I Hational^ureaTr^l

Library, R.W. Bldg

MAR 1 1 1965

Reference

taken from the library.

^ecknical <^iote 257

PRACTICAL ASPECTS OF THE USE OF

AC-DC TRANSFER INSTRUMENTS

E. S. WILLIAMS

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assuring

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* NBS Group, Joint Institute for Laboratory Astrophysics at the University of Colorado. ** Located at Boulder, Colorado.
NATIONAL BUREAU OF STANDARDS

Technical Note 257

ISSUED MARCH 9, 1965

PRACTICAL ASPECTS OF THE USE OF

AC-DC TRANSFER INSTRUMENTS

E. S. Williams

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 15 cents
CONTENTS

PAGE

1, GENERAL 1

2, AC-DC DIFFERENCE 2

3, GALVANOMETER CALIBRATION- ~~

: —

4

4c A-C MEASUREMENTS 5

5. PRECAUTIONS 7

6« SAMPLE CALCULATIONS- 8

LIST OF FIGURES

1. Circuits for AC-DC Difference Measurements

(a) for voltage, (b) for current

2*. Galvanometer Scale Calibration

3c Circuits for AC Voltage Measurements

4c Circuits for AC Current Measurements

li
PRACTICAL ASPECTS OF

THE USE OF AC-DC TRANSFER INSTRUMENTS

E. So Williams

Electrothermic transfer instruments may be used to

make direct measurements of ac-dc difference or frequency

influence o With a somewhat similar procedure an "unknown" a-c voltage or current may be accurately measured by determining the difference between it and a preset and accu- rately measured d-c equivalent. Test circuits and procedures are described and data taking and calculations are illustrated.

1 . GENERAL

The term "transfer instrument" is the name used here to designate

an instrument used to transfer direct current standards to alternating

quantity measurements. It is analogous in a way to the chemist 1 s balance. It compares a direct current or voltage established directly

in terms of a basic standard with the similar a-c quantity to be

measured. In use it is ordinarily transferred or switched between

a-c and d-c circuits. The primary considerations in its design, therefore, are not convenience nor direct readability but high sensitivity of

readout or indication and small ac-dc differences.

The thermoelement is the current sensing element in the transfer

instruments to be considered here B The short straight heater gives it

minimum reactance and makes it an excellent component in a circuit

where a very small frequency influence is important. The thermocouple

is attached to the mid-point of the heater and is insulated from the a-c circuits by a small ceramic bead. For currents up to one ampere

the assembly is mounted in an evacuated glass bulb. Thermoelements for

currents higher than one ampere usually do not have insulating beads or vacuum enclosures

For current measurements, the transfer instrument may consist

simply of a single thermoelement whose heater carries all the current
to be measured, or it may consist of a thermoelement in parallel with

a-c shunts o For voltage measurements, the thermoelement usually

carries only a few milliamperes<> It is connected in series with a multiplier resistor which is very often tapped to give several voltage

ranges, although single-range transfer voltmeters are widely used at

higher frequencies.

AC-DC difference tests are often used to advantage. Electrodynamic

and electrothermic instruments respond equally to d-c and low-frequency

a-c currents „ They are more readily calibrated with d-c voltage or cur- rent using the conventional d-c techniques the potentiometer, volt

box, and shunt. Since the scale accuracy depends on the accuracy of

series multiplier resistors or shunts, it is subject to changes or longtime drifts, and frequent recalibration with direct current is advisable,

On the other hand the ac-dc difference or frequency influence is

much more constant. It is the result of residual reactance in the a-c

circuit. If the instrument is well designed, and constructed, so that

multiplier, shunt and coil components are rigidly mounted, there should

be no appreciable change in the frequency characteristics unless these

parts are repaired or altered. Ordinarily ac-dc difference corrections

are redetermined only once in several years and, if significant, are applied in addition to scale calibration corrections determined in d-c

tes ts

This calibration scheme has the advantage of separating the types

of errors. It simplifies the maintenance of the calibration since the

more difficult a-c techniques are required infrequently.

Many important instruments, of course, cannot be standardized in

this way. Among these are instruments with built-in transformers and

many types of moving-iron instruments, vacuum tube voltmeters, rec- tifier instruments, and some multirange electrostatic voltmeters.

These usually require direct measurements of a-c quantities, but a digital instrument or a volt-ampere converter having an accuracy of 0.03

to 0.05 percent is usually suitable for the calibration,,

There is, however, increasing reliance on a-c digital voltmeters

and a-c voltage standards of rather high accuracy and reliability. If

these are to be tested with appropriate accuracy, they must be compared

as directly as possible with the best d-c standards. The ac-dc transfer instrument is the comparator. But care must be taken to avoid

inaccuracies that can arise from drift, d-c reversal difference, and

other causes if we are to realize its full potential accuracy.

