Are You Factoring in Repeatability and Reproducibility When Evaluating Your Metering System?
In an effort to bring you more informative and technical information, the next blog post summarizes an article that was written for Flow Control back in August 2014 by Karl Stappert, Flow Solutions Advisor. This article defines and explores repeatability and reproducibility – terms that are very common when measuring flow and are commonly misunderstood or confused.
Let’s start off with repeatability – what is it, exactly? By definition, repeatability is “the closeness of agreement between indications or measured values obtained by replicated measurements on the same or similar objects under conditions of measurement that include the following: short period of time, same measurement system, same procedure, same set of conditions, same operator, same location” (source: http://www2.emersonprocess.com/siteadmincenter/PM%20Articles/Emerson%20Flow%20Solutions%20-%20Flow%20Control%20-%20Aug%202014.pdf). Being able to measure flow under unchanging process conditions (conditions of repeatability) is a popular measurement procedure in the flow metering world due to the accurate measurements it helps produce. Of course, it is very hard to find these perfect, unchanging conditions in real life. Normally, these conditions are only achieved in a calibration laboratory.
One thing repeatability does not factor in during measurement are influence quantities. Influence quantities are “influences that do not affect the quantity that is being measured but affect the output of the system performing the measurement.” This is when the term reproducibility comes into play. By definition, “reproducibility is the ability of a measurement system, over a set of changing conditions, to replicate the same measurement” (source: http://www2.emersonprocess.com/siteadmincenter/PM%20Articles/Emerson%20Flow%20Solutions%20-%20Flow%20Control%20-%20Aug%202014.pdf). Reproducibility determines if the measurement system is immune to these influence quantities. The most common conditions of change include different locations, operators and measuring systems. Although repeatability is more commonly discussed and specified than reproducibility, they are both very important in evaluating real-world performance.
Repeatability, Reproducibility and Flowmeter Technologies
So, what flow measurement technology works best when discussing repeatability and reproducibility? Two technologies that stand above the rest are Coriolis and ultrasonic flow meters. Even though their repeatability may not be as good as positive displacement meters, their reproducibility under changing conditions is far better than traditional technologies. For example, Coriolis flow meters have a list of benefits including no wearing parts, and a high insensitivity to flow profile, density, viscosity, velocity, temperature and pressure change, the real-world conditions that labs usually don’t duplicate.
As you can see, it is important to not just look at one method of measurement or system when exploring flow metering technologies. Never forget to evaluate the reproducibility of that metering system as well as the repeatability and beneficial attributes of the technology.
For more information on repeatability and reproducibility, click here.