Reference

NBS

PUBLICATIONS

NBSIR 84-2973

NAT'L INST. OF STAND & TECH

AlllDb D31flT7

An Automatic AC/DC Thermal

Voltage Converter and AC Voltage

Calibration System

K. J. Lentner, D. R. Flach, and B. A. Bell

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20899

August 1 984

Final Report

Issued November 1984

Rnnnsored in part by:

QC Department cf Defense

1QQ bration Coordination Group

,U56

34-2973

1984

NBSIR 84-2973

AN AUTOMATIC AC/DC THERMAL

VOLTAGE CONVERTER AND AC VOLTAGE

CALIBRATION SYSTEM

K. J. Lentner, D. R. Flach, and B. A. Bell

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, MD 20899

August 1 984

Final Report

Issued November 1984

Sponsored in part by:

U.S. Department of Defense

Calibration Coordination Group

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

FOREWORD

The development of the system described in this report was partially

funded by the U.S. Department of Defense Calibration Coordination Group

(DoD/CCG). Members of the DC and LF Committee were from the U.S. Air Force

Aerospace Guidance and Metrology Center, Newark Air Force Station, Newark, OH

43055; the U.S. Army TMDE Support Group, Redstone Arsenal, Huntsville, Alabama

35809; and the U.S. Navy Metrology Engineering Center, Naval Weapons Center

(Seal Beach), Pomona, California 91766.

The DoD/CCG project number assigned was CCG 79-125, and work was

performed under NBS cost center 7220463.

In order to adequately describe the system discussed in this report,

commercial equipment and instruments are identified by manufacturer's name and

model number. In no case does such identification imply recommendation or

endorsement by the National Bureau of Standards, nor does it imply that the

material or equipment identified is necessarily the best available for the

purpose.
CONTENTS

Page

Foreword iii

LIST OF TABLES v

LIST OF FIGURES v

Abstract 1

1. INTRODUCTION 1

2. TECHNICAL APPROACH. ... 2

2.1 System Hardware 2 2.2 AC/DC Difference Test Procedure ... 3

2.3 Other System Capabilities . 9

2.3.1 AC Voltage Calibrations 9

2.3.2 Stability Tests 13

2.3.3 TE Tests 13

3. MEASUREMENT RESULTS AND UNCERTAINTIES 17

4. CONCLUSIONS 21

5. ACKNOWLEDGEMENTS 22

6. REFERENCES 22

APPENDIX A. System Equipment List 23

APPENDIX B. Major Programs 25

APPENDIX C. Typical Software Documentation 28

TV
LIST OF TABLES

Page

Table 1. 3 -V TVC ac/dc difference (ppm) 20

Table 2. 10-V TVC ac/dc difference (ppm) 20

Table 3. 50-V ac/dc difference (ppm) 21

Table 4. ATS software summary 27

LIST OF FIGURES

Figure 1. System block diagram 4

Figure 2. Automatic TVC and AC Voltage Calibration System 5

Figure 3. Present measurement procedure for obtaining average

values of TVC emfs that compensates for drifts and

obtains nearly simultaneous values for the mean value. . 10

Figure 4. System block diagram when used to calibrate an external ac source 12

Figure 5. Plots from stability testing program 14

Figure 6. Plot from n test curve fitting program 15

Figure 7. Plot from dc reversal difference program . 18

Figure 8. Plot from TE thermal response time program 19

v

AN AUTOMATIC AC/OC THERMAL VOLTAGE CONVERTER

AND AC VOLTAGE CALIBRATION SYSTEM

K. J. Lentner, D. R. Flach, and B. A. Bell

ABSTRACT

An automatic ac/dc difference calibration system is described

which uses direct measurement of thermoelement emfs. In addition to

ac/dc difference testing, the system can be used to measure some important characteri sties of thermoelements, as well as to calibrate

ac voltage calibrators and precision voltmeters. The system

operates over a frequency range from 20 Hz to 100 kHz, covering the

voltage range from 0.5 V to 1 kV. For all voltages the total

measurement uncertainties expected (including the uncertainty of the

specific reference thermal converters used) were 50 parts per

million (ppm) at frequencies from 20 Hz to 20 kHz, inclusive, and

100 ppm at higher frequencies up to 100 kHz. The results of initial

intercomparisons between the new system and the manual NBS

calibration system, using single-range, coaxi al -type, thermal

voltage converters as transfer standards, are reported. The results

show that the agreement between the two systems is better than the

uncertainties originally expected, since the intercomparison of

ac/dc differences differed by no more than 15 ppm.

Keywords: ac/dc difference; ac voltage measurement; automation;

calibration; metrology; thermal voltage converter.

1. INTRODUCTION

Until recently, techniques for performing precision ac/dc difference

measurements generally made use of manual testing methods which utilize

photo-cell preamplifiers and light-beam galvanometers as votage detectors, in

conjunction with manually balanced voltage comparators [1,2] . Careful

attention to the reduction of systematic and random uncertainties in

measurement methods, thermoelements (TEs), and thermal voltage converters

(TVCs) has resulted in sufficient confidence in test data to allow results to

be reported with total uncertainties at the 10-100-ppm level (or better, in

special cases) over wide voltage and frequency ranges [3,4]. These manual

test methods, however, are very time consuming and subject to errors due to

operator fatigue and lack of skill. More recent work [5,6] overcame some of

the deficiencies of manual test methods. However, the referenced systems

still require the use of a TE voltage comparator which is not available

commercially. Because of its special design, the comparator adds to a system's cost and complexity.

Stringent requirements are placed on the measurement system to resolve

low-level voltages (about 10 nV), and to overcome the effects of large

temperature coefficients and dc reversal differences of the TEs, as well as instabilities of the voltage sources. However, with the advent of stable and

Numbers in brackets refer to the literature references listed at the end of

this report.

1
accurate analog voltage amplification techniques in combination with digital

electronics, digital voltmeters are available with sufficient stability,

linearity, resolution, and accuracy to meet the requirements of automated

ac/dc difference measurements. Programmable ac and dc voltage sources also

are available which lend themselves to adaption for automatic testing.

Advances in analog/digital interfaces have improved the ease with which

computers can be used to control these instruments.

The system to be described in this report was designed to overcome the

deficiencies of present test methods by taking advantage of the technological

advances described above. Specifically, some objectives were to: 1) utilize a desktop computer; 2) use commercially available programmable instruments

whenever possible; 3) eliminate as much operator intervention as possible in

order that testing could be done completely automatically at one fixed voltage

range of the test unit; 4) eliminate the additional cost and complexity of

using a TE voltage comparator; 5) use test procedures which assure that the

test TE's output emf is held as constant as possible; 6) eliminate the effects

of operator fatigue and lack of skill; and 7) achieve about the same level of

uncertainty as is presently possible for general purpose ac/dc difference

manual testing methods.

