscope" jack is 
 also provided, so that a cathode-ray oscilloscope can be used as an in- 
 dicating instrument if desiredo (On early loung instruments this Jack 
 was connected to the demodulated and filtered output of the amplifier, 
 and consequently could not be used for establishing a reactive balance. 
 It is xuiderstood that in cuinr'ent models this has been changed^ this jack 
 is now connected to the amplifier output,) Current models of the Young 
 Static Strain Indicator have a switch arrangement by which the internal 
 circuit can be modified to accommodate a four»arm external bridge if 
 desiredo 
 
 All of these instruments* employ the phase-sensitive character- 
 istic of the output circuit to establish the balance condition of the 
 strain-gage bridge. All of them are capable of good work in the labora- 
 tory, and (under most conditions) in the field. But under certain con- 
 ditions they can give quite inaccurate results, or refuse to operate at 
 all. Under ""border-line conditions", the inaccuracies may be smaller, 
 but still too great. In these circiimstances, the output circuit may be 
 the source of the errors. 
 
 - All of the a-c bridge instruments now supplied as "portable* 
 instruments are of this general type 5 there have been other types a^)•ail- 
 able in the past, and there may be again, in the future. For special 
 applications a Kelvin-bridge type of instrument may be procured through 
 Baldwin-Southwark, The Cox and Stevens Aircraft Company at one time 
 produced a load-measuring system which employed a modified Foxboro in- 
 dicatory the present Type "L" has much the same characteristics. 
 Several laboratories make their own instruments, designed to suit their 
 specific needs. Some of these employ a dual bridge circuit which re- 
 duces error from the resistance of the connecting cables j, but this 
 circuit does not eliminate any errors that might come from uncompen- 
 sated reactances of lead wires and in general it is less accurate than 
 the Kelvin-bridge circuit. 
 
I 
 
 I 
 
 hi 
 
To explain this, it is necessary to describe the operaticg 
 principle of the phase-sensitive outpit circuit used in these instruments, 
 and to show how sometimes it can act to conceal the true balance point of 
 the strain-gage bridge » 
 
 As stated in the first section of this report, the true balance 
 of an alternating-current bridge circuit occurs only when the output of 
 the bridge circuit is zero at al^ timeg during ^he cycle of the exciting 
 wave. Accurate data may be secured;, however^ without ever accomplishing 
 a perfect balance ^ if the same unbalance is used for erery reading of the 
 instrument,. This, actually, is the normal procedure „ The principal 
 reasons for this are as follows s 
 
 Ic There will often be some amplifier output when no input is 
 applied. If tnis (or its harmonics) is at or near the exciting frequency 
 the reading will be affected, 
 
 2 8 The ring-connected rectifier unit can give an output when 
 there is no amplifier output, 
 
 3o Either of these, or both together, can cause an Instrument 
 deflection, and consequently a possible error „* 
 
 Instrxunent indication form these saorces will not introduce any 
 appreciable error if it remains constant in raagnitude„ However, over long 
 periods of time the instrument deflections from these sources will not 
 remain constant; those resulting from effects in category (1) are caused 
 by such things as changes in characteristics of the vacuum tubes used in 
 the amplifier,, positions of connecting wires, and the like; and those 
 from category (2) because they are usually caused by changes in the in- 
 dividual units which make up the ring-connected rectifier unit, or changes 
 in the characteristics of the transformer which feeds this unite 
 
 These very slow changes ordinarily do not distrub short-time 
 tests o Occasionally an individual instrument will show a systematic error 
 over its slide-wire range; this is likely to result from variations in position of 
 the flexible leads from the slide-wire unit as this unit is operated; a 
 variable "error signal* can be introduced into the amplifier in this way. 
 Some users of these instruments have reported that they can secure good 
 results only over a certain portion of the travel of the slide-wire , 
 Over any long period of time it must be assumed that all of these errors 
 can be presento 
 
 * This effect can usually be demonstrated by observing the instru- 
 ment with no gages connected. An instrument will often show a steady de- 
 flection, which can be changed by rotating the slide-wire dial; as the 
 positions of certain connecting wires within the instrument case are 
 changed the spurious signal sent to the amplifier is also changed. Like- 
 wise, if the amplifier tubes are removed entirely there is often some de- 
 flection of the indicating instrument remaining, this indicates that the 
 ring-connected rectifier unit does not perfectly balance out the reference 
 voltage. This imperfection may occur either in the transformer or the 
 rectifier unit. 
 
f 
 
A more detailed explanation of the reasons for these actions, 
 and for other sources of error in instrtunents of this typep follows? 
 
