A111D5 b2MD0M

NBS

PUBLICATIONS

NATL INST OF STANDARDS & TECH R.I.C.

All 102624004

Williams, Earl S/The practical uses of A

QC100 .U5753 NO. 1166 1982 V19 C.1 NBS-P NBS TECHNICAL NOTE 1166

^"tEAU O* *

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

QC —

100

.U5753

#1166

1982

The Practical

Uses of AC-DC

Transfer Instruments
NATIONAL BUREAU OF STANDARDS

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The Practical Uses of AC-DC

Transfer instruments

Earl S. Williams

Electrical Measurements and Standards Division

Center for Absolute Physical Quantities

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

^''ffeAU 0* *

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrlge, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1 982
National Bureau of Standards Technical Note 1166

Natl. Bur. Stand. (U.S.), Tech. Note 1166, 37 pages (Oct. 1982)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982

For Sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402

Price $4.50

(Add 25 percent for other than U.S. mailing.)
Contents Paae

1.0 Introduction 1

2.0 Thermoelements 2

2.1 Output Characteristics 3

2.2 DC Reversal Difference 3

2.3 Temperature Coefficient 5

2.4 AC-DC Difference 5

2.5 Radio Frequency Interference 5

3.0 Voltage Measurements 6

3.1 A Null Method 6

3.2 .A Deflection Method 8

3.3 Electronic Galvanometers 9

3.4 D'Arsonval Galvanometers 11

3.5 Calculations 11

3.6 Galvanometer Calibration 12

3.7 Reading Deflecting Detectors 13

3.8 Peak and Average Responding Instruments 15

4.0 Switches and Potentiometers 15

4.1 AC-DC Switches 15

4.2 Lindeck Potentiometers

16

4.3 Detector Keys 16

5.0 AC-DC Difference 17

5.1 AC-DC Difference Corrections 17

5.2 AC-DC Difference in Thermoelements 18

6.0 AC-DC Difference Measurements 19

6.1 A Null Method 20

6.2 A Deflection Method 20

6.3 TE Comparators 21

6.4 Voltage Control Circuits 22

6.5 Verifying AC-DC Difference Corrections 25

7.0 Measurements of n 27

8.0 Current Measurements 28

References 32

m

THE PRACTICAL USES OF AC-DC TRANSFER INSTRUMENTS

Earl S. Williams

ABSTRACT

Alternating currents and voltages are measured most accurately at this time

when they are compared with nominally equal and known dc currents and voltages.

The comparisons are usually made with thermal transfer instruments which

respond nearly equally to ac and dc quantities. Practical information and

recommended procedures are given for using these instruments along with

diagrams of apparatus and examples of typical data and calculations. Methods

for minimizing difficulties caused by dc reversal differences, thermal drift,

energy picked up from local electromagnetic fields, and the deviation from

square-law response of these instruments are considered. Causes of ac-dc

differences are discussed, and methods for measuring them and applying corrections are also described.

Key words: ac current measurements; ac voltage measurements; ac-dc comparator; ac-dc difference; thermoelement.

1.0 Introduction

The basic units for electrical measurements are derived at the National Bureau

of Standards in absolute terms from the units of mass, length, and time. The

standards primarily used to transfer these units for direct current measurements from NBS to other users, and to preserve them in the standards laboratories, are the saturated standard cell (the volt) and the standard

resistor. With these and a variety of shunts and resistance ratio instruments

for range extension, direct current and voltage can be measured with high

accuracy.

Since there is no ac counterpart to the standard cell, measurements of

alternating current, voltage, and power are made relative to these same

standards. The chain of measurements is extended to these alternating quantities by ac-dc transfer instruments, which have a flat and known frequency response and hence may be calibrated on direct current and then used

for alternating current measurements. They provide an accurate transfer from

direct voltage and current standards to alternating current and voltage

measurements, hence the name.

Electrodynamic instruments have been used to transfer voltage, current, and

power; however, they are limited in frequency to a few thousand hertz due to

their relatively high inductance. Electrostatic moving systems are usable to

higher frequencies, but they have severe low voltage limitations. Instruments

which respond to peak and average values of ac voltage have been employed

successfully over certain voltage and frequency ranges (see section 3.8). The

instruments most widely used for voltage and current transfer measurements,

and the ones to be discussed here, are electrothermal. They are usually

called thermal voltage converters (TVCs) and thermal current converters (TCCs)

to distinguish them from other types of transfer instruments [1].
2.0 Thermoelements

The key component in these transfer instruments is a thermoelement (TE),

consisting of a heater which carries the current to be measured and a thermocouple attached to its midpoint. The output from the thermocouple is relatively low (7 to 12 mV); however, heater current differences, and differences

between ac and dc currents, as small as one part-per-million (ppm) can be

detected, if changes in the output are monitored with a sensitive detector--a

galvanometer or microvoltmeter. Thermoelements for currents from 1 to 1000 mA

are available with evacuated glass-bulb enclosures and with insulation (a

small ceramic bead) between the thermocouple and heater (fig. 1). TEs are available for currents up to at least 20 amperes, but vacuum enclosures are

not feasible at these currents because the elements require heavy copper

conductors which are very difficult to pass through a vacuum seal. Insulation

is not generally available in TEs for currents greater than one ampere. They

are difficult to construct, and the demand for them is not great. Most

current comparisons are made with special ac shunts in parallel with lowrange TEs (see section 8).

Figure 1. Vacuum thermoelement. (Heater and thermocouple wires retouched.)

The TE heater is short and straight thus having minimal reactance, and the

insulation of the thermoelement prevents any appreciable interaction between

the ac current and the read-out instrument. TEs are therefore well suited

to measurements at audio and higher frequencies. They are used with series

resistors in multi range TVCs up to 100 kHz on most ranges and to 1 or 2 MHz at

low voltages. Single-range units with carbon or metal film resistors coaxially

mounted in tubular casings are used up to 100 MHz.

Multi junction thermoelements [2,3,4,5], with a relatively low heater

temperature and 50 to 200 thermocouples in series distributed evenly along the

heater, are capable of higher accuracy in transfer measurements, and their

structure minimizes most of the thermoelectric effects that cause errors in
other types of TEs. However, they are difficult to construct, and therefore

expensive and not readily available.

2.1 Output Characteristics

The thermocouple output of the TE is a function of the power dissipated

in the heater, and therefore varies approximately as the square of the heater

current. However, the device does deviate significantly from a square-law

response as the heater current approaches the rated value. The relationship

of the output, E, to the heater current, I, may be expressed as

E = kl". (1)

The response appears to be nearly square-law (i.e., n=2) at very low currents,

but n is usually 1.6 to 1.9 at rated heater current. The factor k varies

somewhat with large changes in heater current, but this is not a constraint.

The TE is a substitution device, and not normally used to measure ac current

as a function of thermocouple output. The factor k is constant over a narrow

range where nearly equal ac and dc currents are compared.

The relationship between a small change in TE heater current (Al) and the

corresponding change in output (AE) is expressed as

AI/I = AE/nE. (2)

If the thermoelement is in a TVC,

AV/V = AE/nE. (3)

It is often advantageous to make measurements of n and use the data in

calculations as suggested in sections 3.2 and 6.2. The measurements are not

difficult, and the values are quite permanent. Methods for measuring n are described in section 7.

2.2 DC Reversal Difference

DC reversal difference is generally (and in this document) defined as the

percentage difference between the dc current and its reversed counterpart,

when they both produce the same output from a TE. This difference is not

necessarily constant and may increase in some TEs as the heater current is

lowered. A low dc reversal difference is, of course, advantageous. The

difference between ac and dc voltage and current is often measured by

observing the change in indication of an instrument which responds to changes

in TE output as ac and dc are applied. If the dc reversal difference is

large, it may be necessary to reduce the sensitivity of the read-out instrument to keep all readings on scale.

It is recommended that both directions of dc be used in the procedures

described here. However, it is not often necessary to have an explicit

measurement of dc reversal difference. A measurement may be desirable

sometimes, as for instance when making sure that new TEs conform to specifications. A low- range TE can be connected in series with a resistance of 50 to

200 kQ, and the appropriate voltage applied for rated heater current. The

value of the two direct voltages which give equal output can be read from a
calibrated source or measured with a digital voltmeter. Common currentmeasuring equipment, usually a potentiometer or a digital voltmeter and a shunt, is used to measure currents in higher range TEs. In any method, it is

advisable to use a test sequence which will minimize the effect of drift in

the TE output. Readings should be taken with DC+, DC-, DC+ after time

intervals which are about equal. The average of values obtained with the two

positive dc voltages should be used in the calculations.