The system operates over a frequency range from 20 Hz to 100 kHz,

covering the voltage range from 0.5 V to 1 kV. For all voltages the goal was

to achieve total measurement uncertainties (including the uncertainty of the

reference TVCs used) of 50 ppm at frequencies from 20 Hz to 20 kHz, inclusive,

and 100 ppm at higher frequencies up to 100 kHz. In addition to performing

ac/dc difference tests, the system can be used to calibrate ac voltage

calibrators or precision ac voltmeters. Furthermore, as will be discussed

later, automatic measurements of a TE's dc reversal difference, response time,

and the value of the exponent "n" in the expression relating a TE's heater

current and output emf can also be made.

2. TECHNICAL APPROACH

2.1 System Hardware

The new system differs from most present techniques for ac/dc difference

measurements in that no TE voltage comparator is used. Eliminating the

comparator places additional requirements upon the stability of the system's

ac and dc voltage sources, the digital voltmeter (DVM) which measures a TE's

output emf, and the switches which select a TE voltage to be measured by the

DVM. The hardware development problem of obtaining low-thermal -emf-switches

which could be automatically controlled was solved during previous work [5].

Manually operated 1 ow-thermal -emf-switches were modified by linking them to

solenoids with 24-V operating coils energized by means of programmable relays.

TE voltages of about 10 mV must be measured to within about 10 nV to

achieve 1-ppm comparison precision, and the DVM used to measure these voltages

has the resolution necessary to make such a voltage measurement when extensive

averaging of data is used. The DVM has an input resistance of about 10 Gfl, a 60-Hz common mode voltage rejection >40 dB, and a dc common mode voltage

rejection >150 dB. On its most sensitive range (10-mV), the peak-to-peak

low-frequency noise (dc to about 10 Hz) of the DVM, with its input shorted,

was measured to be about 50 nV.

2
The dc voltage source has 10-V, 100-V, and 1-kV ranges with six-digit

voltage setting capability on each range. Its specified accuracy is 50 ppm of

setting to 10-V, and 100 ppm above 10-V. The ac voltage source also has

six-digit voltage setting capability for voltage ranges of 1-V, 10-V, 100-V,

and 1-kV, with specified accuracies in the range of about 0.02 percent to

0.1 percent, depending on the voltage and frequency ranges. Some typical

results of stability tests made on these sources will be discussed later.

A block diagram of the system is given in Fig. 1, and an overall view of

the system hardware shown in Fig. 2. The desk-top computer serves as the

system controller and has about 60 kbytes of user-available memory. Magnetic

tape cartridges are used for program and data storage. A video display

terminal (CRT) is used for data display during testing and also serves as an aid during software development. Since the computer only has three

input/output slots, an I/O expander is needed to accomodate the required

instrumentation. The real-time clock is used for precise control of the time

interval between applications of ac or dc voltages during a test, and the

printer and plotter provide test records.

As indicated in Fig. 1, communication between the computer and the

stimulus and measurement instrumentation is via the IEEE 488 bus. The digital

panel meters (DPMs) monitor the voltage outputs from the dc and ac sources.

During testing, readings of both the dc and ac source's output voltages are obtained by the DPMs. These voltage readings are made before the voltages are applied to the test circuit. If the measured voltages differ by more than

0.5 percent from the programmed values, the test is automatically aborted.

High accuracy in these voltage readings is not necessary, since TVCs can readily withstand an applied voltage which is 120 percent of rated value. For

the ac source, the output voltage is attenuated by a factor of 100 before

being connected to the DPM. The attenuation is necessary in order to overcome

the voltage-frequency product limitation of the DPM. The counter shown in

Fig. 1 monitors the frequency of the ac source. If the counter reading

differs by more than about 10 percent from the programmed frequency, the test

is automatically aborted. The relay actuator is used to apply 24- V dc to the

operating coils of the high-voltage relays as well as to the coils of the

solenoids which operate the low-thermal -emf switches. 2 These switches select

the output voltage of the standard TE (E s ) or the unit-under-test TE

( E-t ) . The voltages, E s and Et» are applied directly to the input of the

DVM, and the DVM voltage readings are then stored in the computer's memory for

later processing.

2.2 AC/DC Difference Test Procedure

One of the advantages of using a TE voltage comparator is that, since

small differences in the ratio of two TE emfs are measured directly, the

output emf of the test TE can be allowed to drift slightly from its reference

emf value when ac, +dc, or -dc voltages are applied [2,6]. In this new system, since no TE voltage comparator is used, the requirement for holding

the test TE's emf constant is quite stringent. However, with automation and

the rapid response time of the voltage sources (typically 50 us

2 Construction details of the high-voltage relay and low-thermal -emf modules

can be found in [7].

3
Figure 1. System block diagram

4
5

Figure

2.

Automatic

TVC

and

AC

voltage

calibration

system.
for the dc source and a few milliseconds for small voltage changes for the ac source), the constant output requirement can be readily satisfied. The

techniques used will be discussed in this section.

For a TVC with response defined by E = KV n

, where E is the output emf

of the TE in the TVC, V is the voltage applied, and K and n are parameters

characteristic of the particular TVC, the ac/dc difference is defined as

( 1 )

where V a is the applied ac voltage, and V<j is the average of the two

polarities of dc voltage; i.e.,

V + V v

d

* 2 ~ • (2)

V a » V+, and V. are voltage values which give the same output emf of the

TE. In a manual test system using a TE voltage comparator, the voltage

difference, V a

- Vj, is determined by a second TVC, the standard. If the

standard's response is defined as

E

s KsV ( 3 )

it can be shown [2] that the ac/dc difference of the test TVC is

E AE„ ca - E . 6 = §1 + 6 = -

t n E , s n E + 6

s sd s sd (4)

where E sa is the standard TE's emf when the voltage V a is applied,

E S(j is the standard TE's average emf when voltages V+ and V. are applied

(cf. eq. 2), n s is the standard's dimensionless characteristic n, and <$ s is the ac/dc difference (in ppm) of the standard determined as described in

[1,2]. Equation 4 is numerically correct if dE s is in nanovolts, E S d in

millivolts, and the "6's" are im ppm. During a test, V a , V+, V_, and

V a are adjusted until the same emf of the test TVC is obtained in each case when these voltages are applied in sequence.

