Ratio Measurements; Additional Errors - Fluke 8508A Service Manual

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 measurement 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 200m VDC
range and 100 V on the 200 VDC range, applying the 365 day 1 C Absolute
specifications:
⎛
⎜ ⎜
4 5 . 0 5 .
⎝
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 measurements and effectively cancel, leaving
short term noise and linearity as the dominant errors. The 20 minute Transfer Uncertainty
Specifications 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.
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:
⎛
⎜
. 0 12 0
⎝

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 specifications. 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:
⎛
⎜
3 0 . 0
⎝
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 specification to be applied when making DC measurements on the AC Voltage
function when DC coupled, and the High Voltage adder when making measurements
above 300 V on the AC Voltage function.
2
⎞
3
200 x 10 ⎛
⎟ ⎟
⎜
x 4 5 .
3
100 x 10 ⎝
⎠
2
⎞ ⎛
20
⎟ ⎜
1 . x . 0 12 0 1 .
5 ⎠ ⎝
⎞
20
⎟
0 2 . 25 x 3
10 ⎠
Introduction and Specifications
2
⎞
200
⎟
0 2 . x . 7
100 ⎠
2
⎞
20
⎟
x . 0 61 ppm
10 ⎠
0 . 50 4 . 53 4 . ppm
Applying the Specifications
37 ppm of the ratio
of the ratio
of 10 V
1
1-21