DC reversal difference can also be measured conveniently with a second or reference TE of about the same range and with known n values. The two heaters

are connected in series through a switch which reverses the test TE input but

not that of the reference one (fig. 2). Direct and reversed currents are adjusted for equal output from the test TE (null on detector Dl). The emf of

the reference TE (E) is read from its potentiometer (Pot'r) and changes in emf

(AE) are computed from readings taken from the detector (D2), which must be a microvoltmeter or a calibrated galvanometer. DC reversal difference, AI/I, is

then computed from eq. (2).

Reference TE

DC+ DCDC Current

Supply

L

Pot'r D2 (Sz Pot' D1

Test

TE

Figure 2. Circuits for dc reversal difference measurements.

As pointed out in the previous section, a TE output varies approximately as the square of the heater current. If dc reversal difference were to be

defined as the percentage difference between emfs obtained with equal currents

in opposite directions, the figure would be about twice as large.

DC reversal differences range from near zero to several hundredths of a percent. They are largely due to Peltier heating and cooling at the ends of

the heater, and to Thomson heating in the two halves of the heater [6,7].

Both of these effects cause asymmetry in the temperature rise of the heater.

The asymmetry is affected when the current is reversed, and therefore the

temperature at the point where the thermocouple is attached is changed. DC

reversal difference is minimized by placing the thermocouple at the thermal

center of the heater. If the heater wire is not quite uniform, this may not

be the exact mechanical center. DC reversal difference is therefore not

easily controlled. Some manufacturers sort TEs according to dc reversal

difference and market those with low dc reversal difference as a premium

product.
DC reversal difference rarely changes with use, and the few changes that

have been observed are believed to be due to the heater having been overloaded. One possible explanation for this is that one-half of the heater was stressed more than the other, and its thermal center was shifted relative to

the thermocouple's position. If this model is correct, the dc reversal

difference could be either increased or decreased by an overload.

2.3 Temperature Coefficient

The temperature coefficient of most TEs (percentage change in output for a temperature change of 1 °C with constant heater current) is relatively

high--often 0.1%/°C. Elements should therefore be mounted in a thermally

lagged enclosure to minimize the effect of ambient temperature changes. The

output of a TE can be very stable at a fixed temperature if the heater is

carrying a constant current for several hours. However, if the instrument is

switched back and forth between ac and dc after a brief warm up, as it is in

most measurements, the output will usually drift. The drift may be negligible

during the short time required for a measurement, especially at low voltages.

However, on many ranges where there is some self-heating effect, the drift

rate is apt to be quite constant but significant. Accurate and consistent

measurements can be made under these circumstances, if readings are taken at

time intervals that are about equal and in a sequence which tends to average

out the effect of drift (e.g., [a] +DC, AC, -DC or [b] AC, +DC, -DC, AC).

2.4 AC-DC Difference

Thermoelements, as well as TVCs and TCCs, sometimes have a significant ac-dc

difference, 6, particularly at higher frequencies,

6 = (V^ - V,)/V, .

where V and V . are the ac voltage and the average of the two directions of dc

voltage required for equal response, or output emf. For voltage measurements,

Vg = V^ (1 + 6). (4)

Similarily, for current converters, I = I . (1 + 6).

AC-dc differences, methods for measuring them, and the application of

corrections are discussed in sections 5.0 to 6.5.

2.5 Radio Frequency Interference

Thermoelements have a frequency response which extends to 100 MHz or more, and

measurements made with them can therefore be affected by local electromagnetic

fields. Interference from television stations can be particularly troublesome

with low-current TEs, because the length of leads used in calibration laboratories often makes them an effective antenna. Energy picked up by a measure- ment circuit can usually be detected by shorting the TVC input terminals with

all connections in place as in normal use, but with the power supplies off.

Any change in the indication of a detector in the TVC output circuit as the

short is opened and closed will indicate a pick up problem. Interference can usually be avoided by using coaxial leads and shielded circuit components. In

extreme cases it may be necessary to work in a shielded room.
3.0 Voltage Measurements

AC voltage is measured most accurately at this time by comparison with a nominally equal dc reference voltage using a thermal voltage converter. The

TVC may consist simply of a series resistor and a thermoelement mounted

coaxial ly in a tubular casing, usually a plated brass tube, with input and

output connectors [8]. As noted earlier, this configuration is well suited to

rf measurements. In another arrangement, the resistor is mounted in one tube

and the TE in another. The TE can then be attached to any one of a set of

resistors to make a TVC with the desired voltage range [9]. The number of

resistors in a set can be reduced to about half, by using two TEs with different current ratings (e.g., 2.5 and 5 mA). Each resistor is then used for

two ranges [10]. These sets, which are commercially available, have two

distinct advantages. The ac-dc difference corrections are very small, and

they can be determined relative to any one range, which has known corrections,

by a step-up or step-down intercomparison of adjacent ranges.

Multirange TVCs usually contain 10 to 14 ranges between 0.5 and 1000 volts,

and ranges are selected with a rotary switch which connects the TE in series

with one of the ranging resistors. Some commercial models also incorporate an ac-dc switch, as well a null detector, and a balancing emf source for

monitoring the TE output. (The "balancing emf" referred to here and elsewhere

usually consists of the voltage across a fixed resistor with current supplied

by a battery and adjusted with a resistive divider or series resistors. The

circuit may be quite similar to the Lindeck potentiometer shown in fig. 3, but

without the mill iammeter. It balances, but does not measure, the emf.) If

these components are not included in the TVC, the relatively simple Lindeck

potentiometer (fig. 3) can be used with a detector, which may be either a microvoltmeter or a galvanometer, to monitor the TE output.

The detector can be simply used as a null detector when an ac voltage is

adjusted to be equal to a reference dc voltage (3.1), or a calibrated detector

can be used to measure the changes in TVC output emf as ac and dc voltages are applied alternately. The difference between the ac voltage and the dc

reference is then computed from the emf differences (3.2). Provisions are

made, and instructions furnished, by some manufacturers for using either

method.

3.1 A Null Method

In the "null" procedure, the ac voltage to be measured is applied to the TVC

and the balancing emf is adjusted for a detector null. The two polarities of

dc voltage are then applied in turn, and each one is adjusted to produce a null indication on the detector. The two voltages are measured (or read from

a calibrated source), corrected, and averaged. The average is corrected for

the ac-dc difference, 6, of the TVC to give the RMS amplitude of the ac voltage. For example, from eq. (4), with 6 = -0.012%, V.+ = 200.012 V, and

V^- = 199.960 V.

V = [(200.012 + 199.960)72] + (V. x -0.012/100)

^ = 199.986 - 0.024

= 199.962 V.
AC voltage should be reapplied after the two dc voltage measurements, to

evaluate any significant drift from the original null position. To find the

magnitude of the drift, the detector can be calibrated by making a small change

in the dc voltage, observing the change in detector indication, and computing

the scale sensitivity in percent per division. If the drift is large, a longer warm up period should be allowed, or the "deflection method" (3.2) may be used.

In the example above, a small part of the 0.026 percent difference between the

two dc voltages may be due to drift in the TVC output, but most of it is due

to the dc reversal difference. These measurements are sometimes carried out

with only one direction of dc voltage, and a correction equal to half the dc

reversal difference is then applied. This must be done very cautiously. If

the correction is applied with the wrong sign, the error is twice as large as with no correction. Of course, if the dc reversal difference is small relative

to the desired accuracy, satisfactory measurements may be made using only one direction of dc current flow.

Figure 3. AC-dc switching, TVC and potentiometer for ac-dc comparisons.

NOTES:

The "DC Supply" may be a calibrator and serve as both source and standard,

or the voltage may be measured with a "DC Std."--a digital voltmeter or a volt

box and a potentiometer.

R4 and R5 (see section 3.3) and Re and R7 (section 3.6) are optional. They

are used to adjust and measure the detector sensitivity, respectively.
The null procedure has certain disadvantages which should be kept in mind:

(A) If the dc standard is one of the widely used five or six dial calibrators,

adjusting it for a detector null may be tedious and time consuming; (B) the

wear on the lower two or three control switches will be accelerated; (C) the

calibration of the dc standard may be in some question if a number of readings

are obtained above and below the round-number value where a calibration test

was made; (D) if the TVC output drift is significant during the time required

for the null adjustment, it may be difficult to obtain consistent results.

Where the drift rate is constant, and the measurements are made in equal time

spans, the results may be consistent but in error; and (E) if the dc voltage

is supplied by a power supply and measured with a volt box and potentiometer,

the time required for the two adjustments (the voltage adjustment for a TVC

null and then the potentiometer balance) may be even longer. The two adjustments can be made in less time if two operators work together.

3.2 A Deflection Method

A calibrated detector may be used with a TVC to measure the difference between

a dc reference voltage and an ac voltage. The difference can be applied as a correction, C, to the dc voltage to obtain the value of the ac voltage.