In the automated system the test TVC's output emf is held constant in

the following manner. With the nominal dc voltage applied to the test TVC,

an output emf, Eset, is measured which is called the "set-point" voltage.

In all subsequent applications of ac and dc voltages to the test circuit of

6
c ’g. 1 the computer is used to adjust the voltage applied so that the test TV

C

has an output very nearly equal to E se t* A four-step procedure is used for

making ac/dc difference measurements. The procedure consists of applying a sequence of ac, +dc, -dc, and ac voltages to both TVC's at approximately equal

time intervals. The following operations are performed for each step in the

procedure presently used with the system.

First, the nominal ac or dc voltage is applied, the test emf, Et» is

measured, and a difference voltage, dEt, is calculated as

(5)

If the difference voltage is not sufficiently small, as discussed below, a voltage correction, dVt, is calculated from

d V"t Et

= dv

t

/v

t

• < 6 >

Hence,

dV

t

- ( dE

t /nt

E

t>

v t

• < 7 >

The correction, dVt, is added to the nominal voltage, V n , and a "reset"

voltage* Vr

= V n + dVt, is applied. The test emf, Et, is measured

again, and a second voltage correction, dV2, is calculated using equations 5 and 7. The output emf of the standard TE, E s , is now measured. A

correction for the emf of the standard TE, dE s , is calculated, using dV 2 to compensate for any deviation of the actual emf of the test TE from the set

point E se t ; 1.6.,

dE

s - (n

s Es dV

2

)/V

r

. (8)

The corrected emf for the standard TE is then

7
A discussion of the exact measurement procedure used is given below.

After completing the sequence of four steps by applying successively ac, +dc, -dc, and ac voltages, the ac/dc difference of the test TVC, o-t, is

calculated as

6t

( 10 )

where E a is the average of the standard's corrected emfs determined for the

two applications of ac voltage; E S(j, n

s , and <S s are as defined in

equation 4; and Ej is

E

d (ID

where E s i+ is the corrected emf calculated as in equation 9 when +dc is

applied, and E s ]_ is the corrected emf when -dc is applied.

In the computations previously discussed, account is taken of the finite

resolution of the ac and dc voltage sources. The voltage setting remains

unchanged if the output emf of the test TVC is close enough to the set-point

voltage (E se t) that a least-significant-digit change in the voltage in the

voltage source's output will provide no decrease in the magnitude of the

calculated difference, dEt.

A test run consists of four determinations of ac/dc difference and the

average of the four determinations is calculated and printed on the system's

printer. In general, three runs are normally made at each test voltage and

frequency.

In order to increase the reliability of the emf measurements made by the

DVM, and, therefore the reliability of the results, a unique measurment

procedure is used. The procedure is necessary to compensate for instabilities

in the ac or dc voltage sources or linear drifts in the standard or test

TVCs. After the dc reset voltage Vr has been applied to the test TVC, five

readings of its emf are obtained as illustrated in Fig. 3. The standard's

average emf is obtained next by taking ten readings of its TE emf. If the

standard deviation of an observation of the ten readings is > 300 nV, (300 nV

= 17 ppm for a 10 mV TE emf with n = 1.8) the first reading is discarded and

an eleventh reading is taken. The average of readings two through eleven is

calculated and the standard deviation test limit again applied. The process

continues until the average of any ten consecutive readings passes the

standard deviation limit test. Up to ten attempts are made to obtain an average emf value with a standard deviation of an observation £ 300 nV. The

test is aborted if the tenth attempt is unsuccessful.

8
When the ten reading average of the standard's emf is within the standard

deviation test limit, five readings of the test TVC's emf are again obtained.

As illustrated in Fig. 3, the average test TVC's emf is calculated as

( 12 )

where Eti is the average of the first of the five test emfs, and E^2 is

the average of the second of the five test emfs. The value of Et obtained

from equation 12 is used to calculate dE s using equations 5, 7, and 3.

As the preceding equations show, the values of the exponent n for both

the test and standard TVCs must be known. The exponent varies with applied

voltage, and its variation can be determined by doing "n-tests." These tests

will be discussed in the next section. For the standard TEs, the coefficents

in the equation expressing the value of n as a function of output emf are stored in a data file in the system software. Once the operator has specified

which standard TE is being used, the standard TE's output emf is measured with

a +dc voltage applied, and the value of n s is automatically computed. The

values of the test TE's coefficients for n must be manually typed into the

computer at the beginning of a test. To guard against mistakes in entering

data, the computed values of n for the test TE must be within the limits 1.4

to 2.1.

An additional safeguard is contained in the ac/dc difference program to

avoid application of voltages great enough to damage the TEs. When the reset

voltage^ Vr , is calculated, its value must be less than 120 percent of the

rated voltage of the TVC. If the calculated reset voltage is outside this

limit, the program aborts.

2.3 Other System Capabilities

2.3.1 AC Voltage Calibrations

As mentioned above, the system can readily be used to calibrate ac voltage calibrators using TVCs with known ac/dc differences. The output

voltage of the test calibrator replaces the output voltage of the system's ac source, and the test calibrator's correction can be determined in terms of a calibrated dc source and the known ac/dc differences of the standard voltage

converters used.

The correction to the test calibrator (in ppm) is

C t

6 s + C dc * (13)

9
0) £

H

C

CO

a> 2

>» c

<0

<DC

Sjui0 oa± p9JnsB0|/^

10

Figure

3.

Present

measurement

procedure

for

obtaining

average

values

of

TVC

emfs

that

compensates

for

drifts

and

obtains

nearly

simultaneous

values

for

the

mean

values.
where E a

is the average of the measured emfs of the TVC with ac voltage

applied, E<j is the average emf with direct voltage of both polarities

applied, n s is as previously defined, S

s is the correction (in ppm) to the

standard, and C(j c

is the correction (in ppm) to the dc voltage source's

nominal output obtained from calibration data. As in the case of ac/dc

difference tests, the correction is in ppm if E a

-E<j is in nanovolts and

Ed in millivolts. A simplified block diagram of the system connected for

such an application is shown in Fig. 4.

For ac calibrations a program called ACCAL is used (see Table 4, Appendix

8). As illustrated in Fig. 4, only one TVC is required and, as for ac/dc

difference tests, the ten reading sequence of the TE's output emf is used

(cf. E s of Fig. 3). However, a different sequence of applying voltages to

the TVC is used. The voltages are applied successively as aci, +dc,

ac2, -dc, and ac 3

. The standard TVC's n coefficient is calculated using

the TE average output emfs measured with +dc and -dc applied. A first

correction, <5^, is then calculated using

(E

al + E a2

- Ed+

nE (14)

E d+

+ Ed- where E^ =

^ » and the other terms are as defined above.

second correction is computed as

( E a2 + E a3)

- E C

2 = dnE (15)

and finally, the first average correction, C^i, is computed as

C

1 + C2

'tl (16)

11
Fiqure 4. Block diagram of system when used to calibrate

an external ac source.

