Accuracy is paramount to the success of a pressure transducer. However, the output of a pressure transducer isn’t linear, meaning the signal deviates from reality by varied amounts across the full measurement span. When graphed, the output looks like a curve rather than a straight line.
This behavior is expected. But it can’t get out of hand. The curve of the output can never get farther away from reality than stated in the spec.
At APG, we spec the error band of most of our pressure transmitters at 0.25% of full scale (most also have 0.1% option). So the gap between the true linear progression of pressure and the measurement thereof can never exceed 0.25% of the full scale. If that scale is 100 psi, it has to remain within 0.25 psi at all times.
To ensure our pressure transducers perform within spec, we test each one in the manufacturing process. Each sensor will have a uniquely curved output. We graph each output and apply the Best Fit Straight Line (BFSL) calculation. This means that the output of the sensor can meander above or below the real pressure as long as it stays within 0.25% of the full scale – even at the end points.
That’s why the accuracy specification states ±0.25%. The plus or minus is a meaningful part of the specification. In fact, our spec quite purposefully mentions BFSL as our testing method. It reads something like this:
±0.25% of full scale (BFSL)
Another quality calculation, called Straight Line, draws a line between the end points, but allows the middle portions of the full-scale range to deviate more. This also has the effect of widening the error band. More on that in a bit.
It’s easy to get a little mixed up when sorting out accuracy specs and the calibration process. To the uninitiated, the terms can all start to sound the same.
To be clear, BFSL is not a calibration method. Neither is Straight Line. They’re comparisons. They’re quality tests. They serve to tell you if your sensor is within spec, or if it’s a reject.
BFSL is a way of measuring the sensor’s accuracy.
You might wonder why it’s important for you to know about BFSL. After all, most manufacturers use this standard to test their sensor accuracy. But not all.
There are more ways than one to measure sensor accuracy. Now you know how we do it, so you can understand us when we state ±0.25% full scale (BFSL).
If you have questions about pressure transducer accuracy, calibration, or accuracy quality tests like BFSL, let us know. We're happy to take a deep dive to make sure you're getting the accuracy you need.
top photo credit: Thomas Leth-Olsen via flickr cc
