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
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.
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.