12
As for ac/dc difference tests, four determinations constitute a test

run. The average correction Ct of equation 13 is computed as

C

t = (C tl

+ C

t2

+ Ct3 + Ct4 )/4 + 5

s + Cdc ' (17)

This measurement technique is very useful also for calibrating precision

ac DVM's. Since a correction, say Cti, has just been determined for the ac source, the correct ac voltage applied to the standard TVC has just been

determined. With a DVM connected at the input of the TVC, the DVM's reading

can be recorded, and using the correction Ctl, a correction for the test DVM

can then be determined.

2.3.2 Stability Tests

To control the variability in the test data, it is important to make sure that the voltage sources are quite stable. Thus, a progam called STABLE was written which uses the system's DVM to measure the stability of the voltage

sources with time. Figure 5 illustrates typical stability test data for the

dc source. At a nominal setting of 10 V, 20 voltage readings were taken over

a time interval of about 20 minutes. The deviation of the measured voltage

(in ppm) from the nominal voltage is defined as

A

v

V

X 10

6

, (18)

where Vj_ is the measured value, and V2

is the nominal value. The maximum

deviation measured was 1.3 ppm and the mean deviation was 0.6 ppm. The

standard deviation of an observation was 0.5 ppm, and the 3a limits for the mean were ±0.3 ppm. The results indicate that the dc source has acceptable

stability and is well within its specified accuracy of 50 ppm.

Typical results for a test of the ac source's stability are also shown in

Fig. 5. Again, 20 readings were obtained during a 20-minute time interval.

The test voltage was 10 V at a frequency of 20 kHz. Here the range of

deviations was 40 ppm with the mean being 19 ppm. The standard deviation of

an observation was 9 ppm, and the 3a limits for the mean were ±6 Dpm. The

discrete 10-ppm steps in the plotted data are due to the fact that the system

DVM's resolution on its 10-V range is limited to 10 ppm.

2.3.3 TE Tests

As mentioned in the previous section, one important test that is readily

performed automatically is that of determining a TE's "n" character!' Stic

.

Typical results for a 2.5-mA, VHF-type TE which was tested with a nominal 3-V

range resistor are illustrated in Fig. 6.

13
s

-CM

CO

Ijj 3

CvJ <n

CDa<

LU

o:

u_o

cr

ujaozZ

aoO) s co (\J c\j *-1 »- - 3 o: n ui

CD UJ

CM Z CD (0 Z z Z < 3 S

CD DC

11 Z>—1 3 >- a ’'T < t-H -CD < _I LD UJ 11 UJ QC a C

-CO

U

X< U1 E

-- < z sI—1 3

(- CM » 3 -C

CM 0 3 z u

t—i Z NI 1-

Wdd

N I 1VNIW0N HOdd N0I1VIA3Q

U< s

CM

LdI— I—H m<

co

cl

CL<3

UJI-<a

<33IZ

UJ>

QC

UJ

<S) CDO

Wdd

N I “IVNIWON WOdd NO I IV I Add

(D

UJ

CDz<

CLX<

CD• 3Inua

>-uz

UJza

UJCL

u.>

13

UJ

CD<

h-

_Ja><z

HHZoz

_J

I—I

CD<

hCD

cla

u.a

UJ

cn0. <

_jUJ

UJz

_l<

14

Figure

5.

Plots

from

stability

testing

program.

The characteristic n is defined by

( 19 )

where ae is the measured change in output emf for small changes in applied

voltage AV, V is the nominal test voltage for the TVC, and E is the measured

emf at the nominal test voltage, av is programmed to be ±0.5 percent of

nominal test voltage V. Sufficient time is allowed after each voltage change

for the TE to reach its final emf value.

For the results shown in Fig. 6, the TVC was tested from 50 to 110

percent of rated voltage. The total elapsed time to obtain the seven data

points was about 1.25 hours.

Each value of n is computed as the average of four determinations at any

given voltage. A least-squares analysis of the data provides the option of

obtaining a fitted curve using up to a fourth-degree polynomial. An analysis

of variance table and residual standard deviation are computed and printed so that the "goodness-of-fit" of the computed equation for n as a function of TE

output emf can be determined. The printed data sheet also shows the values of

n(measured) - n(computed). The figure shows an example of a linear fit which,

in most cases, provides sufficient accuracy. The coefficients 2.00 and -0.04

are either stored in the computer's data file (for a standard TE) or are typed

by the operator into the computer program for the TE under test at the

beginning of an ac/dc difference test.

The dc reversal difference (DCRD) of a TE can also be readily

determined. DCRD is defined as the difference in applied values of both

polarities of dc voltage required to produce an equal output emf (or "constant

output") of the test TE [7]. In order to facilitate DCRD measurements,

"constant input" emf, a, is actually measured. This a is related to the DCRD

by a factor of 1/n as

where E+ and E_ are the TE output emfs with equal +dc and -dc (or

"constant input") voltages applied, and n is as previously defined.

During acceptance tests of some TEs, the data showed much greater

variation of DCRD from unit to unit than had been expected. The only

information available indicated that the DCRD should reach a maximum value

between 70 and 80 percent of rated heater current and should approach a zero value close to 100 percent of rated current. The results obtained did not

E+

- E

. DCRD = (l/n)(a) = (1/n) ~-^

/ 2

( 20 )

16
substantiate this information. Hence, a program was 'written to determine the

DCRD as a function of applied voltage. Typical results for a 5-mA, UHF-type

TE tested with a 60-V range resistor are shown in Fig. 7. In this test the

voltage was varied from 50 to 120 percent of its rated value in 2-percent

increments. It should be noted that the plotted data show a; hence, the DCRD

values referred to a constant output emf would be smaller by the factor of n (see equation 15). In this test, the maximum a was about 700 ppm at 58

percent of rated voltage, and the minimum was about -330 ppm at 120 percent of

rated voltage.

At the time the DCRD testing program was developed, the information on DCRD variation was limited. However, in the intervening time Inglis [9] has

published a substantial amount of information in this area. Investigations at

NBS have not been as extensive as those of Inglis, but, in general, the

results obtained are in agreement with his published data.