2. AC-DC DIFFERENCE

Figure la shows a multirange transfer voltmeter and the circuits

for an ac-dc difference test. Instead of the usual millivoltmeter
connected to the thermocouple output, as in electrothermic indicating

instruments, the transfer Instrument has a Lindeck potentiometer or other balancing circuit. This is adjusted to equal or balance the thermocouple output o A galvanometer null indicates the balance

To make an ac-dc difference or frequency influence test on the

"TEST Vm" (it may be an indicating instrument or another transfer instrument with its Lindeck potentiometer and galvanometer) it is connected in

parallel with the transfer instrument as shown so both instruments can

be switched to a-c or d-c voltage (AC-V or DC-V, Fig. la). The i test

voltmeter is set to a certain test point on d-c voltage and the Lindeck

potentiometer is adjusted so that the galvanometer is balanced near midscale. This need not be a null balance. Then, without further Lindeck

adjustmient, the test and transfer voltmeters are switched to a-c and then

to d-c voltage. The test instrument is set to the same test point in

each case, and the galvanometer indications corresponding to the two

voltages are read. The difference in the indications is a measure of the

difference between the a-c and| d-c voltages required to give equal

deflection or response to the instrument under test.

The transfer instrument usually has two shortcomings which must be

taken into account for highest accuracy. The thermoelement has a small

reversed d-c difference (that is, the emf output is not quite the same

for equal heater current in opposite directions) and it frequently has a drift in output emf even after several minutes of warm-up. The drift is

particularly noticeable on higher voltage ranges where there is considerable heating in the multiplier resistors.

To overcome these two difficulties, the following procedure is

recommended: after an initial galvanometer reading on a-c voltage (AC-V)

we take two on DC-V one for each direction of current and then, in case there is a drift, a fourth reading on AC-V. If the drift rate is

reasonably uniform, as it usually is after a short warm-up, and if the

readings are taken at rather regular time intervals, the effect of drift caa be practically eliminated. Even with a large drift the determinations

can be repeated surprisingly well with this procedure.

An alternative procedure is to take readings on DC-V, AC-V and

reversed DC-V. This will also compensate for drift, but a second AC-V

reading is preferred as a check on the first one as a precaution.

For current measurements (Fig. lb) the transfer instrument—in this case a single thermoelement—has its output connected to the same Lindeck

potentiometer and galvanometer as that shown in Fig. la. Again the "TEST

Am" may be an indicating instrument or a transfer instrument. The

procedure described above for voltmeters is also used here.

The ac-dc difference is indicated in terms of galvanometer scale

divisions (cm or mm). We therefore need a calibration of the galvanometer
scale for each thermoelement to be used. If the calibration is made and

a curve drawn as described below, then a dividing factor may be found to

conveniently convert scale divisions to percent difference,

3. GALVANOMETER CALIBRATION

Before making the galvanometer scale calibration (or using it in an

instrument test) the galvanometer must be connected so an increase in

heater current gives an upscale deflection or a deflection to the right on the galvanometer scale. This is important to assure a proper sign with

the data in Instrument calibration.

Figure 2 shows, on the left, a simple circuit by which we can pass

a direct current through the heater of the thermoelement, and a shunt

and a potentiometer to measure the current. Again the output emf of the

thermocouple is balanced against the Llndeck potentiometer as indicated

by the galvanometer. Actually this balance should be such that the

galvanometer deflects a little to the left of mid-scale» Having measured

the current accurately, we then accurately measure an increased current

AI of perhaps 1 percent. This Increase in current will deflect the

galvanometer upscale (beyond mid-scale) and this change in deflection we

designate AD.

From the denominator terms of the equation (Fig. 2), we see that

this 7<AI gives an increased deflection AD. Now another change in current

which we designate in the numerator as %DIFF (or more exactly, a small

difference between a-c and d-c currents) will then give another change

in deflection which we designate Dac - Ddc . This gives us a simple ratio

%DIFF Dac " Ddc

%AI AD

If AI was one percent, the denominator term in the first expression

(%AI) is unity, and the percent difference between the a-c and d-c current (or voltage) is given by the equation %DIFF » (Dac - Dac)/AD and

AD is a sensitivity factor which we may call S. The complete formula

including the percentage ac-dc difference correction (Cs ) for the transfer

instrument is

D - D , %DIFF ~

aC

- dC

+ C_„

S s

The calibration may be made with larger or smaller Al, but S should

be equal to the AD obtained with one percent AI» That is, if AI is 2%

then S « AD/2 (see sample calculation). A one-point calibration is, 6f

course, good only at that particular current level. We have found it

convenient to make four or more determinations and to draw a curve connecting each point as shown in Fig. 2. (Values of "TC mV" and "S"
on the figure are used in the sample calculations, page 9 ,) We then

find a value of S corresponding to any current level or thermocouple

output emf o If the thermocouple output is measured with the Lindeck

potentiometer, it is convenient to use it as the abscissa in drawing

and using the curve. Otherwise, one may use heater current or percentof-rated-voltage as abscissa.

