I initially thought interpreting electrical specifications was a little daunting. But once you get the basics, it’s really pretty straightforward. Specifications in dc/low frequency electrical calibration are based around a simple “y = mx + b” linear equation for both sourcing and measuring instruments. Manufacturers’ use terms like “% of reading + % of full scale” or “% of reading + units”, all based on two terms, a scale factor error (“m”) and an offset or floor “b”. Really, pretty straightforward stuff once you figure it out. One caveat though, always, always read the footnotes. You may be surprised that your 5 ppm + 3 uV specification only applies on certain days of the month, after doing “xyz” procedure, and when the instrument is sitting in a horizontal position. Of course I’m exaggerating, but you get the point.
Compared to electrical calibration, my introduction to pressure calibration was a rude awakening! In pressure calibration, specifications sometimes refer to the “m” and sometimes to the “b”, sometimes together but not always. Plus there are terms that are unique to each manufacturer. Having said that, there are reasons for all of this. Any specification should fully represent the true performance of the instrument. Pressure measurement devices use a variety of different technologies, each with their own unique performance peculiarities. Thus, a number of different terms and techniques are used when creating pressure specifications. While there is commonality in some of the terms with the electrical world, some terms are unique to pressure (and some are even unique to a specific manufacturer). The key is to make sure you are always comparing like to like (apples to apples as the saying goes in the United States). Here’s a sampling of accuracy terms used by various manufacturers to describe their respective products:
- % full scale
- % reading
- % span
- % autoranged span
- % Q-RPT span
- And the list goes on, and on, like % BSL, % terminal point accuracy, % span terminal point accuracy, % total error band
I can explain at least the more commonly used accuracy terms in the pressure world, again with one caveat: As in the electrical world, be sure to read ALL of the footnotes, so that you are comparing apples to apples. And, don’t hesitate to fire up a spreadsheet to really keep things in order.
My small glossary of pressure accuracy terms:
% Full Scale – Specification is a percentage of the maximum pressure. Example: +/- 0.05% FS on a device with a range of -10 to 10 psi. The value would be 0.05% X 10 psi = +/- 0.005 psi throughout its entire range.
% Reading – Specification is a percentage of the current pressure. Example: +/- 0.05% on a device with a range of -10 to 10 psi. If the pressure is 5 psi, then the value would be +/- 0.0025 psi. When the pressure is 10 psi, then the value would be +/- 0.005 psi. You do have to be careful when evaluating a % Reading spec for instruments whose range includes zero or points near zero, since pressure at and near zero must include a constant accuracy term (the offset or “b” in the electrical world).
% Span – Specification is a percentage of the overall span. Example +/- 0.05% span on a device with a range of -10 to 10 psi. The value would be 0.05% X (10 – (-10)) = +/- 0.01 psi.
% autoranged span – While the instrument is vented, the user can specify an “autoranged span.” During the resulting test, the user would not be able to go to pressures outside of this span. The specification would be a percentage of this autoranged span. For example, you are using a controller with this type of specification and a full scale of 1000 psi in order to calibrate a device with a full scale of 500 psi. You can set the autoranged span to 500 psi instead of 1000 psi. Your specification would now be a percentage of 500 psi instead of 1000 psi, cutting it in half.
% Q-RPT Span - Q-RPT refers to the physical sensor that is installed in the instrument. It’s the same as % span, but adds clarification when there is more than one Q-RPT installed in the instrument or to differentiate from autoranged span.
What are your thoughts on pressure calibration specs? Is there a preferred method that works for you?