The response time of a TE is also a characteri Stic which can readily be

measured using the automatic system. Response time is obtained by making

measurements of a TE 1 s emf when full rated voltage is suddenly applied to the

TE. The calculated response time, t, is the time taken for the output emf to

reach 1 - (1/e) of its final value. Typical results for a 2. 5-mA, VHF-type TE are shown in Fig. 8.

For the interested reader, detailed documentation (see Appendix C for an example) is available upon request for most of the automatic test programs

which are described in this report.

3. MEASUREMENT RESULTS AND UNCERTAINTIES

Manual calibration systems have been in operation for many years at NBS,

and their uncertainties have been documented [1-3] . DC reversal difference,

self-heating and ambient temperature effects, drift effects due to nonequal

time intervals between emf measurements, unbalanced lead impedances, induced

voltages from electromagnetic fields, and operator fatigue and lack of skill

are some of the more common sources of error. The process of determining an estimated total uncertainty for a system is based upon 1) a history of

measurement results, 2) careful measurements of standards with accurately

known corrections, 3) theoretical analysis of sources of errors, along with,

preferably, 4) intercomparison tests using two independent test methods [3].

A combination of processes 2 and 4 was used for the initial evaluation of the

present automated system. The primary emphasis was placed on comparison of

results obtained with a well -characteri zed manual system and those obtained

with the new system.

A single-range (3-V), coaxial -type TVC was tested using established

manual techniques, followed by tests on the automated system. The results

obtained from the two test systems are shown in Table 1.

17
c\j cm

03 -i

ZD

•-> q:

o

CD h-

.. CO

LlI *-* h- CO

< LlI Q QC

(S3 (S3 (3 (3 (3 (3 (3

S d

• (S3

• (3• (3

« (3 d

ID n CD t—

4

00 ID 00 ID 00 (\J iH 00

“IVSddAdd

I

I

3

o> iZ

Wdd N

I

18

PERCENT

OF

RATED

VOLTAGE
A

m N I 3W3 indino 31

19

Li.

TIME

IN

SECONDS
TABLE 1

3-V TVC AC/DC DIFFERENCE (ppm)

Frequency

kHz Manual Automatic Difference

0.02 0 2 -2

20 1 -9 10

50 -1 -9 8

100 -1 -4 3

For the automatic system, the values reported are the average of 12

determinations of the transfer standard's ac/dc difference. The random

uncertainty was calculated as three times the standard deviation of the mean

of the 12 observations, with the result that 3a = 21 ppm for the worst case. For the manual system, three times the standard deviation of a comparison of

two TVCs, plus an allowance for systematic errors in the comparison process,

ranges from 6 ppm at 1 kHz to 12 ppm at 100 kHz [10]. In each case the

differences between the results obtained with the two test methods is less

than the combined uncertainties for a comparison of the same TVCs by the two

methods.

A second test was mSde using a 10-V range TVC as a transfer standard.

The test results are listed in Table 2.

TABLE 2

10-V TVC AC/DC DIFFERENCE (ppm)

Frequency

kHz Manual Automatic Difference

20 -4 -17 13

50 -3 -7 4

100 -1 6 -7

Both the 3-V and 10-V TVC transfer standards had small ac/dc

differences. Therefore, a third intercomparison test was conducted using a transfer standard with large, but accurately known differences. The results

of this test are listed in Table 3.

20
TABLE 3

50 -V TVC AC/DC DIFFERENCE (ppm)

Frequency

kHz Manual Automatic Difference

5 29 26 3

10 56 52 4

20 121 121 0

50 320 310 10

The test results summarized in the three tables suggest a possible

systematic uncertainty since the automatic system gave lower values for all

but three test frequencies. Further investigation is required to evaluate

whether or not this uncertainty is significant.

As was mentioned previously, the results reported are initial

intercomparison tests. Neither sufficient tests were run nor data obtained to come to any definite conclusions regarding the systematic or random

uncertainties of the automatic test system. However, since the difference of

the ac/dc difference measurements for the three voltage converters tested did

not exceed ±15 ppm, it appears that the new system should be able to achieve

total uncertainties much less than had originally been anticipated. Further

testing'will be done to verify this hypothesis. Such testing will include a "build-up" process from a reference voltage of 10 volts to about 200 V at various frequencies. Also, a "step-down" testing process from the 10 V level

to about 2 V will be performed.

In order to investigate the effects due to random uncertainties, repeat

tests need to be done each day, for several tests. Analysis of the data from

the replicated tests should provide statistical information on the "within

day" and "between day" test data variability.

In any event, the small magnitude of the differences obtained in the

intercompari son tests are significant in that each TVC transfer standard was tested on two completely different calibration systems using different

converters as the reference standard.

4. CONCLUSIONS

The results indicate that it is feasible to make fully automatic

precision ac/dc difference measurements at any one voltage. The principal

advantage of the system is the elimination of the need for any type of

manually- or automatically-balanced TE comparator, permitting the system to oe assembled using commercially available instrumentation, except ^or the

switching modules. This approach, therefore, should make automation

attractive to many calibration laboratories faced with the need to make ac K

difference measurements. Moreover, the system can be used as a near

1 /

self-calibrating meter calibration system. Measurements can be done with

minimal operator intervention, eliminating tedious and fatiguing manual

21
operations. Hence, personnel presently engaged almost exclusively in manual

ac/dc difference tests can be freed to do other work. Experience gained

during many years of testing with manually operated systems at NBS has shown

that the precision of the test results is dependent upon operator skill. The

automatic system's precision is independent of operator skill, elminating this

variable in the experimental uncertainty.

5. ACKNOWLEDGEMENTS

The authors are indebted to M. S. Carter who wrote much of the software

and solved many difficult programming problems encountered in developing the

system. S. G. Tremaine, L. Freeman and E. V. Byland also made valuable

contributions to some of the programs which have become part of the total

system software. C. B. Childers helped greatly by doing the measurements on the manual system.

6. REFERENCES

[1] F. L. Hermach, E. S. Williams, "Thermal Converters for Audio-Frequency

Voltage Measurements of High Accuracy," IEEE Trans. Instrum. Meas.,

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

[2] E. S. Williams, "Thermal Voltage Converters and Comparator for Very

Accurate AC Voltage Measurements," J. Res. Nat. Bur. Stand. (U.S.),

Vol. 75C, pp. 145-154, Dec. 1971.

[3] F. L. Hermach, "AC-DC Comparators for Audio-Frequency Current and

Voltage Measurements of High Accuracy," IEEE Trans. Instrum. Meas.,

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

[4] R. S. Turgel, "A Comparator for Thermal AC-DC Transfer Standards,"

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

[5] K. J. Lentner, et al , "A Semiautomatic System for AC/DC Difference

Calibration," IEEE Trans. Instrum. Meas., Vol. IM-29, pp. 400-405,

Dec. 1980.