V^ = V^ (1 + C) (4A)

(see section 3.5, Calculations)

This procedure will largely avoid the problems listed above. It should also

save time, as the adjustments are fewer and less tedious. The data and

calculations may be a bit more complicated; however, once routine procedures

are established, this method can be used with confidence. If the correctness

of the results is in question, they can be verified by relatively simple extra

tests, as described in section 3.5.

In the deflection method, the dc voltage is adjusted to a round-number value

which is nominally equal to the ac voltage to be measured. The TVC is

connected to the dc voltage (see fig. 3), and the balancing emf (or

potentiometer) is adjusted for a near null on the detector. The TVC is then

switched to the ac and dc voltages in the sequence indicated in fig. 3 (DC+,

AC, DC-), and the small emf variations are read from the detector directly or computed from the detector readings,* as explained below. The difference

between the ac and dc voltages is computed from these readings.

An equation very much like (3) in section 2.1 is used. Instead of a simple

change in TVC input voltage (AV/V) , the difference between the ac voltage (V ) and the corrected dc voltage (V .) is measured by observing the change in TVC

output. Therefore, AE in eqs. (2) and (3) becomes E - E ., the emfs

corresponding to the voltages V^ and V .. The eq. (4A) above may therefore be

written

* The detector should be connected so the indication increases, or deflects up

scale, when the input voltage to the TVC is increased.

8
where, as before, n is the exponent in eq. 1, E is the output of the thermoelement, and 6 is the ac-dc difference correction to the TVC.

The emfs E and E . are the TVC outputs (E , fig. 3) with ac voltage and the

two directions of dc voltage applied to the TVC. However, all but a small

fraction of these emfs are balanced by the emf source or the potentiometer.

The difference between the two emfs can be computed from imbalance emfs, e, read from a microvoltmeter.

The procedure is probably most straightforward when a digital microvoltmeter

is used. Emfs are indicated directly in volts (pV or nV). AC and dc

voltages should be applied, and emf readings taken, in time intervals which

are about equal. Apply DC+, AC, and DC-, read ei, e2 , and e^, and compute

^a

- ^d = ^2 (^1 ^ ^3^/2.

The complete calculation is discussed in section 3.5. The digital microvoltmeter indicates both positive and negative readings with the sign displayed, but mistakes are less likely to occur if the balancing emf is adjusted

so that all readings have a positive sign.

Emfs may also be read directly from a deflecting microvoltmeter. However, it

is recommended that readings be made in divisions, a deflection change, AD,

computed, and the emf difference E - E . be computed from the deflection

change. Techniques for reading detector scales are described in section 3.7,

and multiplying factors, K, for converting AD to emf differences are also

discussed. Calculations from data taken by this procedure are shown in table

1, column 5, where

Emf differences may also be measured by a calibrated galvanometer.

3.3 Electronic Galvanometers

An electronic galvanometer with appropriate sensitivity may be used to measure

an emf difference, if a method is provided to determine its voltage sensitivity, K, as described in section 3.6. Electronic galvanometers are available

with continuously adjustable gain control over a wide range, and this feature

provides a significant advantage. The sensitivity can be adjusted for maximum

resolution for each measurement. On the other hand, a deflecting microvoltmeter's sensitivity drops typically by about 60 percent if the range is

increased (e.g., from 10 to 30) to keep all readings on scale.
Table 1. Examples of Calculations

In these examples, data are listed in columns 3 and 5 and identified in

columns 1, 2, and 4. Test parameters are listed in the first eight lines. An

emf difference, ave AE, is computed from emf readings e, to e^ in column 3 and

from deflection readings D, to Dr in column 5. See text for further explanation.

1 2 3 4 5

Freq kHz 50 Freq 50

Range test inst. 200 Range 200

^d

DC voltage(V) 200.006

^d

200.006

TVC range 200 TVC 200

E TVC emf (mV) 8.8 E 8,8

n 1.83 n 1.83

nE 16.1 nE 16.1

pVm range 10 uVm

K

10

2 x lO'^

^1 dc 6.5 "l 23

^2 ac 4.4 °2 36

^3 dc (rev) 7.1 "3 73

^4 ac 4.1 "4 35

^5 dc 5.8 "5 20

AE-|_ * mV -2.4 AD, -12

AE^ *(jV -2.3 AD

2

-11

ave AE mV -0.0024 ave AE -0.0024 (AD X K)

100 AE

percent -0.015 100 AE -0.015

nE nE

6 TVC corr % +0.003 6 +0.003

C Final corr % -0.012 C -0.012

Notes: *AE^ ~ ^2 " ^^1 ^ ^3-^^^ ^"^ "^^2 ~ ®4 " ^^3 ^ ®5-^^^

e, and e^ differ because of drift in the TVC output, e, and e^ differ partly

because of drift but mainly due to dc reversal difference in the TVC. In the

statements above, substitute D for e where the readings are in divisions.

10
If a "one-range" instrument intended mainly as a null detector is used, an arrangement to adjust its sensitivity is often desirable. Such detectors can

be shunted by resistors, such as R4 and R5 in fig. 3. When the potentiometer

is balanced, the detector responds to currents through these resistors. The

switch, which must have contacts with very low noise and thermal emfs, may

select one or several shunting resistors. The resistor values should be

chosen experimentally, depending on the basic sensitivity of the instrument

and the sensitivity desired. Difference ratios of 2/1 or 3/1 between

sensitivities are recommended. These instruments may be calibrated as suggested in section 3.6, and the scale reading techniques of section 3.7 are

useful also.

3.4 D'Arsonval Galvanometers

D'Arsonval (permanent magnet, moving coil) galvanometers are often considered

to be quaint, if not obsolete, by electronically oriented metrologists, and

buying one for these measurements is not recommended. However, if an appropriate galvanometer is readily available, as is often the case, it should not

be rejected out of hand. It is probably superior to most electronic instruments in two important respects--noise rejection and zero stability. Its

sensitivity will probably be quite low unless a photoelectric galvanometer

amplifier is used to increase its input voltage. However, some galvanometers

may be sufficient for certain measurements where the voltage differences are

large, or where there is less need for accuracy.

It is advisable to test a galvanometer for linearity if a deflection method is

to be used by applying small equal changes to the potentiometer input and

seeing that the resulting changes in indication are about equal on all

portions of the scale. Some instruments may be nonlinear at the extreme ends,

but satisfactory over most of the middle portion.

3.5 Calculations

Table 1 is intended to illustrate data logging and calculations and to suggest

a data sheet format. Different data arrangements and line labels may be

preferred by some users, but it is advisable to develop a more or less standardized data sheet for each type of measurement. The line labels in column 1 are brief to conserve space (E - E . is abbreviated to AE as in eqs. (2) and

(3)), but they should be sufficient for a practical data sheet. Labels are further explained or defined in column 2.

In column 3, a correction, C, is determined for a test instrument indicating

200.000 V at 50 kHz. The measurements are made relative to a corrected dc

reference voltage of 200.006 V. Two emf differences, AEi and AE2 , are computed from five readings of a digital microvoltmeter. The average AE is

changed to millivolts to be in the same units as the TVC output, E, before

being divided by nE. (Measurements and plots of n are discussed in section

7.0.) AE/nE is multiplied by 100 to obtain a percentage value, and the ac-dc

difference correction to the TVC (6) is added to determine the correction, C,

to be applied to the dc voltage to obtain the ac value.

n
The same test is repeated in columns 4 and 5, but the data are read in

divisions from a deflecting microvoltmeter. The line labels in columns 1 and

4 are very similar. Deflections Dj to D5 are read rather than emfs, ADj and

AD2 are computed, and the average is multiplied by conversion factor K = 2 x 10-^ (from table 2) to find the average AE in millivolts. The rest of the

calculation is the same as in column 3. If data in ppm are preferred, the

average AD can be multiplied by K=200 from table 2 to obtain -2400 nV. Then

AE/nE + 6 = -2400/16.1 + 30 = -120 ppm .

The correction, C, expressed in percent, can be applied to the reference

voltage, V., in two ways, as illustrated below, to obtain the measured value

of ac voltage, V . The calculation on the right may be preferable where a

a machine calculator is not used.

^a " ^d ^'- " ^^^^^^

^a "^ ^d

" (V-^°° ^ ^^ = 200.006 (1 + (0.01 X -0.012)) = 200.006 + (2 x -0.012)

= 200.006 (.99988) = 200.006 - 0.024

= 199.982 V = 199.982 V

where C is expressed in percent

*** If the accuracy of the procedure or the correctness

of the sign is in question, it can be verified as follows: Increase the dc voltage by 0.020 percent after

a measurement, and repeat the procedure and calculation.

