Fluke 8508A Extended Specifications page 4

Applying the specifications
Ratio measurements
The 8508A Ratio mode will automatically take
measurements of inputs applied to the front and
rear terminals and display the result as a ratio in
the voltage and resistance functions. The
measurements can be made on the same range
or different ranges. When making measurements
on different ranges, the error in each measure-
ment is evaluated by applying the relevant
specification for each range and combining the
two specifications in an RSS summation,
expressing the contributions in parts-per-million
of the measured values. For example, making
measurements of the ratio of 100 mV on the
200 mVDC range and 100 V on the 200 VDC
range, applying the 365 day ±1 °C Absolute
specifications:
Making measurements on the same range
will eliminate range-to-range errors, such as
drift since the time of calibration, and improve
the result. When making measurements on the
same range, these errors will affect both meas-
urements and effectively cancel, leaving short
term noise and linearity as the dominant errors.
The 20 minute Transfer Uncertainty specifica-
tions are provided to describe the performance
obtained when making ratio measurements on
the same range. The error in each measurement
is evaluated by applying the relevant 20 minute
Transfer Uncertainty Specification for each value
and combining the two specifications in an RSS
summation, expressing the contributions in
parts-per-million of the measured values. If the
measurements are made within the same range,
but independently (not using the ratio mode)
with an elapsed time greater than 20 minutes
but less than 24 hours between the measurements,
then the 24 hour specifications should be
applied instead.
4
Fluke Corporation 8508A Extended Specifications
For example, making measurements of the
ratio of 5 V and 10 V on the 20 VDC range,
applying the 20 minute Transfer Uncertainty
Specifications:
Additional errors
The 8508A specifications are listed for the
maximum resolution in each function, using the
Normal reading mode. For measurements taken
in other resolutions or the Fast read mode
additional error contributions listed in the Read
Rate and Additional Uncertainty table must be
included. These additional contributions must be
added algebraically to the relevant specifica-
tions. For example, measuring 10 V on the
20 VDC range at 5 digit resolution in Fast mode
and applying the 365 day ±1 °C Absolute
specifications:
Other additional contributions apply in certain
situations and are also to be added algebraically
to the relevant specifications. These additional
contributions include the DC Accuracy specifica-
tion to be applied when making DC measure-
ments on the AC Voltage function when DC
coupled, and the High Voltage Adder when
making measurements above 300 V on the AC
function.