[6] E. W. Williams, "A Thermoelement Comparator for Automatic AC-DC

Difference Measurements," IEEE Trans. Instrum. Meas., Vol. IM-29,

pp. 405-409, Dec. 1980.

[7] K. J. Lentner, S. G. Tremaine, "A Semiautomatic AC/DC Thermal Voltage

Converter Calibration System," NBSIR 82-2576, Sept. 1982.

[8] "American National Standard for AC-DC Transfer Instruments and

Converters," NASI C100.4 - 1973, Amer. Nat. Stand. Inst., Inc.,

1430 Broadway, New York, NY 10018.

[9] B. D. Inglis, "Errors in AC-DC Transfer Arising From a DC Reversal

Difference," Metrologia, Vol. 17, pp. 111-117, 1981.

[10] F. L. Hermach, "Analysis of Uncertainties of NBS Thermal Voltage and

Current Converters", NBSIR 84-2903 (In Press).

22
APPENDIX A

System Equipment List

The use of trade names or company products does not constitute

endorsement or recommendation by the National Bureau of Standards, and does

not imply that the products named are necessarily the best available for the

purpose.

1. Computer and peripherals

a. Hewlett-Packard 9825T Desk Top computer (with 64 kbytes of memory)

b. Hewlett-Packard 98034A 488 interface (2 each)

c. Hewlett-Packard 98035A real time clock with options 002 and 025

d. Hewlett-Packard 9866B option 025 thermal printer

e. Hewlett-Packard 9872B option 025 plotter

f. Hewlett-Packard 98036A serial interface with 98241 test adapter

g. Hewlett-Packard 9878A 1/0 expander

h. Hazeltime 1500 video display terminal

i. Hewlett-Packard 98226A printer cradle

j. Hewlett-Packard documentation software PN 09825-10020

k. Hewlett-Packard statistical software PN 09825-15030

l. Hewlett-Packard 59300-10001 HPIB verifications cassette with service

notes 5312A-2 and 59306A-4

m. Hewlett-Packard 59405-66503 HPIB test card

n. Hewlett-Packard I/O extender board assembly PN 98124-67901

2. Tektronix P6007100 probe PN 010-0165-00 (shown as 100/1 attenuator in

Fig. 1.

3. Hewlett-Packard model 3438 Digital Multimeter, option 15 with rack flange

kit, PN 5061-0074, (2 each, shown as DPM in Fig. 1)

4. a. Hewlett-Packard 5300B counter mainframe with service note 5300B-1

b. Hewlett-Packard 5301 10 MHz counter module with service note 5301A-1

c. Hewlett-Packard 5312A HPIB interface module

d. Hewlett-Packard 10853A rack mount kit for 5300B counter

e. Hewlett-Packard 10548A service support package with service note

5300B-1

5. a. Hewlett-Packard model 59306A relay actuator (2 each)

b. Rack mount kit, PN 5061-0094

c. Rack flange kit, PN 5061-0074

6. a. Guildline Model 9577 precision digital voltmeter (shown as DVM

in Fig. 1

)

b. Option 03 three terminal input lead (2 each)

c. Option 07 low thermal lead kit

d. Input connectors (2 each no part number)

23
e. Option 08 service manual

f. Option 09 service kit

g. Option 10 calibration cover

h. Option 11 488 interface

7. a. Electronic Development Corporation (EDC) model 501J rack

mount dc voltage calibrator with minimum 25 mA current output

b. Option B, sixth decade

c. Option C, standby/ready signal

d. Option D, 100 mV range

e. Option J, bipolar operation

f. 488 interface kit, PN KT-48

g. Option AM-1, manual control panel and display (BCD)

8. EDC, model RA4 high voltage amplifier with lkV output at 25 mA

minimum current

9. a. John Fluke Manufacturing Co. 5200A Programmable AC Calibrator and

5205A Precision Power Pmplifier

b. Option 05, 488 interface for 5200A

c. Rack kit for 5200, PN M07-205-600

d. Rack kit for 5205, PN M10-205-600

10. Low-Thermal -Emf switching module (NBS design)

11. High-Voltage-Relay switching module (NBS design)

24
APPENDIX 3

Major Programs

In this appendix, a list of the major programs used in the ATS is given.

A brief explanation of the purpose of’ each program is also given. Detailed

documentation, such as the example given in Appendix C, is available for most

of the programs. A summary of the programs is given in Table A.

1.

Program name: STABLE

STABLE is a program which is used to check the stability of the system's

dc and ac sources. These stability tests are useful to determine whether or not the sources are functioning properly. Occasionally, ac/dc difference test

results appear to be erratic, and the stability tests are a convenient method

to ascertain whether the erratic results may be due either to the ac or dc

source, or both. Stability of the sources is measured by using the system's

real time clock and DVM whose function, reading rates, and ranges are remotely

controlled. The program computes the deviation from nominal voltage in ppm,

and prints the values of measured voltage and the deviation.

When a stability test is complete, the operator has the option of

plotting the measured deviations as a function of time using a proaram named

PLOTTER.

2. Program name: n-TEST

As discussed in the text, this program is used to determine a TE's

dimensionless characteristic "n." To determine the equation for "n" as a function of TE output emf, (in mV), a program called "STATIS" is loaded and

executed at the end of an n-test. The operator has the option of obtaining up

to a fourth-order polynominal equation using a least-squares curve fitting

algorithm. A plot of the actual measured n versus TE emf and the fitted curve

can also be obtained as an option.

3. Program name: DCRD

This program is used to determine a TE's dc reversal difference as defined by eq (16) of the text. The operator has the option of testing at any one fixed value of a TE's heater current. Both the constant input and

constant output values of dc reversal difference are computed. Finally, tne

response time of the test TE is determined. Response time is defined as the

time it takes for the TE to reach 63.2 percent of its final (99 percent level 1 emf when full rated heater current is applied as a step funtion. If the

operator desires, a program called EMF PLOTTER is loaded and executed, ’his

program plots the data obtained during a response time test.

4. Program name: ACCAL

This program is used to calibrate an ac voltage calibrator, *nether tnat

of the ATS or an external unit, or a precision DVM. The program ; s discusse:

in Section 2.3.1 of the text.

25
5. Program name: ACDC

This program is used for ac/dc difference testing, and is discussed in

Section 2.2 of the text. At the beginning of the test, the operator types

into the computer the parameters of test voltage, frequencies, and number of runs per frequency. One run consists of four determinations of ac/dc

difference, with each determination and the average of the four values

printed. The test is made completely automatically at any one fixed voltage.