The correction, C, should be 0.020 percent more NEGATIVE

than in the first measurement.

3.6 Galvanometer Calibration

An obvious method for calibrating a galvanometer scale in these test circuits

is to introduce a small measured change in the TVC input, observe the resulting change in indication, and compute a multiplying factor, K, for converting

changes in indication to voltage differences (e.g., percent/div or ppm/mm).

This method has certain advantages. No n data are required, and the TVC

output, E, need not be measured. It is also quite feasible in voltage measure- ments if the small voltage change can be measured with the dc standards being

used. The change should be large enough for good resolution, but of course,

the galvanometer must not deflect off scale.

This calibration method is, however, more tedious and time consuming than some alternate methods. It may also require additional equipment in ac-dc difference measurements (see 6.2) where no dc standards are ordinarily used.

Several calibration methods have been developed to introduce a known change in

emf directly in the detector circuit. In one of these, a change in resistance

(AR) in the balancing emf circuit produces a change in current and hence a change in emf. However, this requires a rather sophisticated circuit with

practically constant resistance (except for the AR) at all emf levels.

12
A relatively uncomplicated calibration circuit, which requires its own battery,

is shown in fig. 3. It consists of Re and R7 , an on-off switch and a 2400-

mAh mercury battery. A computable emf, e, is inserted across R7 (a one percent, 1-ohm resistor is recommended) by closing the switch. A round-number

emf of 10 pV (R6=135 kfi) will be satisfactory in many instances; however, the

calibrating emf should be commensurate with the detector sensitivity. A

rotary switch with several contacts can be used to connect any one of several

resistors in the Re position. It may be desirable to have more than one calibrating emf if the sensitivity of the detector is adjustable over a wide

range.

A multiplying factor, K, (pV/div) is computed from the deflection observed

when the voltage is inserted. The deflection change, AD, (sections 3.2, 6.2

and 8.0) resulting from differences between ac and dc voltages or currents, is

multiplied by K to obtain the emf difference, E " E .. This factor K is

equivalent to K in table 1 (column 5). But in that example the detector is a microvoltmeter, and K is obtained from table 2.

Table 2. Conversion Factors K

Microvolts Divisions

(full (full

K (x by ) for 100-div scale

mv div to div to div to

range scale) scale) mill i volts microvolts nanovolts

30 60 60 6 X 10"^

2 X 10 ^

6 X 10 ^

2 X 10"^

0.6 600

10 100 20 0.2 200

3 60 6 0.06 60

1 - 100 2 0.02 20

The current drawn by the battery is, of course, minimal, and the mercury cell

maintains a nearly constant voltage for most of its useful life. A new battery should last a year unless it is inadvertently left on for an extended

period. It is advisable to check its voltage occasionally by connecting a voltmeter between the positive input connection and the test point "P" (fig.

3). The potentiometer switch may be either on or off.

3.7 Reading Deflecting Detectors

The scales on most suitable microvoltmeters and galvanometers have the zero indication at the center, so that positive and negative readings can be made

to the right and left. Microvoltmeters with intermediate ranges —3, 30, 300,

etc. --usually have scales with 30 divisions to either side of zero as well as 50-di vision scales for the decade ranges. This arrangement is convenient for

most measurements and for null detectors. However, it is very desirable to

have the zero at the far left for the procedures described here. Typical

microvoltmeter scales are shown in fig. 4, but with a number plate in front of

the 50-division scale. The substitute numbers are engraved on a plastic plate

and filled with black wax. A similar plate can be made by cementing cut-out

numbers on a transparent plate. The instrument is read with zero at the left

and a full scale range of 100 divisions. It can be read in microvolts;

however, some conversion to other units will probably be advantageous in the

13
calculations. It is probably more straightforward, and mistakes are less

likely, if readings are recorded in divisions. A deflection difference, AD, can be computed in divisions and then converted to an emf difference, AE,

using the appropriate factor, K, from table 2 below.

Figure 4. Typical microvoltmeter scales with three deflections marked.

Three readings are marked on the scale in fig. 4 at 23, 36, and 73 divisions

as an illustration. If the reading at 36 divisions corresponds to an ac voltage or current and the others to direct and reversed-dc, then

AD = D, (D,. 03)72 = 36 - (23 + 73)/2 = -12 div.

Of course, the same result is obtained when the regular scale is read, but the

sign must then be observed very carefully. The readings are -27, -14, and

+23, and

AD = -14 - (-27 + 23)/2 = -12 div.

Obviously, mistakes are less likely with the substitute numbers, where all

readings are positive.

The scales shown in fig. 4 are typical of several instruments in which the zero and full scale indications on either scale coincide. It is feasible—and

advantageous--to read the 100-division scale with any range on most of these

instruments. The same substitute numbers can be used with any range, and the

reading precision is better with 100 divisions. The feasibility of this

procedure must, of course, be verified for any particular instrument. In the

example in fig. 4, the indications marked on the 100-division scale are extended to the 60-division scale and read as 14.0, 21.5, and 43.5. Then AD = 21.5 - (14 + 43.5)72 = -7.25 div. The emf difference is -7.25 |jV on the 30-mV

range (60 div full scale). If the corresponding reading of -12 divisions on the 100-division scale is multiplied by 0.6 from table 2, essentially the same result is obtained.

14
The table gives factors, K, for converting deflection changes to mV, |jV, and

nV for the four most-used ranges. As indicated in the text, a conversion to

mV is preferred for results in percent while nV are more convenient for ppm.

3.8 Peak and Average Responding Instruments

Peak and average responding instruments are widely used for certain applications, and their principles can be adapted for ac-dc comparisons. They

are not electrothermic and are therefore outside the intended scope of this

writing. However, since at least one peak comparator [11], a peak calibration

method [12], and one average responding ac-dc comparator [13] have been

developed, they are noted here and referenced. The accuracy of these com- parators is sufficient for many applications; however, they are more limited

in voltage and frequency range than electrothermic instruments. These com- parators are not commercially available at this time.

4.0 Switches and Potentiometers

As noted earlier (3.0), some commercially available multirange TVCs have

provisions for ac-dc switching as well as an adjustable emf source and a detector for monitoring the thermoelement output. If these components are not

included, or if single-range TVCs, thermoelements or TCCs are to be monitored,

the equipment described in this section is not difficult to build, and the

parts are readily available. The circuits are shown in some detail in fig. 3,

and of course, variations can be made to meet particular needs. The diagram

of the potentiometer includes two optional features, as noted below the

caption, which were discussed in previous sections.

4.1 AC-DC Switches

The ac-dc switch shown in fig. 3 connects the TVC to either direction of dc

voltage, or to the ac voltage to be measured, in the order suggested in

section 3.2. Other switch arrangements, such as reversing dc with a separate

switch, should be satisfactory also. The ac input to the switch, and the

cable connection to the TVC, should be coaxial to minimize electromagnetic

interference and circuit impedance. The switch and the potentiometer should

be in separate shielded enclosures, and the cable to the detector, D, input

should also be shielded.

The dc voltage may be supplied by a calibrator which serves as both source

and standard, or a separate standard (a volt box and potentiometer or a digital voltmeter) may be connected at the terminals for "DC Std." The

standard should not be appreciably affected by changing the polarity of the

applied voltages. At very low voltages, where the voltage drop across the

line resistance may be significant, the dc voltage should be monitored at

the "DC Std." terminals. The cables with conductors c and shields s (fig. 3)

should have equal resistance (identical cables are preferable) to equalize the

voltage difference between the ac and dc voltage terminals and the com- parison points at the switch contacts.

The ac-dc circuit should have only one ground point, usually, but not

necessarily, at the ac source. The switching circuit shown in fig. 3 provides

15
a direct connection from the TE to ground. It is preferable to have no switch

in this part of the circuit. If a switch is used, and if it opens an instant

before the switch contact in the input side, the insulation between the

thermoelement heater and the thermocouple will probably be punctured by an excessive voltage.

4.2 Lindeck Potentiometers

A two-range (10- and 20-mV) potentiometer is shown in fig. 3. It is powered

by a 1.35-V mercury battery and controlled by two helical resistors, which may

be either the 3- or 10-turn devices currently available. If the "deflection

method" proposed in section 3.2 is used, the fine (10-ohm) control need not

have resolution sufficient for an exact null. The controls are adjusted for a detector indication in the portion of the scale where the first reading (e^,

section 3.2) should be made.

If the 1-mA meter (M) is omitted, the instrument is a balancing emf source which cannot measure the TE output. However, as noted earlier (3.2), the emf

can be measured by the detector if the detector is a microvoltmeter. Select a range to measure 10 mV or more, turn the balancing emf off, and close the key

(ki).