 The circuit (shown in block diagranij, Fig„ 4) embodies a ring- 
 connected rectifier unit, fed by two separate sources of alternating 
 current. It is assumed that all four of the rectifiers which make up 
 the complete unit are alike^, and also that the two center-tapped trans- 
 former windings which feed the unit are symmetrical about their center 
 taps a (In another form of this circuit „ resistance elements are intro- 
 duced to permit improvement of the symmetry of this unit,,) 
 
 If a signal is fed to the rectifier unit from either trans- 
 former alone, there will be no instrument deflectiono There are actually 
 pulses of rectified current flovrlng through the indicating instrument 
 at each cycle, but they are equal in magnitude and in opposite directions 
 and consequently the average instrument current is zeroo But this 
 assumes that all conditions are correct | if for any reason these pulses 
 are not equal in size, their algebraic sum will not be zero, and there 
 will be an instrument deflectiono This effect may be produced by (1) 
 unbalanced rectifier units^, (2) highly unsymmetrical wave-form, and (3) 
 differences in resistance of the several paths through which the current 
 flows o The practical instrument may contain any or all of these dis- 
 turbing factors, but so long as they are small and do not change much 
 in operation their final effect may be negligible o 
 
 If, however;, any one of these three disturbing elements changes 
 appreciably during a testy an error will be introduced into all data 
 taken with that instrument; this is a consequence of the fact that the 
 apparent zero of the instrument has shiftedc. There is no obvious external 
 indication of this shift, but such shifts are often observed in field 
 tests and can easily be reproduced in the laboratory,* 
 
 When this circuit is operating normally, two simultaneous sig- 
 nals reach the ring-connected rectifier unit from the two transformers „ 
 Under these circumstances the two signals (the reference or polarizing sig- 
 nal coming direct from the oscillator and the datum signal coming from the 
 strain-gage bridge and through the amplifier) combine in the ring- 
 connected rectifier unito If the polarities of the two signals are alike 
 when they reach a rectifying element^ a considerable current can flow; 
 if they are opposite, very little current is permitted to passo 
 
 * For example, replacing a rectifier unit often causes a shift of in- 
 strument zeroo Unsymmetrical wave-from (often encountered in older models 
 of the SR-4 Indicator as tubes age) can likewise cause a large change in 
 instrument zerOo Defective output transformers can produce an effect like 
 that produced by a large increase in resistance of rectifier units or their 
 connecting leads. Some instruments have been found to show a tendency to- 
 ward a high-frequency parasitic oscillation superimposed on the 1,000 cycle 
 carrier wave; this likewise can produce a shift of instrument zerOo Care- 
 ful maintenance of equipment can minimize many of these troubles. 
 
 These phenomena have been observed with a number of different instru- 
 ments, of different models and makes » They are not peculiar to one instru- 
 ment or model. 
 
8, 
 
 It is usual to assume that there is no phase shift through the 
 amplifier; that is, if the reference voltage and the datura voltage are 
 in phase it is an invariable indication that the reference voltage and 
 the bridge-output voltage are also in phase o This is not necessarily 
 true. Sometimes phase adjusting arrangeaents are built into the instru- 
 ments scraetimes the phase-shift within the amplifier is small and can 
 justifiably be Ignored, In null instnuaentSj, like those described here, 
 there is little likelihood of serious error ftrom this source, unless the 
 phase-shift through the amplifier changes during a test, or unless the 
 deflection of the pointer of the indicating instrument is used as an in- 
 dication of degree of unbalance — this is often done for small changes 
 in loado The current flovdng through the indicating instrument is quite 
 accurately proportional to the product of the two voltages (reference 
 and datum) and to the cosine of the angle bet wen thema 
 