26
Table

4.

ATS

Software

Summary

</> O) OJ d ^ o O o o o o +-> o o E CQ o o o o o O D L0 o o

-r- LO o CO CO o an 3 DD an o X 4- * A n A r— -a -l> »» O O CM d" LO LO an 4-> o

s_ t—H 3 c a i—

H

Q. • o id oc

CL

O s_

U CJ < Z jO 1—< 3 JC

</l <j

d

•r— rd -i ? LO •r— •i— Q O

<D qc -a

Ol TO J= CJ

+-> o S3

O +J

d CJ i_ 3

cr on 4—

O

•<- 4- o o o o o O o o .sc o o X O o LO o o LO CO s_ 4-> PO o o d* I—1

cm CM CM CL c •<— ro

S_ OS o s

CL (1) LO P"

CL 3 4-1

C

<=c -- JZ CL 3

H—

O SD on • • cn LU d 1 c 1— +-> 4—1 O id 1 o • • d

LO S- 3 r— 1—

O >s

s_ CL on ••

u

d • «* CJ +->

D

D

D 3

+j oo > d esc CJ 4-> cn

D

+j 1—1 1—

D

D

d 3 o 1— CL •• LU r— 4-4 cr

OS i

-aD 4-> S D

D 3 CL 1— D DC O —i a O S- o U0 4-> r— 1— 1— > 4-

•r— • ad •• CL o • • 4-> -L-> 4-> 4-> • • 4-4

DD

CL id o c id •• 3 CL

d cn on o d A— 3 D Q 3 o D D 3 3

CL CL

L

O CLDC S- DC U_ c CL

d

d

CD

D

D CM

D CJD CJ s: o D s: .3

QC DC DC qc

a DCQ LU zc QC CJ CJ

s_o d 4-4

4-> 4->d 3 (J • o D d s_ cl 

a LU D E -C JD 3 JC 'T~ • r— LU i— •r— -u> s 4-> D A— 4- 1— 5- OO 3 (d d O 00 D LU 4-> CL Q D i

a

O) id o 1— 3 4-4 4— QC L0 LJ a 4— S_ 1 id 3 O CJ c u

4-J sO 4—

O c -u> o CD o D

d

•i

3 O LO 4- CL OS 1

o c/l -C 4_ 3 4-> 4- -u> LO c c d •r— os 4-> c 4-J o o c *r— <j D o d

OS -O i— s_ <J id T3

D S— CL o (d CJ 3 D 3 4-> 4-> i_ LO

D

CL 4-4 -a 00 3 O •<“ L0 LO r— S- LO

D 4-4 s_ C/I D • r

4- 4-

D 3 d o D LU S_

d

-a 1- LU E C. o LOu S_ I— s_ Q_ -X 3 s_ O D 3 u i_ 3 LO JD u •d 4-> CD D -U> > id LO D o • LO Id 4—1 r—

O)O

C 4-> S- +j id TJ > LO CM rd o r—

jO CJ i— 1

D Q. 3 <d

D 3 D r— HD 4- (—

d

CJ id o_ on

a 35Oa

d s_ c 3 s; o Q_ CJ

H—

on 1

cr DC z:

O

d LU LU i—•

OO CJ 1 i_ y— on >—i

a_ cn a; CD i— LU J— O LU < O E o (— < QC S CJ 3 d 1

_j 1 1— CJ i—i

CJ

CL, zz O0 CL 3 on o t— LU <

27

AC/DC

DIFF

Calibrate a

UUT

for

ac/dc

10

runs/frequency:

550

16,500

difference

automatically

20

frequencies/test
APPENDIX C

Typical Software Documentation

Generally, the documentation available for the major programs used in the

ATS consists of 1) a narrative description of the program operation, 2) a list

and explanation of variables used in the program, 3) a program listing, 4) a listing of variables by program line numbers, and a listing of labelled

subroutines, 5) a program flow chart, 6) a list of special function keys (if

any are used) and the commands which each key executes, and 7) sample data

output.

In this appendix an example of the documentation for the program called

EMF PLOT is presented. The programming language used is the Hewlett-Packard

language commonly known as HPL.

28
EMF PLOT PROGRAM DOCUMENTATION

NARRATIVE PROGRAM DESCRIPTION

EMF PLOT is a program which is called by the program DC REVERSAL WITH

TIME CONSTANT to plot the emf values obtained in the time constant turn-on

test and to plot the time constant. EMF PLOT can only be used when the

maximum emf value obtained in the time constant test is less than or equal to

13 mV, and the time constant is less than or eaual to 10 seconds.

The program begins by opening all relays and clearing the IEEE 488 bus,

which were used in DC REVERSAL WITH TIME CONSTANT. A message is then

displayed instructing the operator to load the plotter paper before

continuing. The plotter select code is set to 805 and a plotter clear is

executed to reset the plotter to its default values. The scale is then set.

Pen number four (blue) is selected before the labeling begins. The

plotter then labels the graph using the observer's name, the date, and the TE

number input in DC REVERSAL WITH TIME CONSTANT. Pen number one (black) is

then selected to draw and label the X- and Y-axes. A character size of one is

executed, and pen number three (red) is selected to plot the points.

The loop which plots the emf value vs the time of the reading in seconds

is initialized after the first point is plotted. The points are plotted until one of two conditions is met, either 17 points have been plotted, or the time

and emf value are equal to zero. When either of these conditons is met,

control -jumps out of the loop. Pen number three (green) is selected, and the

time constant value is plotted in the form of a straight line and labeled.

The pen is returned to its holder, an ending message is displayed, and

the tape is then rewound.

29
VARIABLE LIST FOR EMF PLOT

Note: All variables except the simple variables X and R are dimensioned if

necessary and assigned values in the program "DC REVERSAL WITH TIME

CONSTANT"

SIMPLE VARIABLE DESCRIPTION

K Time constant in ms.

R Loop control variable

used to label the

X-axis.

X Loop control variable

used to plot the emf

values vs. time.

ARRAY

T[*]

U[*]

STRING ARRAY

F$

H$

N$

DESCRIPTION

Time of emf

reading in ms.

emf value.

DESCRIPTION

TE number.

Date of test.

Observer's name.

30
p q nr - r ’ '.I'?: TING FOP EMF PLOT.

