I, ^.■Pf\-'1: s^ jo\\A\.'/<ry 
 
 UNITED STATES ATOMIC E 
 
 ORNL-1707 
 
 LOW RESISTANCE RECORDER 
 
 ISSION 
 
 By 
 
 George Ritscher 
 
 June 4, 1954 
 
 Instrumentation and Control Division 
 Oak Ridge National Laboratory 
 Oak Ridge, Tennessee 
 
 
 UNiV. 3- • 
 
 
 
 '" " ~ - I-.- 
 
 
 1 
 
 U.S. DEPOSITORY 
 
 
 Technical Information Service, Oak Ridge, Tennessee 
 
Page ii OENL-I707 
 
 Subject Category, rwSTRIBffiNTATIOW . 
 
 Work performed under Contract No. 'W-^kO^-eng~26 . 
 
 Issuance of this document does not constitute authority 
 for declassification of classified material of the same or 
 similar content and title by the same author. 
 
 This report has been reproduced vlth mini mum alteration 
 directly from manuscript provided the Tectualcal Information 
 Service In an effort to expedite availability of the informa- 
 tion contained herein. 
 
 Reproduction of this information is encouraged by the 
 United States Atomic Energy Commission. Arrangements for 
 your republication of this document in whole or in psirt 
 should be made with the author and the organization he 
 represents. 
 
 Printed in USA, Price 10 cents. Available from the 
 Office of Technical Services, Department of Commerce, Wash- 
 ington 25, D. C. 
 
 Operated by 
 Carbide and Carbon Chemicals Campany 
 for the 
 U. 3 . Atomic Energy Commission 
 
 AEC, Oak Eldge, 1eim..-Mk3h9h 
 
Page 1 ORML-ITO? 
 
 LOW RESISTANCE RECORDER 
 George Ritscher 
 
 ABSTOACT 
 
 An instrument for recording resistances of fractions of an oha has laeen developed 
 by the Industrial Instrument Engineering Section. The instrument has several 
 advantages over the Kelvin bridge, which is commonly used in measurement of 
 resiststnces less than one ohm; the more important are independence of resistance 
 of leads and therefore lead lengthy simplicity of recorder circuit, and ease of 
 providing multiple ranges . 
 
 The recorder circuit ccsnpares the resistance of the specimen under test to that 
 of a standard resistance by comparing the potential drop across the specimen to 
 that across the standard resistance with the same current flowing through both. 
 This is similar to a potentiometer measurement in which the unknown EMF is compared 
 to the EMF of a standard- cell (through a working battery) . A modified recording 
 potenticaneter is the heart of the apparatus. 
 
 IMTRODUCTION 
 
 A recorder was needed for making a continuous plot of resistance of a specimen under- 
 going tensile tests in a creep test machine. Values to be measured were less than 
 five ml Hi ohms. Flexibility in application was de8ired--that is, the recorder 
 should be easily set into operation by technicians, and several resistance ranges 
 provided. The ranges decided upon were from zero to one, two and five milliohms 
 full scale. 
 
 The Wheatstone bridge circiiit is not suitable for this application because attain- 
 able accuracy falls off rapidly as the resistance being measured decreases below 
 one ohm. The Kelvin bridge circiait can lead to results of high accuracy in the 
 low resistance ranges when properly set up. However, the balancing of lead 
 resistances necessary for accurate readings makes this circuit impractical except 
 for laboratory measurements or for fixed recording Installations where leads re- 
 main consteuat. In addition^ a recording Kelvin bridge requires a second slide - 
 wire with accompanying resistors | this calls for considerable modification to re- 
 cording potentiometers readily available. Because of these difficulties, and the 
 simplicity of the measuring circuit to be described, the Kelvin bridge circuit 
 was discarded for this application. 
 
 The method adopted makes use of the fact that one resistance may be measured in 
 terms of another resistance by comparing the potential drops across the resisteuaces 
 with the same current flowing through botho This method is applicable for any 
 value of resistance (within reason), and lead lengths are of little importance. 
 
Page 2 
 
 CRWL-ITOT 
 
 ■A/y*^W- 
 
 VNAAAAAAAAA/VNAA^Y^AAAAAA/^ 
 
 Servo 
 Amplifier 
 
 Motor 
 
 V5-_Std._ _Meas._^\ ^ 
 
 Rs 
 
 AAAAA- 
 
 Rx 
 
 0.005 ohms 
 
 Constant 
 
 Current 
 
 Supply- 
 
 Figure I 
 
 A modified potentiometer recorder makes a record of the potential drop across 
 the \inknown resistance, with cvirrent from a constsmt current supply flowing throxogh 
 the unknown resistance. Automatic stsindardization of the potentiometer current 
 takes place not by comparison with a standard cell, but by comparison with the 
 potential drop across a standard resistance in the constemt current circuit. Thus, 
 the potentiometer standardizes to compensate for variations in the constant current 
 supply as well as changes in its working battery. A simplified circuit is shown in 
 Figure 1. With the circ\iit component values shown, and after the potentiometer 
 circuit is standardized against the potential drop across the O0OO5 ohm standard 
 resistance, the recorder can plot resistance values from zero to O.OO5 ohms. 
 