 In normal operation the circui . ^ ^--y to use, and sufficient- 
 ly accurate fcr all ordinary useso Sparlo'as signals are rejected by the 
 circuit unless they happen to be of the same frequency as the reference 
 signals thus, noise and 60Ksycle contamination are almost entirely elim- 
 inated o The direction the indicating Instiounent deflects indicates the 
 direction the slide-wire knob must be tiirned to restore the balance of 
 the strain-gage bridge,, The sensitivity of the circuit is higho There 
 can be a considerable phase<=shift between reference signal and datum 
 signal with no appreciable inaccuracy resulting, although of course there 
 will be a decrease in sensitivity of the instrument as the cosine of the 
 phase-angle becomes smaller o 
 
 But if something goes wrong with i..-- : ireuit the very items 
 which normally promote ease of operation and reduce the probable error can 
 combine to introduce errors of their own? this is the more serious be^ 
 cause the circuit (as with most electrical instruments) is especially sub- 
 ject to systematic errors, which tend to conceal themselves from the in- 
 experienced operatoro Further, under certain cjjrcumstances large errors 
 can be produced by sources entirely without ^'^ ^-° of the ins-i^riimentj 
 which, however, affect the operation cf th^ nsitive output circuito 
 
 One of the most common cf t'neso 0.= . v. i:;ec ting-cable reactances 
 Field tests often require long lead-vires to the gages, sometimes these 
 wires must be located close to conducting material^ or tied in bundles, 
 or even Immersed in water. The capacitance of the lead-wires to ground 
 will be relatively high,, and the stram-gage bridge will be unbalanced 
 reactively even though the resistance balance is perfect. In this con- 
 dition, the only symptom of this imperfect balance may be that the In- 
 strument is "insensitive" 5 tnat is, the instrximent pointer may no longer 
 move away from its mid-position rapidly as the balance condition is 
 changed by moving the slide-wire knob, (Sometimes even this symptom will 
 net be present,) What is happening is thlss although the strain-gage 
 bridge may be perfectly balanced so far as its resistive components are 
 concerned, there is still a large reactive unbalance because of the 
 capacitance of the lead- wires. Thus, the strain- gage bridge outpit is 
 not zero at all times (see the definition for balance in the first 
 section of these notes,) The pliase-sensitive output circuit, however. 
 
J 
 
does the best it canf it tries to ignore the relatir&ly large signal 
 coming through the amplifier because of the reactive' unbalance (it can 
 do this because the capacitive unbalance signal is not in phase vrLth 
 the resistive unbalance signal), and the instrument showns a deflection 
 indicative of the resistive unbalance of the strain-gage bridgeo 
 
 If every part of the instrument is in perfect condition, 
 there may be no error introduced because of this reactive unbalance j 
 that is J, the instrument current will be proportional to the product of 
 the reference and the datum signals j and to nothing elsco Since at 
 balance the instrument deflection is zeroj, the loss in sensitivity vdll 
 be unimportant unless it is great enough to prohibit accurate adjuct- 
 ment of the instrument. 
 
 In practice 5 however, every part of the instrument cannot be 
 considered in perfect conditiono The relatively large ciirrents flow- 
 ing in the output circuit, even though they are 90 deg, out of phase 
 vd.th the reference signal, can cause some instrument deflection (see 
 explanation earlier in this report),, The relatively large capacitive 
 loading of the oscillator may cause some wave-form distortion of the 
 exciting and the reference voltage . * 
 
 If the capacitances in the external circuit are variable, as 
 often happens in moist locations, a very difficult problem arises o It 
 is not unusual, under such circumstances, for the instriunent indication 
 to vary so rapidly that it is difficult for an operator to follow it, 
 and it is obvious that any data so taken would be entirely unreliable. 
 When such conditions are encountered, it is first necessary to remove 
 the variable component, which is usually caused either by changes in 
 position of lead-wires, by sudden changes in temperature of lead-wires, 
 or by variable moisture conditions such as random condensation or drip- 
 ping of water on lead-wires » The remedies to be applied include these 5 
 change the location of the wires, increase the thickness or quality of 
 the insulation used, or even immerse the entire length of the lead- 
 wires in wax or oil. In emergencies j, it may even be necessary to flood 
 the work-area and the lead-wires with water, although this obviously 
 increases the danger from leedcage as well as the capacitive losses in 
 the system. 
 