-4 10

1 1

. M F M fi R M U I-' r L U i 1 t K V E K 3 1 U N I PI fi R U H • 3 1 . V = 4 R ? F — 1 ? h == 1 7

1 c 1 7 ? hi rt " RFlR “

? " B 1

“

5 w rt " RfiL "? “ B2 “? c 1 i 7

tii s P' ? d s P

1

1 L U fi D P fi F' E R ? P Pi ITM 6 3 u u H T I N U E

”

i> s 1- P' dsp "NATCH PLOTTER \ ! p s c 8 8 5 j p c 1 r

i :« ? i o ? — 4 i» i r p s n ’ P €' n # 4

c s i z 3 5 p It 2 ? - 3 . 5 ? 1 ? 1 b 1

;: TI M E I N S E C 0 N D 8

“ CS 1 Z 3)2!) 1 ? 40 5 P 1 1 ~’2 )

u ~ „ 2 ? 1 ? 1 b i "It UU I P'U i Ll’lr i N iv

i V

' C S i Z 4 ? P 1 1 2 ? 1 b „ b ) 1

I Li 1 " T E EMF N A R M U P

'

'

r, It I » 1 4 .

: . 1 5 f H 0

csiz 1.65 1 b 1 "DATES "

? H$

13 Pit 7.5? 14.3)

1

14 1 b 1 "OBSERVERS "

, NT

H C

1 p

I

t 7 . b ) 1 3 ) 1 ? 1 b 1 "It NUMBERS "

= FT

1 6 p s n# 1 5 f x d 9

17 x a x 0)1)0)11

4 O t 0 r R —

0

t o 1 1 5 p i t K i( jdL O 4 a y ) 1 ) i b

19 t x d 2 ) y 0. x 0 ) 1?0? 1 3 ? 1 20 C -• 1 Z 1 ) P S' n ) P S' r'l 4 3 ? P 1 t TCI]/ 10 0 0 ? U L i 21

:

f 0 r X = 2 1

0

117

cl uL

" if T [ Ik ] = 0 0. r"i d U [ l

:

' :

I ] * 1 0 0 0 = 0 j 5 1 0 2 b

d -J 2 Pit T [ X ] 1 0 0 0 h i_i [ X ] * 1 0 0 0)2) p S n

r. ) n S' r.

24 '• c P 1 1 -

.

6 6 ? ~ . 2 5 5 1 b 1

" +

"

j

• . i”. p 1 1. ~ « 6 7 ) . 2 b 5 n S' x t X

I. . -+ P' p n ? P S' Pi rt 2 ? P 1 1- K ' i 0 0 0 h 0 i ~ 2

! f X d 3 ) P

1 1 t K 1 0 0 0 h 1 3 ) “ 1 ) P i t K 1 0 0 0 ) b

> s csiz 1 , b ) p S' n ? c p it .2)0) 1 b 1

" T I Pi E U U N 3 l fi N T =

'

' ? K • 1 0 0 0 ? " 3tL

1 9

’ p s n # ? b S' S' p 5 d s p

" E N D U I- E X E u U i i U N

’ ? r s w 5 s n d

4 0 y b y t S' s 2' 0 1 b 3 c h S' c k s u ivi

31
V fl R I H b L E L I b i 1 M b r U K t M i- P L U i »

i 8 1 8 1

8

Tl

F$

Hi

v 1

VL-1

12

N $

14

32
DEFINITION OF FLOWCHART SYMBOLS

DEFINITION

Terminal

Decision

Manual Input

SYMBOL SYMBOL DEFINITION

Magnetic Tape

Preparation

Connector

Process

Input/Output

Document

Display

Auxi 1 iary

Operation

Comment

Branching

Tabl e

V

Program Flow

33
34 0
6 35 0
Plo

R-.

1

t23, -.8,

> , 18

Label

R

\ / 18

Next

R

t 19

Set fxd2

i

19

/ Draw & Label

Y-axi

s

v

20

Pen

Select

40

TT O

20

PI ot

T [lj/1000

U [1]*1000>1

PI ot

T[x]/1 000

U[x]*l 000,2

23

Lift

Pen

36

Char

Size 1

\/ 24

Char Plot

-.33, -.25

©
37

©
0 33
(J

CO

Cl

3

on

LlI >

on

LlI (f) CDO

DC

<

i

U

U.

cm

LUCD

LU

h3 s 3 (3 S 3 3 3 3 3 3 3 3 3

S (3 (3 3 3 3 3 3 3 3 3 3 3 3

CO

1 CM8 r—4 3 CT) CO CO in co C\J rH 3

n i JIA3 indino si

39

TIME

IN

SECONDS
NBS-1 14A ( rev. 2 -ac> U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions/

1. PUBLICATION OR

REPORT NO.

NBSIS 34-2973

2. Performing Organ. Report No.| 3. Puolicacion Date November 1984

4. TITLE and SUBTITLE

AN AUTOMATIC AC/DC THERMAL VOLTAGE CONVERTER AND AC VOLTAGE CALIBRATION SYSTEM

5. AUTHOR(S)

K. J. Lentner, D„ R. FTach, and B. A. Bell

S. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C- 20234

7. Concract/Grant No.

S. Type- of Report S> Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE AQDRESS (Street. City. State, ZJP)

Sponsored in part by:

U. $. Department of Defense

Calibration Coordination Group

10. SUPPLEMENTARY NOTES

j 1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. A8STRACT (A 200-word or less factual summary of most significant in formation. If document includes a significant

bi bl io graph y or literature survey, mention it here)

An automatic ac/dc difference calibration system is described which uses direct

measurement of thermoelement emfs. In addition to ac/dc difference testing, the system

can be used to measure some important characteristics of thermoelements, as well as to

calibrate ac voltage calibrators and precision voltmeters. The system operates over

a frequency range from 20 Hz to 100 kHz, covering the voltage range from 0.5 V to 1 kV.

For all voltages, the total measurement uncertainties expected (including the uncertainty

of the specific reference thermal converters used) were 50 parts per million (ppm) at

frequencies from 20 Hz to 20 kHz, inclusive, and 100 ppm at higher frequencies up to

100 kHz. The results of initial intercomparisons between the new system and the manual

NBS calibration system, using single-range,, coaxial -type, thermal voltage converters as transfer standards, are reported. The results show- that the agreement between the two

systems is better than the uncertainties originally- expected, since the intercomparison

of ac/dc differences differed by no more than 15 ppm.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

ac/dc difference; ac voltage measurement; automation; calibration; metrology;

thermal voltage converter

13. AVAILABILITY

i Y ! Unlimited

j

j For Official Distribution. Do Not Release to NTIS

|

! Order From Superintendent of Documents. U.S. Government Printing Office, Washington, D.C.

20402.

i xj Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

45

15. Price

$8.50

USCOMM-OC S043-P30