The curve is not quite straight so that one numerical factor cannot

be used to accurately determine S from TC mv directly,, The error would

sometime be 20 percent or more at the upper end of the curve

The curve is easily determined and drawn and is reliable until the

thermoelement is replaced if the circuit components and galvanometer

sensitivity are unchanged,. The galvanometer should be checked now and

then to see that its sensitivity is unchanged,, The scale should also be

reasonably linear over the portion being used. We have found that most

galvanometers are satisfactory in this respect and that sensitivity

rarely changes.

We should also note that where the differences being measured are

small, we can tolerate surprisingly large inaccuracy in the galvanometer

and its calibration. For instance, if we have a 10 percent error in a measurement of a 0,1 percent voltage difference, we are wrong by only

0,01 percent of the total voltage.

This calibration curve may be made for a transfer voltmeter on any

range and the voltage and voltage change (AV) can be measured with a volt

box and potentiometer (see sample calculation). Also if a calibrated

d-c supply is available with which small incremental changes can be set

by decade switching, it can be used to make the galvanometer scale

calibration.

Incidentally, some commercially available transfer voltmeters now have built-in arrangements for determining galvanometer sensitivity,

4, A-C MEASUREMENTS

With the galvanometer scale calibrated, we are also prepared to make

accurate a- c voltage and current measurements. Figure 3 shows the circuit for voltage. The a-c source shown on the left may be the voltage

applied to a deflecting or digital voltmeter or it may be the output of a calibrated a-c voltage standard. The transfer voltmeter can be switched

back and forth between the a-c and d-c voltages with its output con- nected to the Lindeck potentiometer and galvanometer. The d-c voltage

supply is shown on the right with the reversing switch and the volt box

and potentiometer,

A measured d-c voltage could be supplied, of course, by a calibrated

d-c supply, or the volt box and potentiometer could be replaced by an

accurate digital voltmeter,

5
A procedure often used in this test, and one that may be

satisfactory in many cases is as follows: switch to AC-V (Fig. 3) and

adjust the Lindeck potentiometer to balance; then switch to DC-V, adjust

it to rebalance the Lindeck potentiometer and measure the d-c voltage

with the volt box and potentiometer. The procedure is then repeated with

reversed d-c voltage. The average of the two d-c voltages should then

equal the a-c voltage when corrections are applied for the volt box and

potentiometer, and for any ac-dc difference in the transfer voltmeter.

However, as stated earlier, we very often have a drift in

thermocouple output, mainly because of heating of the transfer voltmeter*

Since we must make two exact adjustments after the AC-V is switched off

(we must adjust the d-c voltage and then measure it), an apparent but

unreal difference between the two voltages can appear because of the

drifto It is also quite difficult to determine just how much effect the

drift is causing. If the d-c adjustment and measurement require varying

amounts of time, it may be difficult to obtain repeatable results.

The procedure suggested here will largely avoid the drift effect

and may, at the same time, actually make the test a bit faster and less

tedious The potentiometer is set to a round-number value which is

nominally equal to the a-c voltage and the d-c voltage is adjusted to

balance the potentiometer at this setting. The Lindeck potentiometer is

then adjusted to give a near mid-scale galvanometer reading (not

necessarily a null reading) . Three galvanometer readings are then made

at time intervals which should be nearly equal: DC-V, AC-V and reversed

DC-V. The d-c voltage is adjusted to nominal value for each reading on DC-V. The difference between the a-c reading and the average d-c

readings is a measure of the voltage difference. When converted to

percent with the galvanometer calibration curve, it can be applied to

the potentiometer reading to determine the a-c voltage, or it can be

recorded as a correction to be applied to the a-c instrument under test

with due attention to sign (see sample calculation).

One precaution here may be worth mentioning. The leads between the

volt box and ac-dc switch should be equal in resistance to those between

this switch and the terminals of the test instrument to equalize the

voltage drop in these leads. This can be important when working with

low voltages.

The use of two galvanometers with spots on one scale (Fig. 3)

illustrate an arrangement we have found to be convenient and efficient.

If a stable a-c voltage standard is being tested at certain settings or

if an a-c digital voltmeter under test is supplied from a stable source,

then one observer, who has the d-c voltage controls at hand, can make the

test. After preliminary setting of the potentiometer and Lindeck potentiometer, he sets the DC-V as indicated by a null reading on the top

galvanometer spot and reads the position of the spot on the bottom of

the scale. A second reading on AC-V and a third on reversed DG-V, each
made after readjusting the input voltage to obtain a null on the top

light spot complete a determination.