 The first step toward the correction of errors from this 
 sotirce must be taker! in the original installation of the gages. It 
 is desirable always to install the reference gages as near as possible 
 to the working gages, in order t-o provide temperature compensationo 
 If the connecting wires to both sets of gages follow the same route, 
 their resistances will be comparable,, and likewise their changes of 
 resistance with temperature „ This will also tend to add similar 
 
 » Similar effects sometimes result from varying phase-shift in 
 the amplifier, but this problem will not be discussed here. 
 
1 
 
 I 
 
 1 
 
 I 
 
10, 
 
 amounts of capacity to both sides of the straia-gage bridge, although, 
 usually, good capacltive balance cannot be obtained as easily as this^ 
 The appropriate procedure in such a case is to introduce a capacitor. 
 unit vdth a switching device permitting it to be connected in parallel 
 with that side of the gage-bridge which requires added capacitance to 
 provide a balance, A decade capacitor unit whose smallest steps are 
 OoOOOOlO mfd, is usually satisfactory? often air-dielectric -mriable 
 capacitors are used. Occasionally it proves necessary to add a small 
 fixed capacitance opposite the variable, in order to counteract resid- 
 ual effects in the capacitor unit. 
 
 To determine when the reactive balance is satisfactory, some 
 device must be used which indicates the total unbalance sigiial from the 
 strain=gage bridge. One of the most convenient methods for use with 
 the portable instruments considered in this report is to connect a 
 cathode^ray oscilloscope to the output of the amplifier circuit, ahead 
 of the phase-sensitive output circuito The connection can be made in 
 several ways? one which is both convenient and satisfactory is to add 
 an external connection^ through a 50 mfd, mica condenser ^ to the con- 
 trol grid of the output tube, as indicated in Fig, 4o The cathode-ray 
 oscilloscope input is connected at this point, and the metal cases of 
 both strain indicator and oscilloscope should be connected together. 
 Sometimes better results are gotten if they are also connected to 
 earth I sometimes not. 
 
 In using this devicep the procedure is as follows s first 
 balance the st.rain-=gage fcjidge for both resistance and reactance, ad- 
 justing the two alternately until the signal appearing on the screen 
 of the oscilloscope is at its minimum. The gain of the oscilloscope 
 amplifiers must be low at the beginning of this process, but must be 
 turned high at its end, A true zero wo^ald be ideal, but may not be 
 obtained with this simple system. Then, ignoring the oscilloscope, 
 finish making the resistance balance by observing the indicating meter. 
 Only a small adjustment should be required to bring the meter pointer 
 to its zero position? if all conditions were perfect the proper bal- 
 ance condition would be indicated simultaneously on both instruments. 
 If there is any very large difference between the point of reslstance- 
 and-capacitance balance of the strain-gage bridge^ as indicated by 
 the oscilloscope, and the point of apparent resistive balance >, as in- 
 dicated by the meter, there is some relatively large source of error 
 within the strain indicator. 
 
 Since the ordinary laboratory oscilloscope requires 110 volt 
 power for operation, this method for adjusting reactive balance is 
 often inconvenient or impracticable for field use. In such circuits 
 stances, it may be possible to substitute a sensitive a-c voltmeter 
 for the oscilloscope (especially if the instrument is connected to the 
 plate of the output stage of the amplifier instead of the intermediate 
 stage) 5 such sensitive meters are not easily obtainable as catalog 
 items, but may be easily constructed with i small dry-disc rectifier 
 
• 
 
 I 
 
 f^ 
 
lie 
 
 of the Instrument type and a 0-50 micrc-ampere d-c instnmento Alter- 
 natively, a battery-operated vacuxun-tube voltmeter may be used, Ob- 
 viously, with either of these instruments the balance adjustment is con- 
 tinued until the instrument indication is a smll minimam.g obviously^ 
 too, such instruments will indicate contaminating signals as well as 
 desired signalSj, and will not discriminate between them. 
 
