Applying The Specifications; Introduction; Absolute And Relative Specifications - Fluke 8508A Service Manual

Applying the 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 contributions when performing measurement uncertainty analyses and
compiling uncertainty budgets. Contemporary metrology practices, including ISO17025
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:
⎛
⎜
3 0 . 0 2 .
⎝
Second, expressing the contributions in volts:
6
3 0 . x 10
The 8508 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 realise the full potential of the
8508A performance 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 measurements. The 8508A specifications assume that these
methods are employed.

Absolute and Relative Specifications

The Relative to Calibration Standards specifications 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.
⎞
20
⎟
x 3 0 . 0 4 .
10 ⎠
6
x 10 0 2 . x 10 x 20
Introduction and Specifications
3 4 . ppm of 10 V
5
3 4 . x 10 34 μV

Applying the Specifications

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