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How to calculate Repeatability of an Instrument?
Calculation of Repeatability

We purchased an instrument with specs of 0.3% repeatability and 1% accuracy. What is the correct way to determine/establish the repeatability of this instrument?

Thanks in advance

By Curt Wuollet on 22 October, 2007 - 5:06 pm

Present it with the same input multiple times and observe how consistent the reading is.



Give the same input at least 5 times and record the output. The deviation between any two outputs should not be more than 0.3% of the range if your repeatability has been defined as 0.3% of the range.

Take 5 consecutive measurements with same input. The Repeatability percentage will be (max.reading-min.reading/min.reading)*100. This should be within specified limits.

By W.L. Mostia on 15 November, 2007 - 2:45 pm

The below statements are not quite correct as it is not just any five consecutive measurements. Please refer to the ANSI/ISA Standard 51.1, "Process Instrumentation Terminology," pg. 49, which defines repeatability and Figure 23 on pg. 50, which provides a graphical representation of repeatability based on instrument calibration curves.

William (Bill) L. Mostia, Jr. PE

My reply was based on NIST. I have no access to the ISA standard. As the measurements will normally be on the linear portion of scale/range, I guess the Repeatability figures will be about same in that measurement range.

Dear sir,

I do not have access to ISA standards. Can you please send me those two pages (pg.49 and pg.50) of ISA-51.1-1979(R1993) to