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Fluke Corporation 8508A Extended Specifications
Introduction
The Fluke 8508A has been designed specifically
for metrologists. Not only does it provide the
performance metrologists need, but it is specified
in a way to allow users to really understand the
uncertainties of the measurements, and easily
make allowance for those uncertainty contribu-
tions when performing measurement uncertainty
analyses and compiling uncertainty budgets.
Contemporary metrology practices, including
ISO 17025 based laboratory accreditation
schemes, require uncertainty analysis to be
performed in accordance with the statistically
based techniques described in the ISO Guide to
the Expression of Uncertainty in Measurement
(often referred to as the 'GUM'). For convenience,
the 8508A specifications are quoted at a coverage
factor of k=2, equivalent to a confidence level of
approximately 95 %, as required by these
methods. Specifications are also provided at a
confidence level of 99 %.
Performance specifications for the 8508A
consist of two elements; the first is a contribution
expressed as parts-per-million of the Reading,
and the second contribution is expressed as
parts-per-million of the Range. These must be
evaluated and combined for the relevant reading
and range values applicable to the measurement
being made, ensuring that both elements are
evaluated on the same basis, such as parts per
million of the measured value or in absolute
terms (volts, amps, ohms, etc). The two elements
are combined by adding algebraically. For
example, measuring 10 V on the 20 VDC range
and applying the 365 day ±1 °C specifications:
First, expressing the contributions in terms of
parts-per-million of the measured value.
Second, expressing the contributions in volts:
The 8508A is designed to provide accuracy
and stability without the need for internal auto
or self calibration routines which may otherwise
compromise the continuity and traceability of
measurement performance history.
To realize the full potential of the 8508A per-
formance, accepted metrology practices should
be employed, such as performing a zeroing or
null operation to remove any offsets present in
the measurement setup when making DC meas-
urements. The 8508A specifications assume that
these methods are employed.
Absolute and Relative specifications
The Relative to Calibration Standards specifica-
tions describe the performance of the 8508A
itself for the time periods and temperature range
listed, excluding the uncertainty of the standards
used to perform calibration of the 8508A during
manufacture. The Absolute specifications
include the uncertainty of the standards used to
perform calibration of the 8508A at manufacture
and may be used to determine the uncertainty
of measurements made with the 8508A for
periods up to 1 year and over a temperature
range of ±5 °C from calibration. If the user has
their 8508A calibrated with different uncertainties,
the Relative specifications can be combined
with the uncertainties applicable to that
calibration to determine the effective absolute
uncertainty following that calibration.
Applying user’s calibration uncertainties
When the 8508A is calibrated by another
laboratory, the uncertainties of the calibration
standards used may be applied by combining
those uncertainties with the 8508A's Relative to
Standards specifications. The applicable
calibration uncertainties and the 8508A relative
specifications must both be expressed at the
same confidence level, and be combined in an
RSS (Root Sum Square) summation. Accepted
metrology practice mandates that calibration
uncertainties are stated at 95 %. Check the
applicable calibration uncertainties are stated at
95 % and then combine them with the 8508A
95 % Relative specifications. For example, if the
8508A is calibrated at 10 VDC with an
uncertainty of 1.5 ppm at 95 % the absolute
uncertainty at 10 V for a period of 90 days and
±1 °C from calibration is:
Interpreting and applying
the specifications
