ASIA-PACIFIC METROLOGY PROGRAMME (APMP)
BILATERAL DC VOLTAGE COMPARISON
between the National Measurement Laboratory, Australia (NML)
and the Standards and Calibration Laboratory, Hong Kong (SCL)
PROTOCOL
This comparison will use a Zener-based voltage standard (ZVS) to measure the difference between
the maintained units of dc voltage at NML and SCL. The significance of any measured difference will
be assessed, using statistical techniques as appropriate to estimate the uncertainty of the measured
difference.
NML will be the pilot laboratory, so that the sequence of measurements will be: NML-SCL-NML.
The artefact will be a Fluke 732B, serial number 5875101, property of SCL, but will be made
available to NML before the start of the comparison in order that its temperature coefficient, humidity
coefficient (with any significant accompanying time-constant) and pressure coefficient may be
measured using equipment already available at NML for this purpose. This preliminary procedure is
termed the “characterisation” of the ZVS, and will be undertaken as follows:
Characterisation of the ZVS:
(1) At 50% relative humidity, the voltage of both the 1.018 V and 10 V outputs of the ZVS
will be measured at the following temperatures: 17C, 20C, 23C and 26C.
(2) At 20C, the output voltages will be measured at 35% relative humidity, 70% relative humidity
and then again at 35% relative humidity.
(3) The output voltages will be measured at –200 kPa, 0 kPa and +200 kPa relative to atmospheric
pressure and at 20C and 50% relative humidity.
Comparison:
NML measurements: at 23C and 45% relative humidity, two measurements per day of the
1.018 V level for ten working days, and two measurements per day of the 10 V level for ten working
days. If time permits or if this is considered desirable, NML will do these measurements
also at 20C and at 45% relative humidity. The two measurements per day will if possible
be spaced so that one is in the morning, the other in the afternoon.
SCL measurements: at 23C (1C) and 45% (8%) relative humidity, four measurements
per day of the 1.018 V level for ten working days, and four measurements per day of the 10 V level
for ten working days. The four measurements per day will if possible be spaced so that two are in the
morning, the other two in the afternoon.
The date, time of day, temperature, relative humidity and pressure will all be recorded for each
measurement.
Packaging of the Fluke 732B ZVS:
SCL will pack a Testo 171 data logger with the Fluke 732B for continuous measurement and
recording of the temperature and relative humidity. The Testo 171 will record at a rate of 1
reading every 5 minutes. Its calibration will be checked by comparison with temperature and
humidity standards maintained at NML. If the characterisation of the Fluke 732B reveals a
significantly long humidity time-constant, the data logger readings will be used in order to correct
the measured Fluke 732B voltages for this delayed effect.
Powering of the Fluke 732B ZVS:
During all measurements (except possibly for the humidity coefficient measurements) the Fluke
732B will be powered from its internal or a specially provided external battery.
Signal connections to the Fluke 732B ZVS:
The output terminals will not be connected to ground nor to the guard terminals. No connection
will be made to the chassis terminal of the Fluke 732B. The guard terminal of the Fluke 732B will
be connected to system ground.
Reporting of results by each laboratory:
Each laboratory will report the voltage values at both 1.018 V and 10 V together with the
date, time of day, temperature, relative humidity and pressure that pertain to each reported
value.
Each laboratory will present an uncertainty budget for its reported voltage values. This
budget will indicate all sources of uncertainty, their type (A or B) and associated degrees
of freedom. The last row in the uncertainty budget will indicate the overall standard uncertainty
and its associated degrees of freedom.
Bob Frenkel
National Measurement Laboratory.