Fluke 1595A - Manual (Page 115)

105
Calibration
RATIO CALIBRATION (F3)
sured resistance ratios. Ideally the product would be exactly 1.0. The reported error is half the difference of the
combined result from 1.0, in units of 1×10 -6 .
Test 3 (Equal ratio sum--100% scale) - This test evaluates linearity errors at a resistance ratio near 0.5 with
the measurement circuit operating at full scale. The test has two parts. For the first part, one resistor of a 100
W + 100 W resistor network is connected as Rx, and the series combination of the two resistors is connected as
Rs internally. The first measured resistance ratio is reported as result A. For the second part, the opposite of the
two resistors is connected as Rx and again, the series combination is connected as Rs. The second measured
resistance ratio is reported as result B. Ideally the sum of the two measured resistance ratios would be exactly
1.0. The reported error is the difference of the sum from 1.0, in units of 1×10 -6 .
Test 4 (Equal ratio sum--90% scale) - This test evaluates linearity errors at a resistance ratio near 0.5 with
the measurement circuit operating at 90% of full scale. The test is performed in the same way as Test 3, except
the measurement circuit gain is reduced so that the circuit operates at the reduced scale. The reported error is
the difference of the sum from 1.0, in units of 1×10 -6 .
Test 5 (Equal ratio sum--75% scale) - This test evaluates linearity errors at a resistance ratio near 0.5 with
the measurement circuit operating at 75% of full scale. The test is performed in the same way as Test 3, except
the measurement circuit gain is reduced so that the circuit operates at the reduced scale. The reported error is
the difference of the sum from 1.0, in units of 1×10 -6 .
Test 6 (Equal ratio sum--60% scale) - This test evaluates linearity errors at a resistance ratio near 0.5 with
the measurement circuit operating at 60% of full scale. The test is performed in the same way as Test 3, except
the measurement circuit gain is reduced so that the circuit operates at the reduced scale. The reported error is
the difference of the sum from 1.0, in units of 1×10 -6 .
Test 7 (Equal ratio sum--50% scale) - This test evaluates linearity errors at a resistance ratio near 0.5 with
the measurement circuit operating at 50% of full scale. The test is performed in the same way as Test 3, except
the measurement circuit gain is reduced so that the circuit operates at the reduced scale. The reported error is
the difference of the sum from 1.0, in units of 1×10 -6 .
Test 8 (Unequal ratio sum) - This test evaluates linearity errors at resistance ratios near 0.25 and 0.75 (with
the measurement circuit operating at full scale). The test is performed in the same way as Test 3, except the
two resistors are 25 W and 75 W. The reported error is the difference of the combined result from 1.0, in units
of 1×10 -6 .
All tests use a sensing current of 1.0 mA. The timing settings are temporarily set to provide measurements at
two-second intervals and the digital filter is disabled. The reported measurement of each test is the mean of a
set of measurements of the resistance ratio under certain conditions.
For each test a standard error of the mean is calculated and reported. This provides an estimate of the uncer-
tainty of the mean of the reported error due to measurement noise. The standard error of the mean is calculated
from the standard deviation (S
A
, S
B
) of parts A and B and the sample count (n) using the following equation:
SE
a
S S
n
A B =
+
×
1
10
2 2
6
The sample count is the number of samples comprising the result from each part of the test. The divisor a is
2 for Test 1 and Test 2, accounting for the fact that the reported error is calculated as an average from the two
test parts. For the other tests a = 1.
When the Ratio Self-Calibration completes, the time and date (when the last test ended) are shown below the
test results.
To view the test report screen, the user presses the VIEW REPORT (F1) function key. The report screen shows
the specified tolerance, test result error, condition (the percent error relative to the tolerance), and the standard
error of the test result error for each individual test. The tolerances shown are based on the 99% confidence
level resistance ratio specifications found in the Specifications section.
The Ratio Self-Calibration report data are stored in internal non-volatile memory, overwriting the results of
any previous Ratio Calibration. The report of the most recent Ratio Self-Calibration may be viewed at any
time using the VIEW PREVIOUS (F2) function key in the Ratio Calibration screen.