 Ideally, the most accurate system would be one in which the 
 total output of the strain-gage bridge is observed, and all adjustments 
 continued until the bridge output is truly aero. This can be accom- 
 plished only if provision is made for resistive, capacitive, and induc- 
 tive balance^ and with a sensitive and stable indicating instnimento 
 For most purposes it is sufficiently accuraite (and more convenient) 
 to make the initial reactive balance with the cathode-ray oscilloscope, 
 adjusting both capacitance and resistance simultaneouslyj then to 
 finish the resistive balance with the indicating meter. It is not 
 necessary to take down any data on the -values of capacity^ or its ad- 
 justment; it is often convenient, however, to make notes of the values 
 so that any sudden changes may be observed. It is desirable to note 
 occasionally the number of divisions on the slide-wire dial between the 
 resistance-and-capacitance balance point, and the instrument balance 
 point indicated by the meter. If this difference changes appreciably 
 from one time to another, it is an indication of some change within the 
 instrument, and any such sudden change must be viewed as a source of error. 
 
 One of the instruments pictured in Fig, 3 was not available 
 at the time this report was first prepared; it is the Model BA-1 amp- 
 lifier of Ellis Associates o This instrument operates on a principle en- 
 tirely different from that employed by those previously described. In- 
 stead of a carrier-type system, the Ellis instrument contains a re- 
 sistance-capacity coupled amplifier, a direct-current bridge circuit, 
 and a mechanical modulator. It is intended as a general-purpose instru- 
 ments for both static and dynamic measurements. 
 
 With the Ellis amplifier, strains are measured by using the 
 instrument as a simple amplifier, with its output applied to a cathode- 
 ray oscilloscope; calibration data are secured by occasionally introduc- 
 ing known bridge unbalances through the mechanical modulator (a self- 
 contained electrically-dri-ven vibrator or "chopper"). Static or ex- 
 tremely 3 lowly- varying strains are measured by the d-c bridge circuit, 
 with its output fed through the mechanical modulator and amplified for 
 display on the oscilloscope screen. Switching devices and calibrated 
 resistors are contained in the case of the instrument, for calibration 
 purposes, and an indicating instrument is also provided, for battery- 
 checking and similar applications. 
 
 This system is subject to certain fundamental difficulties, 
 partlcularyl (l) the maintenance of vibrator-type amplifiers is often 
 difficult, (2) the system does not permit simultaneous observation of 
 low and high-frequency phenomena, (3) the d-c bridge circuit used is 
 subject to errors from thermal e.m.f's., electrolytic contamination. 
 
 'W'VER.SITY Of ,u,;VO;S 
 
I 
 
 I 
 
 f 
 
12, 
 
 and from stray pick-up which may be partially rectified in imperfect 
 connectionsp and (4.) the use of an oscilloscope as an indicator is un- 
 desirable both because of the relatively low reading accuracy and be- 
 cause it normally requires line power for operationo 
 
 The final verdict as to the desirability of an instrument must, 
 however, come from its users, and the Ellis amplifier has been used only 
 a very little in this laboratoryo 
 
 During the actual reviision of this report^ still another in- 
 strument of this kind has appeared on the marketo It is the Model 129 
 Dyria^Mike made by Industrial Electronics c, Inco While this instrument 
 was intended for use with mutual-inductance gagesj, it is understood that 
 it can also be used with resistance^type wire strain gages. Also, while 
 it operates from 110 volt line currents it seems probable that a modi- 
 fied version may be produced to operate from self-contained power supplies. 
 To date only catalog information has been available j no actual performance 
 data can be given o 
 
 In addition to the carrier-type instruments (and one self- 
 contained d-c„ bridge) for portable usBc there are a number of direct- 
 current-excited bridge units available for laboratory use. No attempt 
 is made to present comprehensive discussions of these instruments j as prev- 
 iously stated, the emphasl<s in this report is on those instruments specially 
 suited to field use and under other difficult conditions « 
 
 The presently available instruments cannot be considered entire- 
 ly satisfactory except for laboratory usej this^ howeverp was the use for 
 which they were originally intended. Better instruments for field 
 applications may be expected in the future. Until they are available, 
 however, and even afterward, many of the present instruments will be us^ 
 in field tests. 
 
 Procedure; 
 
 Any test program should include some means of checking the in- 
 struments which are used;; in additipn, that is, to the built-in circuits 
 for testing battery condition. If data are to be taken over an extended 
 period of time, there must be some means for checking the reference values 
 of the instruments. 
 