As noted in the caption (fig. 3), the resistors R4 and R5 (section 3.3) and Rg

and R7 (section 3.6), and their associated switches, are used to adjust and

measure the sensitivity of the detector, D. These components may be omitted

if the detector is a microvoltmeter, or if a null method (3.1) is used.

The potentiometer and detector should be grounded, preferably at the negative

input to the detector. This is sometimes done by linking the detector input

to ground through the detector's power cord. A few instruments may have the

negative output connected to the instrument casing which is grounded. However, it is preferable to connect the circuits to the grounded potentiometer

shield by closing the switch at the negative input terminal (fig. 3), or by some other link at this point.

4.3 Detector Keys

The detector keys ki and k2 , and the three associated resistors (fig. 3), are

typical of those used in these procedures. Key kj should have low-thermal

contacts and low contact resistance. A two-position lever switch is recom- mended for kg. The contacts should be good, but a quality less than that of

ki should be satisfactory. The resistor R^ is always in parallel with the

detector, unless k^ is closed for a detector reading. This keeps the detector

"quieter" when the key is open. R^ also serves to reduce the detector sensitivity when kg is closed for preliminary balance adjustments. It should not

be necessary to switch a multi range detector to a higher range for preliminary

balances with this key configuration. An Ri of 100 ohms is satisfactory for

most high-impedance detectors. Rg and R3 should be chosen experimentally,

depending on the range most often used and individual preference. However,

20 kQ and 100 kQ, respectively, may be useful as values to start with.

16
5.0 AC-DC Difference

The ac-dc difference in a thermal voltage converter is nearly all due to

reactance in the ranging resistors, the range selector switch, and other

switches and connections in the ac-dc circuit. Series inductance will impede

the ac current, so that more ac than dc voltage is required for equal TE

current—and thermocouple output. The sign of the correction is, by eq. (4),

positive in this case. A positive correction can also result from ac current

being lost to ground by capacitance. However, capacitance between resistors

connected in series, or between turns of wire in a wire-wound resistor, will

increase the TE current for a given ac voltage and result in a negative

correction. Compensation for this effect is sometimes provided by adding

capacitors to ground which bypass the TE and part of the series resistors.

5.1 AC-DC Difference Corrections

AC-dc difference corrections can be plotted conveniently on a semilog scale as

in fig. 5, and such a plot can be used to find good estimates of corrections

at frequencies where determinations were not made. Fig. 5 illustrates a typical set of correction curves where the specified error limits are +a to

-a. Low- voltage ranges in a multi range TVC usually have small corrections

(curves A, B, and C). Intermediate and high-voltage ranges are often limited

to lower frequencies (curves D and E), because the effects of reactive

components are larger and more difficult to control. The useful frequency

range at high voltage is often extended by adding compensating elements as mentioned above. Optimum compensation (curve F) will sometimes result in a change in direction (and sign) of the correction, and the change in magnitude

is usually rapid after the sign change.

+a -

-a -

1 1 1 ..100%

80 %\.^^^

60% -^^^i^:::::^.^

1 > E/_ 1 1 D

1

f\

~^~--^

1 1 1

1 1 1 1 1 2 3

Hz

10 20 50 100

kHz

200

- a

500 1000

Figure 5. AC-dc difference correction curves.

17
It is usually sufficient to make only one determination of ac-dc difference at

low and intermediate- voltage ranges. This determination should be at the

highest frequency of interest. If the instrument is used only at audio

frequencies, a test at 20 kHz is often sufficient, even on high-voltage

ranges. The correction curve can be expected to approach zero rapidly, and if

the instrument is designed for use at 50 to 100 kHz, the correction should be

quite negligible at 10 kHz and lower. Where corrections are relatively large, or if there is evidence of a curve such as F (fig. 5), additional determinations should be made.

5.2 AC-DC Difference in Thermoelements

The low-range thermoelements used in TVCs (2.5 to 10 mA) rarely add significantly to the ac-dc difference of the instrument. The TE response to ac current is affected by two or three thermoelectric phenomena which, incidentally, are quite independent of frequency. However, the result of these

effects is less than 5 ppm for most low-current TEs. It should not be

necessary to redetermine the ac-dc difference of a TVC after a TE is

replaced, and new TEs furnished by the instrument manufacturer should not

require ac-dc difference tests before being installed. However, if they are tested, they should be treated as current converters (i.e., tested in series

with a standard TE), as they respond to the current through the series

resistors. The TE acts more as a voltage converter on very low ranges where

its impedance is a large part of the total.

AC-dc difference in higher- range vacuum TEs is larger due to skin effect in

the current conductors leading to the heater. The copper-coated nickel alloy

widely used to pass current conductors through glass seals is sufficiently

magnetic to cause a small increase in impedance. Therefore, more heating is

produced with ac current than with equal dc current. AC-dc difference resulting from skin effect usually varies from about 0.005 percent at 200 mA to

between 0.01 and 0.02 percent at 1 ampere—the highest range now available in vacuum bulbs. The effect in the low-current TEs used in TVCs is nearly always

negligible. However, it may account for very small positive corrections in some 0.5- and 1-volt ranges, and in the low-voltage TVCs (0.3 to 1 volt) used

with ac shunts for current measurements. The shunt and TVC combination is

ordinarily tested and used as a unit. However, the ac-dc difference of the

TVC can be determined and appropriate corrections applied (see section 8.0 and

reference [16]).

TEs for currents of 10 amperes and higher sometimes have a tubular heater, or

a heater with a C-shaped cross section made from a flat material. The heater

is mounted between heavy copper support blocks with terminals at either end.

The ac-dc difference of these elements can be very much dependent on the

location of other conductors, particularly the return conductor, which is

usually placed close to, and more or less parallel to, the TE heater. AC

current will distribute itself for minimum reactance, and may therefore be

concentrated close to, or away from, the thermocouple which is attached to the

side or top of the heater. This nonuniform current distribution can cause

ac-dc differences as large as 0.03 percent. The ac-dc difference of

these TEs can be made constant by mounting both terminals on one end of the

18
device and connecting a rigid copper conductor between one terminal and the

heater support at the other end. This conductor can also be placed, relative

to the heater, for minimum ac-dc difference.

Practically all low-frequency (below 60 Hz) ac-dc differences in thermoelectric transfer instruments are due to the thermoelement [9]. At most

frequencies the heater temperature, and therefore the TE output, are essen- tially constant. However, at low frequencies the heater is cooled slightly

between peaks of ac current by conduction through the heater supports and

the thermocouple wires. This effect is greater in high-current TEs and may be

detected up to about 60 Hz on some ampere- range TEs. However, in 2.5- to

10-mA TEs used in TVCs the effect usually occurs below 5 or 10 Hz. It also

decreases sharply as the heater current is reduced on all current ranges.

Correction curves for three current levels in one TE are illustrated in the

left-hand portion of fig. 5.

6.0 AC-DC Difference Measurements .

AC-dc difference corrections to a transfer instrument (TVC or TCC) under

test, 6. , are determined relative to a similar standard whose corrections, 6 , are known. Corrections to a standard or reference TVC or TCC are determined

at the National Bureau of Standards and other laboratories. Measurements of

6.-6 may be made by either of the two methods outlined below. Fig. 6 shows a typical test arrangement for comparing two TVCs. AC and the two directions of

dc voltage are connected to the TVCs successively by the ac-dc switch. Note

that both TEs are connected directly to ground as in fig. 3. The switch shown

in fig. 3 can also be used in ac-dc difference measurements. However, the

switching sequence indicated in fig. 6 may be preferable, and the switch is a bit simpler. The TVC outputs are monitored with detectors (D) after being

balanced by the emf sources (B), each of which may be a potentiometer, as in

fig. 3.

C^C^C^)

Figure 6. Circuit for ac-dc difference measurements.

19
The power supplies need not be calibrated; however, if the "null method" is

used, provisions must be made to measure the dc voltage. The dc supply must

have fine controls, with resolution of a few ppm, for adjusting the input to

the test TVC for equal output on both directions of dc voltage. AC voltage

may be supplied by an oscillator, amplifier, and appropriate transformers.

Fine control for the ac voltage is necessary if the deflection method is used.

Control circuits are discussed in section 6.4.

6.1 A Null Method

AC-dc difference can be measured using a procedure in which only null

readings are made on the detectors used with the TVCs. This method is similar

to the "null method" described in section 3.1 for voltage measurements. In

this procedure, ac voltage is applied first, and both balancing emfs are adjusted to null the detectors. DC voltage is then applied, adjusted for a null on the test TVC detector, and measured (or read from a dc calibrator).

