We have a transducer which is rated for 0-2000 psi, and it outputs 0-5 Volts. The specification document states that the accuracy is a combined LHR of +/- 0.5 %FS, how would we check this?
I am trying to create a test to check the transducer to ensure it is working properly. If I test the transducer at every 200 psi, how much allowable error can I expect to see based on the accuracy spec provided?
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In order to check the LHR (linearity, hysteresis, and repeatability) error of a pressure transducer, firstly the reference baseline for linearity errors needs to be chosen from one of the following three error calculation methods:
Absolute Accuracy – Straight line for a 0…2000 psi against 0 and 5 volts output device representing the best possible theoretical performance. This method is mostly used for pressure indicators and calibrators, since they are ready to use without any initial calibration adjustment.
Best Straight Line (BSL, BFSL) – Best fit straight line fit through all the calibration points. This is mathematically more complicated, because the results have to be processed to determine the best straight line. Manufacturers often use this method for millivolt output strain gauge type pressure transducers because it produces the best error for marketing purposes and indirectly shows the potential accuracy achievable if the 0 and 2000 psi reading is set to the output at the points on the virtual best straight line.
Terminal Straight Line (TSL) – Straight line drawn between the output at 0 psi and the full scale output at 2000 psi. This method cancels out any zero or span offset error which is often calibrated out when setting up the measurement instrumentation. However it tends to produce errors which are somewhere between method 1 and 2. This method is commonly used with amplified voltage and current output pressure transducers, since the installer can simply set the reading at zero and full scale pressure, and be confident that the transducer readings will be within spec.
Manufacturers do not always explain which method is used, but as mentioned above one would expect it to be TSL, but there is no guarantee, and if there is no obvious clue to which method was used, then the cautious approach is to to use the absolute accuracy method which will produce the largest errors. If it passes then you can assume it will easily pass using either of the other two methods.
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You can use this tool to calculate the theoretical perfect output for each calibration point to determine the baseline when comparing with the actual readings: 0…2000 psi to 0…5Vdc output calculator.
The fail error of 0.5% FS would be equivalent to 10 psi or 0.025 V at each calibration point.
To ensure you are checking each component of the L. H. R. we would suggest:
Linearity (L) – Check increasing 200 psi increments up to 2000 psi, it is important that each calibration point is taken in one direction.
Hysteresis (H) – Check decreasing 200 psi increments from 2000 psi back down to 0 psi, it is important that these readings are taken following the previous step without releasing the pressure at 2000 psi between the two steps.
Repeatability (R) – Repeat the previous 2 steps for linearity and hysteresis
These are the suggested psi calibration points in order of testing:
- 0, 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, 2000
- 2000, 1800, 1600, 1400, 1200, 1000, 800, 600, 400, 200, 0
- Repeat x 2
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