Figure 4 shows a similar setup for current measurements . The

ammeter under test is set to the test point for the a-c reading and the

d-c current is adjusted to the same nominal value for the direct and re- versed d-c reading o Differences in galvanometer readings on AC and the

average for DC indicates the difference in the currents . The procedure

is the same as before,,

5. PRECAUTIONS

High accuracy can be obtained with transfer instruments only if the

user is aware of certain limitations and takes appropriate precautions.

(1) In current measurements, it is usually better if the

thermoelement is grounded* It should never be more than about one volt

above ground. If the test ammeter must be grounded then the thermoelement can be put above ground by this small potential, but we should

maintain a minimum voltage across the insulating bead between the heater

and thermocouple. For voltage measurements one end of the heater of the

transfer voltmeter should always be connected to the ± or ground

terminal. If binding posts rather than connectors are used, we must be

careful to ground the proper terminal.

(2) The emf of a thermoelement decreases slightly for the first

minute or two that it is energized, aside from any heating effect in

shunts or series resistors. Therefore, it should have a minute or two

to warm up. This is usually done anyway during the preliminary adjustments before the test is started; however, after any large change in the

heater current we should wait a minute or two for the thermoelement output to stabilize,, The multiplier in the voltmeter, as noted earlier,

frequently drifts for some time. The temperature coefficient of the

multiplier resistor is usually quite small, but even so, for highest

accuracy, we should let it warm up until the drift is quite slow and then use a test procedure, such as that given in this paper, to° minimize the

effect of drifts on our determination.

(3) Since thermoelements have a frequency response which extends to

100 MHz or more, they should be used in areas of reasonably low field

strength. Interference from TV stations can be particularly troublesome

with low-current thermoelements. A quick check of lead-borne interference

is easily made by short-circuiting the input terminals, with the con- nections to the standard in place as in use, but with all power supplies

off. There should be no change in indication of the galvanometer in the

output circuit of the thermoelement. Such interference can often be

avoided by using coaxial leads and shielded circuit components. In

extreme cases, however, the instrument may have to be used in a shielded

room.
(4) We should be sure that our transfer instruments have small

ac-dc differences and that they are known so corrections may be applied.

They are usually negligible (<0.005%) at mid-audio frequencies, but then

increase (often as the square of the frequency) at the higher audio

frequencies. They should be determined at the highest rated frequency--

or at the higher frequency to be used and at lower frequencies only as necessary.

Thermoelements also have a low frequency limit, below which they

fail to integrate properly. The ac-dc difference may approach 0,02 percent at frequencies ranging from about 5 Hz for most low- range thermoelements to about 60 Hz for some thermoelements with ratings above 1

ampere. It is independent of the range for multirange converters in

which a single thermoelement is used with shunts or multipliers. One

ac-dc difference test (ordinarily at 20 Hz) is recommended to verify the

low- frequency accuracy,

(5) Replacing the thermoelement should not affect the ac-dc

difference if other a-c circuit components are not disturbed in such a way as to affect their reactance. Replacement thermoelements are generally available from the instrument manufacturer with the necessary

characteristics and with negligible ac-dc difference,

6. SAMPLE CALCULATIONS

Galvanometer Calibration

TC emf 8.9 3.7 mV

V

l

100,00 60.0 voli

V

2

101.00 61.2 it

V

l

100.00 60.00 ti

%AV 1 2

D

l

15.6 14,3 cm

D

2

34.1 33.2 it

D

l

15.8 14.5 ti

AD 18,4 18.8 ii

18.4 9.4 cm/l%AV

(Refer to curve - Fig« 2)

Voltage Measurement

Frequency 20 kHz

d-c volts set 75.000

TC mV 6.2

S 14.2

'dc

Dac

24.1

26.2

Ddc (rev.) 24.7

D -D, + 1.8 ac dc

(Positive sign indicates

AC is larger than DC)

D - D , ac dc

+0.137.

0.0013 x 75 •= +0.098 volts

a-c Volts « 75.000

+ 0.098

75.098

If AC-DC Difference of the transfer voltmeter is

+0.027. at 20 kHz, a-c Volts - 75.113

8
AC-V OC-V

(a)

AC DC

U/J

(£=

TEST

Am.

(b)

Fig. 1. Circuits for AC-DC Difference Measurements

(a) for voltage, (b) for current

—| —wytfw

POTR.

%DIFF D

ac"D

dc

%AI AD

/|FAI = l%\

V S=AD/

AI l-AD

JL

<K LINDECK

POTR.

Fig. 2. Galvanometer Scale Calibration
Fig. 3. Circuits for AC Voltage Measurements

<•»

Am

AC

- j

LINDECK

POTR.

NVf&ArYWW

POTR.

T

Fig. 4. Circuits for AC Current Measurements

10

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