The dc voltage is then readjusted for a null on the standard TVC detector and

measured. In the example below, these voltages are listed on the line labeled

"DC+." DC voltage of the opposite polarity is then applied, adjusted for the

two nulls, and measured as before. ("DC-" in the example.) The ac voltage

should then be reapplied, and any drift from the original null position on either detector should be evaluated for significance. If either TVC output is

drifting appreciably, more warm-up time should be allowed, or the deflection

method (6.2) may be used.

The relative ac-dc difference between the two TVCs in volts is equal to the

difference between the average of the dc voltages required for nulls on the

standard TVC and those required for nulls on the test TVC. The difference is

usually converted to percent (or ppm) as in the following example.

Voltage Voltages required for detector nulls

Std. TVC (Vg)

199.952

200.010

199.981

-0.035 V

-0.018%

+0.003%

-0.015%

or -150 ppm

This procedure requires no galvanometer calibration or measurements of n. However, the adjustments are tedious and time consuming, dc standards are necessary, and the disadvantages discussed earlier (3.1) are present here

also.

6.2 A Deflection Method

The deflection method for making ac-dc difference measurements is similar to

the one described in section 3.2 for making ac-dc voltage comparisons. DC

Test TVC (\)

DC+

DCAve DC

200.000

200.032

200.016

AV = V -V.

Relative ac--dc difference

6 (Std. ) known

6^(Test)

20
voltage is applied to both TVCs, and the balancing emfs are adjusted to null

both detectors. AC and the two directions of dc voltage are adjusted in turn

to obtain a null indication on the detector used with the test TVC. The

corresponding indications on the standard TVC's detector are recorded, and an emf difference, E - E ., is computed as in section 3.2. The ac-dc difference

of the test TVC, 6 . , (defined in section 2.4) is determined by evaluating E

- L a E./nE and applying a correction for the ac-dc difference of the standard TVC,

^a - ^d ^a - ^d

+ 6=6.

V . nE s t

d

In this application, V and V. are the ac and dc voltages required for equal

response from the test TVC. The emf difference, E - E ., can be computed from

readings taken from a digital microvoltmeter or a deflecting detector, as discussed in section 3.2, and the scale reading techniques described in

section 3.7 are useful here also. The reading sequence suggested for voltage

measurements (DC+,AC,DC-) may be used for ac-dc difference measurements.

However, it is probably advantageous to take four readings in the sequence

suggested in fig. 6 (AC,DC+,DC-,AC). Either sequence will minimize the

effects of drift in the TVC outputs, if the readings are taken at time intervals that are about equal. The second reading with ac voltage should increase

the reproducibility.

A measurement made with a deflecting detector can be illustrated using the

indications from fig. 4. If it is assumed that there is no appreciable drift,

so that both readings on ac voltage are equal (i.e., 36 divisions), then

AD = (D2 - D-j^ - D3 + D2)/2 = (36-23-73+36)72 = -12 div.

If nE = 16.1 mV, 6^ = +0.003% and K = 2 x lO"^ mv/div (10 pV rangetable 2), then s

6^ = -0.015 + 0.003 = -0.012%.

6.3 TE Comparators

Stable power supplies are necessary for accurate measurements, and instability

is probably the major cause of nonreproducibility in ac-dc difference com- parisons. This difficulty can be largely overcome by using a TE (thermoelement) comparator. The instrument is not commercially available at this

time; however, it is not difficult to build. A brief description of the

general principle and advantages is given below. A more complete discussion

is left to the referenced papers. TE comparators have been built in at least

three configurations [9,10,14], but they all employ a divider circuit to which

the emfs from the test and standard TVCs are connected. The divider is

adjusted, nulling a detector, and at this point the divider setting corresponds to the ratio of the emfs.

21
The comparator sketch

(Pot' r) is used, with

output. The divider

a ten-turn helical re or standard TVC, is b

the divider. A balan

(pVm) with key kg clo

voltages are applied

to give a test TVC ou

is closed and an emf

difference and dc rev

used, the ac-dc diffe

ed in fig. 7 shows a typical circu

key ki, to measure both emfs and

(DIV) is a two-dial Kelvin-Varley,

sistor. The lower emf, which may

alanced against a portion of the h ce is indicated by a null reading

sed. After this preliminary balan

in the sequence indicated, and eac tput equal to the Pot'r emf. Then

is read from the jjVm. These emfs

ersal difference. If both di recti rence can be computed from them.

it. The potentiometer

to monitor the test TVC

the second dial being

be either from the test

igher emf by adjusting

on the microvoltmeter

ce, the ac and dc

h voltage is adjusted

with key k^ open, kg

differ because of ac-dc

ons of dc voltage are

AC

o

DC+ DC- AC

o o I

Figure 7. TE comparator with two TVCs and ac-dc switch.

Small fluctuatio

proportional cha

appreciably. Th

time constants a not quite matche

second advantage

for equal output

Actually, if the

reversal differe

monitor the test

adjusted to be n adjustments to n ac-dc difference

ns in the power supply output will produce nearly equal

nges in the emfs, and the divider balance will not be affected

e stabilizing effect is greater if the TEs have well -matched

nd response characteristics (n values), but even if they are

d, the effect usually affords a significant advantage. A

is that the ac and dc voltages do not have to be adjusted

from the test TVC so exactly as in other test methods.

test and standard TEs were quite similar, and the dc nces were small, it should not be necessary to adjust or TVC output at all, once the ac and dc voltages are early equal. However, TEs usually differ enough to make

ear equal output necessary for accurate measurements of

An automated model of this comparator is described in reference [15].

6.4 Voltage Control Circuits

Measurements of ac-dc difference require very fine voltage adjustments to

produce equal output from the test TVC. Fine controls are required for dc

22
voltage in the null method, and for both ac and dc in the deflection

method. AC and dc voltages are often supplied by five- and six-dial

calibrators. The lower two or three dials can be adjusted for a detector

null; however, this method is not very satisfactory. The knobs are often

small, close together and not easily turned. An exact null may not be obtainable with the resolution available, and it is sometimes difficult to place the

supply within easy reach of the operator.

The series resistance control shown in fig. 8 is recommended as a substitute

fine control for voltages from 1 to 1000 V. It is placed between the dc

supply and the ac-dc switch for dc voltage control, or between this

switch and the TVCs to control both ac and dc voltage. The voltages are

set to be nearly equal with the regular controls, and the final fine adjustments are made with this control. It can be built into a shielded box and

connected into the line by a two-conductor shielded cable. It can then be

placed where most convenient for the operator. It is important to remember

that the control carries the test voltage which is often high enough to be

dangerous. The casing must therefore be well insulated, and the shield firmly

grounded. This control can, of course, be used with other power supplies

which do not have sufficient fine controls. The voltage across the control,

and hence the voltage at the TVC terminal, is adjusted with the ten-turn,

l-\(n, helical resistor R^. The sensitivity is adjusted by shunting R^ with

any one of 10 resistance values R2. The value of Rg is selected by a rotary

switch, and is increasedas the voltage increases. A resistor R3 of about 250

ohms prevents R2 from being shorted and improves the linearity. The most

appropriate setting for R2 depends not only on the voltage level and user preference, but on the current drawn by the TVCs--usually 5 to 20 mA. The

recommended value of resistance for each switch setting is listed in table 3

below. Each successive step increases R2 by 50 to 70 percent.

Supply

—wvvwv>—

#

i5n

to TVCs

Figure 8. Series resistance fine control for ac and dc voltages.

23
Switch

position

1

2

3

4

5

Table 3.

Resistance of R, (ohms) '

15

25

40

60

100

(see fig. 8)

Switch Resistance of R, position

6

7

8

9

10

(ohms)

150

250

400

650

1000

AC voltage and current from an oscillator- amplifier supply can be controlled

with the circuit shown in fig. 9, which regulates the input signal to the

amplifier. Resistor R^ is a ten-turn helical resistor for coarse control, and

adjusts the amplifier input from zero to near the oscillator output voltage.

The control is less coarse if this resistor is operated near full range. The

oscillator gain should therefore be set for an amplifier output only slightly

higher than that necessary for the desired output voltage. An Rj of 1 kfi is

suitable for most oscillators, but 10 kfi may be necessary for those with

relatively high output impedance.

^K—vyw^

"3 <

Oscillator <

i

<

<

to Amplifier

Figure 9. Control for an amplifier input.

Fine control is provided by a ten-turn helical resistor R2 of 100 ohms and a shunt R3 which regulates the sensitivity. The value of R3 depends on several

factors in addition to personal preference, but 20 to 50 ohms is usually

satisfactory. The control assembly should be well shielded. If it is connected

to the oscillator and amplifier through a shielded three-conductor cable, it can be moved around and placed conveniently for the operator.

24
6.5 Verifying AC-DC Difference Corrections

AC-dc difference corrections are primarily due to reactance in the switches

and resistors in the TE circuit (see section 5.0). They are not likely to

change, if these components are rigidly mounted and spaced so that capacitance

between components, and between components and ground, is not excessive.

