Uncertainty Files for Commonly Used Standards

The information in the uncertainty files is taken from the manufacturer’s specifications for each particular instrument, at a confidence level shown in the specifications and at the top of the uncertainty file.

Note   Uncertainty files contain specifications expressed in specific units. For example, calibrator uncertainty files generally contain ppm output and mV offset, whereas DMM uncertainty files generally contain ppm reading and ppm range. If the specifications for your instrument are not in this format, you must convert them to the same units. For example, if the specifications of your calibrator are given in percent output and mV offset, you must convert them correctly to ppm output and mV offset, respectively. If the confidence level is not given in the instrument specifications, you can assume a 95% confidence level and set the coverage factor to 1.96. For more information, refer to Annex G in ANSI/NCSL Z540–2-1997, U.S. Guide to the Expression of Uncertainty in Measurement.

Template Files

If you use instruments that do not have uncertainty files provided by Calibration Executive, you can create your own uncertainty files by using the template files. Template files named Calibrator Template.txt and DMM Template.txt are provided in their respective directories under Calibration Executive\Uncertainty.

Tip   Instead of using the template files, you can modify an existing DMM or calibrator uncertainty file by making a copy of the file and changing the appropriate information for the standard you are using.

Uncertainty Calculations of Reference Standards

The uncertainty of the calibrator depends on what range it is operating in and what output value it is returning. Calibration Executive calculates the uncertainty of a DC voltage Test Point for a calibrator as follows:

1. Searches in the proper uncertainty file for the row in which the Test Value is less than the range.
2. Retrieves the ppm output and mV offset from that row.
3. Calculates uncertainty using the formula

   

4. Calculates standard uncertainty using the formula

   

   because a 95% confidence level corresponds to a coverage factor of 1.96.

The standard uncertainty is the value that Calibration Executive combines with the other standard uncertainties using the root-sum-square method to determine the combined standard uncertainty. The uncertainty of the DMM depends on what range it is operating in and the value that it is reading. Calibration Executive calculates the uncertainty of a DC voltage Test Point for a DMM as follows:

1. Searches in the proper uncertainty file for the row in which the Test Value is less than the Range.
2. Retrieves the Range, ppm Reading, and ppm Range from that row.
3. Calculates uncertainty using the formula

   

4. Calculates standard uncertainty using the formula

   

The standard uncertainty is the value Calibration Executive combines with the other standard uncertainties using the root-sum-square method to determine the combined standard uncertainty. The combined standard uncertainty is then multiplied by a coverage factor of 1.96, corresponding to a final expanded uncertainty with a 95% confidence level and a normal distribution.

Adding Uncertainty Components

If you want to add other uncertainty components, you can copy and modify an existing uncertainty file. For example, if you want to add an offset uncertainty component to all analog input tests, make a copy of the proper calibrator uncertainty file, rename it, and modify it by adding the offset to the mV offset column.

If the DMM you are using is operating outside of its normal temperature range and you want to account for the temperature difference, make a copy of the proper DMM uncertainty file and add the temperature coefficients to the absolute uncertainty specifications. When you run the Calibration Executive procedure, a dialog box prompts you for information on which calibrator and DMM you are using. If you want to use your modified uncertainty files and they are not in the provided list, click Browse and select your modified uncertainty files.