 CIRCUIT DETAILS 
 
 The complete diagram of the potentiometer circuit and the control panel is shown 
 in Figure 2. The recording instrument used was a "Brown Electronik" potentiometer 
 with modifications. The basic circiilt is standardo Although it appears considerably 
 more complicated than that of Figure 1, it accomplishes the same res\ilts. Alter- 
 ations made to the potentiometer include changing resistor Hq from 509 '5 ohms to 
 
Page 3 ORIIL-17OT 
 
 2.5 ohms, vith addition of Rg to make the total equal 5^9 » 5 ohms, and relocation 
 and adjxistment of contacts on the automatic standardizing switch to give needed 
 switching. A design slidewire span of five millivolts was selected as a compro- 
 mise having sufficient sensitivity for the balancing system, comparative freedom 
 from the effects of the thermal EMF's between leads and specimen, and reasonable 
 current drain requirements <> The potential drop across R is eqxoal to the span of 
 the slidewire . 
 
 The standard resistances, R13^ Rl'^- and R15, were made ten times the full scale 
 resistance for each range to provide approximately 50 millivolts for the current 
 indicator M. A voltage -dividing network., RI7 and RI8, reduces the potential 
 drop to five millivolts at rated current for standardizing the recorder. 
 
 Ciirrent and potential connections to the specimen are made through reversing 
 switches; if thermal EMF's exist, an average of the direct and reversed readings 
 will be the correct reading. 
 
 CONSTANT CURRENT SUPPLY 
 
 The constant current supply was a modified Sorensen Model E-6-5A constant voltage 
 Nobatron. The modification consisted of removing connections to the filament of 
 the regulating diode and bringing the connections to terminals. Leeds from the 
 terminals connect across shunts Rl through R9 in the series loop, and the diode 
 acts to hold a constant current in the loop. The proper shunting resistance is 
 selected by the range selection switch along with the standard resistance. Each 
 shunting resistance is adjustable over a plus or minus ten percent range to take 
 care of variations in diodes. Resistances RIO, Rll and R12. are current limiting 
 resistors selected to hold the output voltage of the constant current supply approxi- 
 mately equal on the three ranges. 
 
 The output of the constant voltage Wobatron was floating^ and with the high 
 voltages which exist in the Nobatron together with slight leakage., voltages from 
 output to ground in the order of i»-00 volts were measureii. Resistor R20 and 
 capacitor CI were added to remove this shock hazard. 
 
 Relay R was installed to short the specimen leads sho\LLd the output voltage exceed 
 eight volts. Otherwise, if the current circuit to the specimen is opened the out- 
 put voltage will increase in an attempt to increase the current. Shorting of the 
 circuit will decrease the output voltage to approximately 4.5 volts, which is above 
 the drop-out voltage of the relay, so the relay will not cycle in and out. Turn- 
 ing off the constant current supply will reset the relay. 
 
 The Sorensen regulating diode has good short time stability, but the long time 
 stability is not comparable to the recording potentiometer. However, with 
 periodic automatic standardization which adjxists for the slow drifts, it has 
 proved satisfactory. The regulating diode is not able to take care of transient 
 changes in line voltage because heating of the filament requires a finite time. 
 By inserting a Sola sine wave regulator aiiead of the Sorensen constant current 
 supply, excellent regulation was obtained against small transient changes in line 
 voltage, and in slower changes frcm 90 to 135 volts. Regulation against change 
 of load is not so good as regulation against line voltage change. However, for a 
 given meas\irement, the load remains substantia3J.y constant so this factor presents 
 no problem. 
 
Page k CBKL-1707 
 
 PHYSICAL ARRANGEMENT OF EQUIPMENT 
 
 The modified recorder was mounted in the top, control panel in the middle, and 
 constant current supply in the bottom of a kl inch relay rack cabinet as shewn 
 in Figure 3 • The cabinet was fitted with casters . 
 
 ACCURACY 
 
 Inasmuch as the components of the measuring circuit axe stable resistances, the 
 basic accuracy is that of the recording potentiometer, or l/^^. Drift of the 
 constant current supply between standardization periods is a source of error, 
 but in tests this error has proved to be less than l/lO^ after a warm-up period. 
 Length of leads will int^roduce no error except as might be caxised by leakage 
 between leads or lack of sensitivity of balance due to excessive resistances. 
 
 POSSIBLE EXTENSION OF METHOD TO OTHER RESISTANCE VALUES 
 
 No problems exist in extension of the method to Jaigher resistance. However, at 
 some value of resistance, for a given application, a Wheatstone bridge circuit 
 will be satisfactory and less expensive. 
 
 Downward revision of the method will require a constant current supply of higher 
 current capacity or a more sensitive recorder amplifier. Both of these factors 
 wovild introduce conplications which would deter extension by more than a factor 
 of five below one milliohm. 
 
 GRimtg 
 U/l9/5i^ 
 
I 
 
 s 
 
 3 
 
 o 
 
 s 
 
 i 
 
 
 li 
 
 'i 
 
 hi 
 
 o 
 
 I 
 
 a 
 
 1 
 
 ii 
 
 ii 
 
 I 
 
 in 
 
 •5: 
 
 |5 
 
 
 5 
 
Page 6 
 
 Unclassified 
 Photo 11723 
 
 Figure 3 
 
Digitized by tine Internet Arclnive 
 
 in 2011 witli funding from 
 
 University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation 
 
 http://www.archive.org/details/lowresistancerecOOoakr 
 
UNIVERSITY OF FLORIDA 
 
 3 1262 08906 3019