Changes are probably more likely, if fixed and adjustable capacitors are added

for frequency compensation. This is often done in multirange TVCs for the

higher voltage ranges. If repairs or alterations are made that might affect

these circuits, the ac-dc difference corrections should be redetermined.

If there is no reason to suspect that a change has occurred, then

recalibration at NBS or other laboratories can be several years apart.

(Five-year intervals are reasonable.) However, it is advisable to make

periodic comparisons between TVCs within a laboratory to detect any changes

that might have occurred. Such tests are especially valuable at high voltages

and frequencies where changes are most likely.

Tests are made most easily between adjacent ranges in a set of single-range

TVCs, or between members of two sets. If these tests are made accurately, it

should not be necessary to send the entire set to another laboratory for

recalibration. A few ranges, perhaps a low, medium, and high range, would

serve as a basis for determining corrections to the other ranges by intercomparison. Of course, other sets and multirange instruments can be compared

with such a set.

If two multirange TVCs are available, one can be calibrated relative to the

other. Such periodic comparisons will assure that no changes have occurred as long as the results are the same from one test to the next. It is very

unlikely that both instruments will change equally and simultaneously.

If only one multirange TVC is available, no intercomparisons can be made.

However, the procedure outlined below should be useful in detecting any

significant changes in the ac-dc difference corrections. It consists of

measuring the difference between a constant ac voltage and the average of the

two polarities of a constant dc voltage at rated voltage on a given range and

then immediately with the same voltages on the next higher range (e.g., 50

volts on the 50-volt and the 100-volt ranges). If the voltages are stable

during the test, the difference between the ac-dc difference corrections of

the two ranges (6^ - 62) can be computed. The voltage difference can be

determined by evaluating (E - E ,)/nE as in section 3.2, and from the equation

in section 6.2,

V, nE

+ 6.

Then, since (V - V.)/V. is the same.

6. - 6^ =

^a2 " ^d2 ^al " ^dl 12 "2 ^2 "1 ^1

where the subscripts 1 and 2 refer to the two ranges,

25
This procedure is more likely to be used on instruments with built-in detectors and balancing emf circuits, so that E and n values may not be known.

However, the detector can be calibrated directly by making a small change in

the dc voltage, observing the resulting deflection change, and computing a multiplying factor, K, (ppm/div) as suggested in section 3.6, first paragraph.

In the example of actual data below, multiplying factors Kj and K2 were determined for the 50- and 100-volt ranges of a typical multi range TVC. The

sensitivity of the 100-volt range was, of course, low (38 ppm/div) as only

50 percent of rated voltage was applied. AC voltage at 100 kHz and the

two polarities of dc voltage were then applied in the sequence

DC+,AC,DC-,AC,DC+,AC,DC-, and seven deflections, Di to D7, were observed.

Then

Dg = D2 - (0^+03)72, Db " ^4 (^3''°5)/2, Dc " °6 ^^''^7^^^ *

and "ave AD" is the average of the three. K is in ppm/div and

6-,^ - 62 = (AD2 X K2) - (AD^ X K^) ,

where the subscripts 1 and 2 refer to the 100- and 50- volt ranges,

respectively.

Range ^ ^ ^ ^ ^ ^ ^ ave AD K AD x K

100 23.0 14.0 25.0 14.0 23.0 13.5 25.0 -10.2 38 -390

50 27.0 12.5 23.5 12.0 25.0 12.0 22.5 -12.2 8.6 -100

The ac-dc differences of the 100- and 50- volt ranges at 100 kHz were +210 and

-90 ppm, respectively, when measured by conventional methods. These corrections differ by 300 ppm, and the voltage comparisons indicate a difference (61 • 62) of 290 ppm. This agreement is closer than in most of these tests; however,

changes on the order of 50 ppm should be detected readily enough by this

method.

Accurate knowledge of the ac and dc voltages is not necessary, and the

need for equality is only that all readings be within the range of the readout detector. It is necessary, of course, to ensure that neither voltage

changes significantly during the few minutes required for a measurement.

Changes are more likely when the TVC range, and hence the load on the

supplies, is changed. However, the loads are very small--usually 2.5 and 5

mA--and most ac and dc calibrators are well enough regulated to remain

stable during these changes. An additional load resistor can be inserted in

parallel with the TVC for a few seconds to roughly double the current drawn.

No significant change in the TVC detector balance should be observed. A

repeat test after a few minutes with consistent results will indicate that one voltage is not drifting relative to the other.

If stable calibrators are not available, the voltages may be monitored with

digital voltmeters. However, most ac DVMs currently available are limited

to 4-1/2 digits, and this resolution is marginal at best in this application.

26
7.0 Measurements of n

The measurements described thus far are made to determine small voltage and

current differences by evaluating (E -E.)/nE. As noted earlier (2.1), the

a Q

factor n is near 2 at very low currents, but usually 1.6 to 1.9 at rated

currents. The expression above can be evaluated if n is predetermined in a special test using dc voltage or current. Values of n corresponding to any

value of E can be found, if a curve is plotted of n versus E. Four or five

determinations between 50 and 110 percent of rated heater current are sufficient.

From eqs. (2) and (3),

" = E (AI/I) ' ^"^ ^^'^ ^ TM' " = E (AV/V)

Measurements of n are made by introducing small measured proportional changes

in TE heater current (Al/I) and measuring the resulting output, E, and changes

in output, AE. Current changes should be large enough for good resolution,

but small enough to keep all detector readings on scale. Changes of a few

tenths to about one percent are usually convenient.

These tests are probably made more conveniently on the 100-V range of a multirange TVC, and n values obtained on one range can be used with any range,

if they are correctly related to the TE output. Low-voltage, single-range

TVCs can be placed in series with additional resistance, and more manageable

voltages can then be applied. Low-range TEs (2.5 to 10 mA) may be connected

in series with 50 to 200 kQ and appropriate voltage applied for the desired

heater currents. The voltages may be supplied, and small voltage changes

"dialed in," from a dc calibrator. Or the voltage may be supplied by a stable dc power supply with fine controls, and small changes measured with a digital voltmeter or a volt box and potentiometer.

In the example below, one determination of n is made by applying 90.000 and

90.500 volts for a proportional change of 0.00556, and E = 8.51 mV. Imbalance

emfs, e, are read from a digital microvoltmeter, and three emf changes are computed from seven readings.

AE^=e2- (e-^+e3)/2 , ^E^=e^-ie^+e^)/2 , AE^=e^-ie^+e-^)/2

Successive readings are made at the alternate voltages--90. and 90.5. The

average of the three emf changes (80.4 pV) is converted to 0.0804 mV for the

calculations.

_y _Ay J_ _!l

^

_!3 _^ _!5_ _f6_ ^7 ave AE AV/V n 90 0.5 8.51 12.3 92.8 11.4 91.2 10.8 90.8 10.4 0.0804 0.00556 1.70

Current changes can also be introduced in a TVC, or in a TE in series with a high resistance, by inserting a known resistance change in the TE circuit. A

relatively low resistance, with a switch for shorting it, is placed in series

with a TVC or TE, and as the switch is opened and closed, Al/I=-AR/R. The

power supply must, of course, be well regulated, and the resistance ratio AR/R

27
should be known to about one percent. The calculations are simplified, as AI/I is the same at all voltage levels. The change can, of course, be a round

number. A 400-Q resistor in series with a 40-kQ ,5-mA, 200-V TVC will

introduce a change of 0.01 (one percent) at all voltage levels.

Currents higher than 10 mA are usually measured with a potentiometer and a shunt in series with the TE.

The TE output emf, E, can be measured with a Lindeck potentiometer (fig. 3), or

it can be measured by the detector, if a microvoltmeter with a 10-|jV range is

used, by turning the potentiometer off and closing key k^. Emf changes, AE, are measured by the detector as described in section 3.2.

It is difficult to measure n with an accuracy better than about one percent.

However, that accuracy is sufficient, as errors in n have a second-order

effect on the end result. A one percent error will result in an equal error

in measuring a difference, say of 0.05 percent, between an ac voltage and a dc reference voltage. However, this will affect the ac voltage measurement by

only 0.0005 percent (5 ppm).

8.0 Current Measurements

Thermoelements are sometimes used as thermal current converters (TCCs), and

they are usually excellent up to about 100 mA. They have ac-dc differences at

higher currents, as discussed in section 5.2, which can be evaluated, and

corrections can be applied. However, TCCs usually consist of a set of special

ac shunts and a low-voltage (0.3 to 1 volt) TVC. In some commercial models,

the shunt output is connected to the TE in a multi range TVC through a special

connector. These TVCs include a balancing emf source and a detector for

monitoring the output of the TE. Instructions for using these instruments

are, of course, furnished by the manufacturers.

