Low resistance recorder


Material Information

Low resistance recorder
Physical Description:
ii, 6 p. : ill. ; 28 cm.
Ritscher, George
Oak Ridge National Laboratory
U.S. Atomic Energy Commission
U.S. Atomic Energy Commission, Technical Information Service
Place of Publication:
Oak Ridge, Tenn
Publication Date:


Subjects / Keywords:
Recording instruments   ( lcsh )
Electric resistance -- Measurement   ( lcsh )
non-fiction   ( marcgt )


Statement of Responsibility:
by George Ritscher.
General Note:
Cover title.
General Note:
General Note:
"June 4, 1954."
General Note:
"Work performed under Contract No. W-7405-eng-26"--P. ii.
General Note:
Work performed at the Oak Ridge National Laboratory.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005254618
oclc - 727948634
System ID:

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Full Text



George Ritscher

June 4, 1954

Instrumentation and Control Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee



Technical Information Service, Oakl Ridge, Tennessee


Page 11


Subject Category, INSTRUMENTATION.
Work performed under Contract No. W-7405-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 with minimum alteration
directly from manuscript provided the Technical 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 part
should be made with the author and the organization be
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. S. Atomic Energy Commission

AEC, Oak Ridge, Tenrn.-W43494

Page 1




George Ritscher


An instrument for recording resistances of fractions of an ohn has been developed
by the Industrial Instrument Engineering Section. The instrument has several
advantages over the Kelvin bridge, which is commonly used in measurement of
resistances less than one ohm; the more important are independence of resistance
of leads and therefore lead length, simplicity of recorder circuit, and ease of
providing multiple ranges.

The recorder circuit compares 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
potentiameter is the heart of the apparatus.


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 milliohms. Flexibility in application was desired--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 circuit 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 circuit 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 constant. 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 drope across the resistances
with the same current flowing through both. This method is applicable for any
value of resistance withinn reason), and lead lengths are of little importance.

Page 2


Figure I

A modified potentiometer recorder makes a record of the potential drop across
the unknown resistance, with current from a constant current supply flowing through
the unknown resistance. Automatic standardisation of the potentiometer current
takes place not by comparison with a standard cell, but by comparison with the
potential drop aerosa a standard resistance in the canatant 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 circuit component values shown, and after the potentiometer
circuit is standardized against the potential drop across the 0.005 dan standard
resistance, the recorder can plot resistance values from zero to 0.005 ohma.


The complete diagram of the potentiometer circuit and the control panel is shown
in Figure 2. The recording instrument used was a "Brown Electronik" potentiometers
with modifications. The basie-eircuit is standard. Although it appears considerably
more complicated than that of Figure 1, it accomplishes the same results. Alter-
ations made to the potentiometer include changing resistor Re from 509.5 ahms to

Page 3


2.5 ohms, with addition of Rg to make the total equal 509.5 ohms, and relocation
and adjustment 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 equal to the span of
the slidewire.

The standard resistances, 813, 816 and Rl5, were made ten times the full seale
resistance for each range to provide approximately 50 millivolts for the current
indicator M. A voltage-dividing network, 817 and 818, reduces the potential
drop to five millivolts at rated current for standardizing the recorder.

Current and potential connections to the specimen are made through reversing
switches; if thermal ENF's exist, an average of the direct and reversed readings
will be the correct reading


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 fran the
terminals connect across shunts RI 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. Resistance 810, 811 and 812 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 Nobatron 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 400 volts were measured. Resistor R20 and
capacitor 01 vere added to remove this shock hazard.

Relay R was installed to short the specimen leads should 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.

15mSorensencegulating diode has good short time stability, but the long time
stability is not comparable to the recording potentiometer. However, with
periodic automatic standardization which adjusts 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 ahead of the Sorensen constant current
supply, excellent regulation was obtained against small transient Whanges in line
voltage, and in alover changes from 90 to 135 volts. Regulation against change
of load is not so good as regulation against line voltage change. However, for a
given measurement, the load remains substantially constant so this factor presents
no problem.

Page 4



The modified recorder was mounted in the top, control panel in the adddle, and
constant current supply in the bottom of a 41 inch relay rack cabinet as shown
in Figure 3. The cabinet was fitted with masters.


Inasmuch as the components of the measuring circuit are stable resistances, the
basic accuracy is that of the recording potentiometer, or 1/4 Drift of the
constant current supplytetween standardization periods is a source of error,
but in tests this error has proved to be less than 1/100 after a varm-up period.
Length of leads will introduce no error except as might be caused by leakage
between leads or lack of sensitivity of balance due to excessive resistances.


No problems exist in extension of the method to hig er resistance. However, at
some value of resistance, for a given application, a Wheatetone bridge circuit
will be satisfactory and leas expensive.

Downward revision of the method will require a constant entrent supply of higher
current capacity or a more sensitive recorder amplifier. Both of these factory
would introduce complications which would deter extension by more than a factor
of five below one milliobm.



Page 6


s e ea m



Figure 3

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