Sooner or later, no matter what the application or the brand of your instrumentation; your pressure transducer is going to need recalibration. Prolonged exposure in a harsh environment, age, and frequency of use contribute to the drift of your sensor, and results in it falling out of calibration.
Some models of pressure transducers have to be sent in to the factory for recalibration. The housing on these sensors needs to be opened to make the adjustments. However, several models come with Zero and Span potentiometers that can be adjusted in the field for minor changes.
This can bring the pressure transducer back into specification, so you don’t have to send it out for re-calibration as often. However, this will NOT replace your regular calibration schedule from your quality program.
Making field adjustments to a pressure transducer is fairly straightforward. All you need is a known pressure source, multi meter, and two small screwdrivers.
The following steps show how to make general adjustments for an APG pressure transducer with adjustment pots.
An ideal zero is indicated by an output of 4 mA (4/20 mA), or 0 VDC (0-5, 0-10 VDC). If you need to adjust the zero point, do the following:
- Remove the protective screw above the Zero pot with a Phillips screwdriver (usually marked with a “Z”)
- Ensure that the transducer is at 0 psig or 0 psia (full vacuum if absolute)
- Using a small flat-head screwdriver, adjust the “Zero Adjust” pot until you have 4 mA (4/20 mA) or 0 VDC (0-5, 0-10 VDC) output.
Do not make changes to the Span adjustment as part of the zero trimming. The Span should only be changed as part of the re-calibration of a gauge with a known pressure source.
To do a span adjustment, you need a known pressure source of at least ±0.1% accuracy in order to get as accurate a result as possible. If this is not available, you can return it to the factory for re-calibration. If you need to adjust the span, follow these steps:
- Ensure that the transducer is at 0 psig or 0 psia (full vacuum if absolute), and adjust zero using the instructions above
- Apply full-scale pressure to the pressure port
- Remove the protective screw above the Span pot with a Phillips screwdriver (usually marked with an “S”)
- Adjust the Span pot with a small flat-head screwdriver until the full-scale signal is reached
- Re-check your zero adjustment and re-adjust as needed
- Repeat all the steps until adjustments are no longer required
By making Zero and Span adjustments in the field, you can keep your pressure transducer in service longer. You won’t need to send it in for recalibration until your quality standards dictate.
However, If at any point you question the accuracy of the pressure transducer after adjustments are made, it is highly recommended to send the pressure transducer to the factory or certified calibration house right away for recertification. As always, let us know if you have any questions about zero and span adjustments or pressure transducer calibration.