 In the past, some workers have used a simple method for establish- 
 ing the "instrxiraent zero*, consisting simply of t-akmg readings on two 
 mounted and unstrained gages,, reversing the gage connections j, and re- 
 readingo It has been considered that the average of the two readings will 
 represent the "instrument zero"; and that so long as this ■instrument 
 zero" shows no change, the instrument remains accurate. This is only 
 parially true. Such a test is satisfactory for the great majority of 
 applications,, if it is properly interpreted. It is permissible to assume 
 that if two gages of nearly equal resistance show little change in their 
 indications over a period of time, there has been little or no change in 
 the gages or in the instrument used to read them^ If>. however, any change 
 
13 < 
 
 is noted, some further check must be made to determine whether the change 
 has occurred in the gage or in the instrumento It is therefore advisable 
 to maintain three pairs of gages n and so long as two of these pairs show 
 little or no change, any change appearing in the third pair may reasonably 
 be considered a change in the gages rather than in the instrument. 
 
 In applying this test, however, it is almost essential that the 
 gage circuits contain reactance comparable to that occurring in the 
 circuits at the test locationo It is also essential that reactive balance 
 be provided. If it is used carefullyp this check can be considered 
 sufficient for practieally all short-time tests „ It does not, of ccurse, 
 indicate the cause of any drift or other change which may be discoveredo 
 
 More reliable checks can be made if a dummy gage circuit is 
 provided^ with which small known resistance changes may be applied to the 
 bridge circuit. Again, provision for reactive balance must be made, and 
 it is not sufficient merely to make the reactive balance complete— it 
 is necessary to insert capacitances, in parallel with the bridge arms, 
 and of magnitudes comparable to those found in the test circuits » There 
 is no other convenient way to make the loading on the exciting oscillator 
 similar to that when under actual test conditions o In such a checking 
 devicBp great care must be taken in construction and in operation, since 
 the unit becomes a secondary standard, and if improperly used can actually 
 introduce errors into the measurement » However, good quality switches 
 and resistors are available, and a high degree of stability may be attained 
 without great cost. 
 
 For ordinary indoor short-time tests, some of these precautions 
 
 are not required. 
 
 Some instrument should be available for determining the point of 
 reactive balance, when such a measurement is takeno For most applications 
 —that is, when relatively short leads are used, under good operating con- 
 ditions— it is not necessary to make the reactive balance for every read- 
 ing of the instrument. For such a test it is sufficient to check the con- 
 dition of the instrument when it is taken from the laboratory, and for thi6 
 check the cathode-ray oscilloscope is usually the most convenient. A 
 vacuum-tube voltmeter may be used, but it cannot distinguish between the 
 desired signal (1,000 cycles per second) and any noise or stray SO-cycle 
 pick-up. Usable results may be had with a sensitive micro- ammeter and an 
 instrument rectifier unit, but it has the same lack of discrimination as 
 the vacuum-tube voltmeter. For field use, when high accuracy is desired, 
 or if the operating conditions are especially difficult, very good results 
 may be had with a vacuum-tube voltmeter fitted with a filtering circuit 
 so that the stray voltages and the noise can be excluded. Such an instru- 
 ment may be portable and battery-powered. 
 
 If a test program is to involve long lead-wires, buried or immersed 
 gages, long periods of time, or if the greatest accuracy is required, the 
 plans for the test must be made carefully. No specific rules can be written^ 
 but these points should be observed o 
 
» 
 
 «.■' 
 
u. 
 
 lo Matched gages should be used whenever possible. If really 
 good temperature compensation is to be provided„ the gage circuits must 
 be as similar as possible. This requires using gages from the same lot 
 (and if possible from the same package) as working and as reference gages » 
 At the best, there will be a gage=factor error of perhaps 1 per cent, and 
 a gage-^-resistance error of perhaps i/2 per cent. 
 
 2o Matched lead-wires should be used for best results. Since 
 the resistance of the lead-wires is effectively added to the gage re^ 
 sistancej the resistance of the lead=wires should be as low, and as con- 
 stant, as possible. If really long leads are used, it is advisable to 
 take all the lead-^vdres for each gag© cireuit from the same roll of wirep 
 and even to match their resistances with a Kelvin bridge. Any lead=- 
 wire which shows a resistance varying appreciably from that of others of 
 the same length should be discardedo It is definitely dangerous to use 
 wires of different types or different makes in one installationj there 
 may be differences in resistivity, in temperature coefficients, or in aging 
 characteristics . 
 
