Measurement Uncertainty
- Updated2023-02-21
- 3 minute(s) read
Measurement Uncertainty
Measurement uncertainty is defined by NCSL Rp3-1988 as an estimate of the range of values about the measured value in which the accepted value is believed to lie. A total uncertainty value represents the combination of a measure of the random error and estimated bounds of the systematic error, and considers all factors that contribute to the measurement uncertainty. For detailed information about measurement uncertainty, refer to ANSI/NCSL Z540–2-1997, U.S. Guide to the Expression of Uncertainty in Measurement. For some devices, Calibration Executive can calculate measurement uncertainties for you. When you run the procedure, the software prompts you to select whether to perform measurement uncertainty calculations. If you select Yes, the procedure prompts you for which uncertainty file you want to use.
How Does Calibration Executive Calculate Measurement Uncertainty?
Calibration Executive does not provide uncertainty calculations for all devices. Only the following devices support an option for uncertainty calculations:
| AT-AI-16XE-10 | AT-MIO-16DE-10 |
| AT-MIO-16E-1 | AT-MIO-16E-2 |
| AT-MIO-16E-10 | AT-MIO-16XE-10 |
| AT-MIO-16XE-50 | AT-MIO-64E-3 |
| DAQCard-AI-16E-4 | DAQCard-AI-16XE-50 |
| DAQPad-6020E | DAQPad-6070E |
| DAQPad-6052E | |
For devices that do not include automatic uncertainty calculations, you can calculate the uncertainty manually using the template files provided in Calibration Executive.
When calculating measurement uncertainty, Calibration Executive accounts for Type A uncertainty and Type B uncertainty. The following table explains the calculations for these uncertainty components.
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Note Type B uncertainty consists of two parts: uncertainty of the reference standard and resolution uncertainty of the DUT. |
| Component | Description |
|---|---|
| Type A uncertainty | Defined as the standard deviation of the measurements divided by the square root of the number of measurements. This calculation is based on different numbers of measurements for different products. The degrees of freedom value is taken into account in cases where the number of degrees of freedom is not considered infinite. |
| Type B uncertainty, part 1: uncertainty of the reference standard |
where N is the number of measurements taken and f accounts for the t-distribution. Including the t-distribution is only necessary in cases where the number of degrees of freedom is not considered infinite. |
| Type B uncertainty, part 2: resolution uncertainty of the device |
where Resolution of the Device is the accuracy of the smallest meaningful displayed digit normalized using a rectangular distribution. The √3 term comes from assuming a rectangular distribution of probabilities of the values of measurements. |
After the standard uncertainty components are calculated, they are combined using the square root of the sum of the squares method to arrive at the combined standard uncertainty. The expanded uncertainty of measurement that Calibration Executive prints on the calibration report is this combined standard uncertainty of measurement multiplied by the coverage factor k = 1.96, which for a normal distribution corresponds to a coverage probability of approximately 95%.