#WishIknew – What’s rangeability?


When you have a question about process automation that needs a short answer, let us know with #WishIknew and #Visaya! We’ll reply with a #WishIknew post. It’ll give a quick explanation, then some related articles, videos, or reviews if you want to know more.

I wish I knew what rangeability is!

Glad you asked this one because the definition gets muddy sometimes. First, we often call rangeability the “turndown (TD) ratio,” and it’s used with measurement devices. It covers the span of the device’s range, usually expressed as the ratio of the upper range limit (URL) and lower range limit (LRL).

Basically, it describes the accuracy of the field device, a very important point. If your device has lost accuracy, you need to calibrate it or replace it, depending on how it performs in your tests.

The equation to find the turndown or rangeability:

TD = URL/calibrated span

Courtesy of Alicat

That means the turndown of your transmitter equals the upper range limit (URL) of the calibrated span. For instance, say you bought a device with a range of 0 to 8950 millimeters of water (mmH2O). Then you need to set up the device to measure from 0 to 895 mmH2O. The calibrated span reflects the difference between the minimum and maximum calibrated range, and the turndown will be a ratio of 10 to 1. Make sense? If not, send me a message, and we’ll work on it some more!

Related tags: #WishIknew Calibration Rangeability
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