 3. Lead-wires should be carefully laid outj and csLrefully tied 
 down. As far as is possibles, all the wires leading to a pair of gages 
 should follow the same route, so that they will be subjected to the same 
 temperature changesj, and the same climatic effects. Wires should be tied 
 down in such a manner that no mechanical strain can affect them. Of the 
 three wires going to a pair of gages, the common lead is the least 
 critical. 
 
 4., In damp locations, bundles of lead-wires should be filled 
 with waxj, if possible? or other arrangements made so that water-pockets 
 can not form. Such water=pockets can cause large and unpredictable 
 changes in the reactance of a circuit. Tying wires in bundles is good 
 practice, in general^ it equalizes temperature effects. 
 
 5. Lead-wires should be as large as possible (considering spape 
 limitations, economies, and required accuracy). In any installation in- 
 volving lead-=wires longer than perhaps 4.O feet, solid wire of No. lA or 
 larger is desirable. Stranded wire is usually less stable,, apparently 
 because of the stretching of some of the individual strands. Its in- 
 feriority usually shows up only when the lead-wires must be tied in vert- 
 ical runs. The insulation must be of good quality, and if it is thick 
 there will be smaller capacity effects from moisture on the outside. Good 
 results have been gotten with General Cable Company's "Gencaseal" wire, 
 Noo 14 solid, with 3/64 in. insulation, used under ?00 feet of water. 
 There are undoubtedly other makes equally good. However, at least one lot 
 of "Habirdur* wire, made by Habirshaw and carrying specifications similar 
 to Gencaseal, gave decidedly poor results, for no obvious reason. 
 
I 
 
15o 
 
 60 Careful handling of both gages and vdre is required,, Sharp 
 bends in plastiC"=insulated wire should be avoided | minor abrasions will 
 be produced which although invisible will have an injurious effect. Rough 
 handling of wire can cause work-=hard6ning which will cause a local change 
 in resistance I this may later be relieved by aging„ with the result that 
 an apparent strain will be registered. Some types of wire are especially 
 susceptible to this effect « Wire should be purchased in standard ceils, 
 and any coil showing signs of rough handling should be set aside for use 
 in locations where less stringent requirements are imposed. These effects 
 are minimised by the use of large wire;, since if the lead^wire resistance 
 is made small enoughp small changes in it will be negligible, 
 
 7o Gages should be handled carefully,, and mounted with even 
 greater care. Sharp bends can change the resistance of a gage, and also 
 its gage factor. Specimen surfaces must be clean and ft:°ee from moisture; 
 heat is the best agent for drying, Cellulose=acetate cement as supplied 
 by Baldwin-Southwark is goed for most applications. The cement should be 
 dried until the leakage resistance is at least 5.0OOO megohms y if possiblee 
 A waterproofing coat of Petrosen© or of Petrolastic should then be applied, 
 the lead~wires attached„ and the waterproofing made complete. Too thick 
 a coating of wax is likely to be brittle, Petrolastic is rubbery, and can 
 be used safely in thicker coats.* 
 
 8, Gage circuits when in place and wired should have leakage 
 resistances of 50 megonms or higher^ if possible. It is entirely possible 
 to operate gages with much lower leakage resistances than thesej, but there 
 is greater safety in the high leeikage resistances p since even a relatively 
 large change in an extremely high resistance will cause little change in 
 effective gage resistance » 
 
 9, Lead-wires should be protected, so far as possible, from 
 anything which might cams® sudden temperature changes, A bundle cf wires 
 with the sun shining on one side may take hours to reach an equilibrium 
 temperature (if it ever does) and during the time the temperature is 
 changing there will be drift of the gag© circuits. Shade from the sun is 
 desirable^ if it is necessary to immerse the wires partially in water it 
 is better to immerse them completely. To indicate the need for uniform 
 temperatures, it may be noted that the change in resistance of a 10- foot 
 length of No, 18 copper wire, if its temperatiire changes 5 deg, Fahrenheit, 
 will be equivalent to that caused by a I'j micro -inch strain in steel, 
 
 10, Switching boxes should also be kept clean and dry, and 
 protected from direct sun and the rain. Portable Strain Indicators must 
 be cared for as Indicated in the preceding section of this report. 
 
