42 34980A Service Guide
2 Specifications
Accuracy Accuracy is a measure of the “exactness” to which the internal DMM’s
measurement uncertainty can be determined relative to the calibration reference used.
Absolute accuracy includes the Internal DMM’s relative accuracy specification plus the
known error of the calibration reference relative to national standards (such as the U.S.
National Institute of Standards and Technology). To be meaningful, the accuracy
specifications must be accompanied with the conditions under which they are valid.
These conditions should include temperature, humidity, and time.
There is no standard convention among instrument manufacturers for the confidence
limits at which specifications are set. The table below shows the probability of
non-conformance for each specification with the given assumptions.
Variations in performance from reading to reading, and instrument to instrument,
decrease for increasing number of sigma for a given specification. This means that you
can achieve greater actual measurement precision for a specific accuracy specification
number.
The internal DDMM is designed and tested to meet performance better than mean ±3
sigma of the published accuracy specifications.
24-Hour Accuracy The 24-hour accuracy specification indicates the internal DMM’s
relative accuracy over its full measurement range for short time intervals and within a
stable environment. Short-term accuracy is usually specified for a 24-hour period and for
a ±1°C temperature range.
90-Day and 1-Year Accuracy These long-term accuracy specifications are valid for
a 23°C ± 5°C temperature range. These specifications include the initial calibration
errors plus the internal DMM’s long-term drift errors.
Temperature Coefficients Accuracy is usually specified for a 23°C ± 5°C
temperature range. This is a common temperature range for many operating
environments. You must add additional temperature coefficient errors to the accuracy
specification if you are operating the instrument outside a 23°C ± 5°C temperature range
(the specification is per °C).
Specification Criteria Probability of
Failure
Mean ± 2 sigma
Mean ± 3 sigma
4.5%
0.3%
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