Shunts can also be connected to a low-voltage, single-range TVC, and its

output can be monitored with an emf source or a potentiometer like the one shown in fig. 3. The detector may be any one of the four instruments discussed in 3.2, 3.3, and 3.4. Techniques for galvanometer calibration (3.6)

and detector readings (3.7) are also useful with TCCs.

AC current may be measured, and ammeters calibrated, with the equipment and

circuit shown in fig. 10. The procedure is very much like the deflection

method described in section 3.2 for voltage measurements. DC current is

adjusted to be nominally equal to the ac current, and the balancing emf or potentiometer (P) is adjusted for a near null indication on the detector (D).

The indication, ej, of the detector is recorded. The TCC is then switched to

ac current, the ac supply is adjusted for the desired indication on the

instrument under test (Test AM-- fig. 10), and the detector indication, e2, is

recorded. Reversed-dc current is then applied to the TCC, adjusted to the

nominal value, and e^ is recorded. The difference between the ac and dc

currents is then computed from the equation,

la - 'd _ h- ^d + «, . Ij nE

28
where 6 is the ac-dc difference correction to the TCC, and

^a - ^d = ^2 (^1 ^ ^3^/2.

If the detector is a deflection instrument,

TCC

DC+ AC DCf

DC Supply

and

Standards

AC

Supply

"esr

AM.

Figure 10. Circuit for current measurements using a thermal current

converter (TCC).

The calculations illustrated in 3.5 are then practically the same as those for

current measurements. I is substituted for V, and TCC for TVC, in table 1.

The ac-dc difference of a TCC can be determined, relative to a standard

instrument, with the circuit shown in fig. 11. The equipment and switch may

be the same as those in fig. 10, but no dc standards are required. The

switching sequence may be as shown (DC+, AC, DC-), although the sequence

suggested in 6.2 (AC, DC+, DC-, AC) may yield greater reproducibility. The

switch shown in fig. 6 may be used if the current-carrying capacity is

sufficient.

Each TCC output is monitored with a balancing emf, or potentiometer (P), and

detector (D). A method similar to that suggested in 6.2 may be used. The ac and dc currents are adjusted for equal response (output emf) from the test

TCC, and imbalance emfs are read from the detector used with the standard TCC.

If four readings are taken, e^ and e4 are read with ac current, and e2 and e^

with the two directions of dc current. Then

29
or

and

^a

- ^d = ^^1 - ^2 - ^3 " ^4^/2,

E3 - E^ = ((D-L

- D2 - D3 + D^)/2) X K,

a d_a a^._j,

T ~ F— • o — o. Ij nE s t

where 5. and 6 are the ac-dc difference corrections to the test and standard

TCCs.

If the shunts have shields or casings (dashed lines in fig. 11), it is recom- mended that they be connected as shown in fig. 11. If one shield were attached between the shunts, a small capacitive current might flow between

that shield and ground. This current would flow through one shunt, but not

the other, causing an error.

Figure 11. Circuit for ac-dc difference tests of thermal

current converters (TCC).

Additional suggestions for building and using TCCs are found in reference [16]

where the following are discussed:

(A) Members of a set of shunts can be intercompared very much as TVCs are (see

sections 6.0 to 6.5), and corrections determined for each shunt and TE com- bination relative to two or three ranges which have known corrections.

(B) A "short set" of shunts can be built in which each shunt in the set is

used with two TVCs (0.3 and 0.6 V) to form two current ranges. Thus the two

30
TVCs and six shunts provide 14 ranges. It is, of course, necessary in this

arrangement that the TVC correction be negligible or that a correction, 6^, be

applied. AC-dc difference in low- voltage TVCs is mainly due to skin effect in

the input leads to the thermoelement heater (5.2). This effect can be

practically eliminated by using TEs with platinum lead-in wires. Such

elements are available from one or two manufacturers upon special request, and

at a reasonable price.

(C) If the TVC corrections are appreciable, they can be determined relative to

other TVCs with known corrections. Then, at high and middle ranges, the ac-dc

difference of the shunt and TVC combination, 6p, is practically equal to the

sum of the ac-dc differences due to the admittance of the shunt, 6^., and the

impedance of the TVC, 8j. As discussed in the referenced paper, this

separation of correction components is not feasible at low currents (less than

0.25 A in most cases) where the TVC carries a substantial portion of the total

current. These low-range TCCs should be tested and used as shunt-TVC

combinations.

31
References

[I] "AC-DC transfer instruments and converters," American National

Standard C100.4, ANSI, 1430 Broadway, New York, NY 10018, 1973.

[2] F. J. Wilkins, T. A. Deacon, and R. S. Becker, "Multi junction thermal

converter and accurate ac-dc transfer instrument," Proc. Inst. Elec.

Eng. , Vol. 112, No. 4, pp. 794-805, 1965.

[3] F. J. Wilkins, "Theoretical analysis of the ac-dc transfer difference

of the NPL multi junction thermal converter over the frequency range dc

to 100 kHz," IEEE Trans. Instrum. Meas. , Vol. IM-21, pp. 334-340, 1972.

[4] F. L. Hermach and D. R. Flach, "An investigation of multi junction thermal

converters," IEEE Trans. Instrum. Meas., Vol. IM-25, pp. 524-528, 1976.

[5] Manfred Klonz and F. J. Wilkins, "Multi junction thermal converter with

adjustable output voltage/current characteristics," IEEE Trans. Instrum.

Meas., Vol. IM-29, pp. 409-412, 1980.

[6] F. L. Hermach, "Thermal converters as ac-dc transfer standards for

current and. voltage measurements at audio frequencies," J. Res. Nat. Bur.

Stand., Vol. 48, pp. 121-138, 1952.

[7] F. C. Widdis, "Theory of Peltier- and Thomson-effect errors in thermal

ac-dc transfer devices," Proc. Inst. Elec. Eng., Vol. 109, Pt. C

(Monograph 497 M) , pp. 328-334, 1962.

[8] F. L. Hermach and E. S. Williams, "Thermal voltage converters for

accurate voltage measurements to 30 megacycles per second," Trans. AIEE

(Communication and Electronics), Vol. 79, Pt. I, pp. 200-206, 1960.

[9] F. L. Hermach and E. S. Williams, "Thermal converter for audio-frequency

voltage measurements of high accuracy," IEEE Trans. Instrum. Meas., Vol.

IM-15, pp. 260-268, 1966.

[10] E. S. Williams, "Thermal voltage converters and comparator for very

accurate ac voltage measurements," J. Res. Nat. Bur. Stand., Vol. 75C,

pp. 145-154, 1971.

[II] P. Richman, "A new peak-to-dc comparator for audio frequencies," IRE

Trans. Instrum., Vol. I-ll, pp. 115-122, 1962.

[12] D. R. Flach and L. A. Marietta, "Calibration of peak ac to dc

comparators," ISA Conference Report No. 14.2-3-65, 1965.

[13] L. A. Marietta and D. R. Flach, "Design features of a precision ac-dc

converter," J. Res. Nat. Bur. Stand., Vol. 73C, pp. 47-55, 1969.

[14] R. S. Turgel , "A comparator for thermal ac-dc transfer standards,"

ISA Trans., Vol. 6, No. 4, pp. 286-292, 1967.

32
[15] E. S. Williams, "A thermoelement comparator for automatic ac-dc

difference measurements," IEEE Trans. Instrum. Meas. , Vol. IM-29, pp.

405-409, 1980.

[16] E. S. Williams, "Thermal current converters for accurate ac current

measurement," IEEE Trans. Instrum. Meas., Vol. IM-25, pp. 519-523, 1976.

Other papers not specifically cited:

F. L. Hermach, J, E. Griffin, and E. S. Williams, "A system for accurate

direct and alternating voltage measurements," IEEE Trans. Instrum. Meas.,

Vol. IM-14, pp. 215-224, 1965.

F. L. Hermack, "AC-dc comparators for audio-frequency current and

voltage measurements of high accuracy," IEEE Trans. Instrum. Meas.,

Vol. IM-25, pp. 489-494, 1976.

«U.S. GOVERNMENT PRINTING OFFICE: 1982 360-997/2265 1-3

33

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BIBLIOGRAPHIC DATA

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NBS IN 1166

2. Performing Organ. Report No 3. Publication Date

October 198a

4. TITLE AND SUBTITLE

The Practical Uses of AC-DC Transfer Instruments

5. AUTHOR(S)

Earl S. Williams

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NATIONAL BUREAU OF STANDARDS

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Alternating currents and voltages are measured most accurately at this

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12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) ac current measurements; ac voltage measurements; ac-dc comparator;

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