 » Notes Petrolastic is a black., tarry, waterproofing material^ pro- 
 duced by Standard Oil of California,, Petrosen© is a micro-crystelline wax 
 produced by the Socony-Vacuum Oil Company, Another popular water-proofing 
 material Is Tygon, a rubbery compound made by United States Stoneware 
 Company, 
 
i 
 
 •ri 
 
 f 
 
l6o 
 
 11, Since the relatively inexpensive instruments used for most 
 of this work are always subject to some drift „ it is advisable to set up 
 the test schedule so that check"=readings may be made frequently. Short 
 intervals between loadings are advisable whenever possible o 
 
 12o It is often advisable to employ mechanical strain-magni- 
 fiers if the strains in the specimen are not large. The "reliable limit* 
 of the ordinary installation of SR-4. gsges is 10 micro-inches or a little 
 more J that is<, even with care in operation an xmcertainty of 10 micro- 
 inches or more may be expected. Often, of coursec, the uncertainty is 
 greater^ and always it will be greater if no reactive balance is used. If 
 the total strain is not large, it is helpful to magnify it mechanically 
 before attempting to measure it with wire=type strain gages. 
 
 Many test programs require the use of more gage circuits than 
 can be conveniently handled without some device for rapid switching of 
 gages. There are available multi^point switching units for this purpose, 
 and some laboratories have constructed their own. Figure 5 shows a phot- 
 graph of a commercially-procurable 20-circuit switch, and also a unit made 
 up in this laboratory^ capable of handling up to 100 gage circuits to be 
 read against any of 10 compensating gages, or of handling 50 gages „ each 
 one against its own individual compensating gage. 
 
 The principal requirements imposed on these switches are (1) 
 low, and constant, resistance, (2) uniformity of resistance. (3) freedom 
 from sources of therassl, electrolytic, or contact potentials,, and (4.) 
 positive action. Of these, the third point is the most important in d-c„ 
 bridge circuits. 
 
 Excepting for their bulk and appearance, good=grade copper knife 
 switches of 30 to 50 ampere capacity are hard to beatj if kept clean and 
 otherwise maintained. They are, howeverp bulky and subject to damage be- 
 cause of their open construction, and consequently are seldom used for 
 more than a few circuits. Many workers have used and recommended the use 
 of multi-point "radio-type* switches, connecting two or more contacts in 
 parallel for lower resistance. The writer's experience convinces him that 
 this practice often leads to a false sense of security, especially in d-Cj^ 
 units I it is better by far to use the best available switches and not 
 parallel contacts. 
 
 There are a number of satisfactory switches available s the writer 
 has used those made by Communication Pro'ducts Coo„ General Radio Co.. Leeds 
 and Northrupf Lewis Engineering Co,, and Shallcross Manufacturing Co., 
 with entire success. There are also, undoubtedly, others not known to the 
 writer, equally satisfactory. 
 
 It should be recognized that any switching unit will contribute 
 to the total probable error in a system, no matter how good the switch. 
 The switch and its extra wiring will increase both the total circuit re- 
 sistance and its reactance, as well as providing more possible paths for 
 leakage and for contamination. Careful design, installation and maintenance 
 of these units can almost, but not quite , eliminate errors from this source. 
 
;%= 
 
 I 
 
Battery 
 
 B 
 
 FIG. 
 
 R], R_3>_Ci__C_3 ^ _Li=Jz3 
 R2 R4 C2 C4 L2 L4 
 
 A-C 
 IN 
 
 FIG. 2 
 
 In most wire strain goge circuits, inductonces (L) are ignored, as 
 ore C_ and C , which appear inside the instrument housing. 
 
* 
 
 } 
 
19, 
 
 4 
 
 /^ 
 
 
 r 
 
 ■ 
 
 1 
 
 
 r 
 
 1 
 
 *. 
 
 Type K. B-S. Indicators Early. 
 
 Fig. 3 
 
 D-C. Bridge 
 
 Ellis 
 
 Young 
 

UNIVERSfTY OF ILLINOIS-URBANA 
 
 3 0112 079432438