Best practice for sensor calibration

Calibration of sensors in a compressed air system should be conducted regularly to ensure accurate measurements. The frequency of the sensor checking should be proportional to the safety-critical nature of the process or dependant on the sensor’s impact on process variability.

For critical measurements, sensors should be checked frequently to ensure accuracy. If a measurement point significantly influences process control - for example, 10% or higher - the sensor should be checked regularly until sufficient data is collected to extend the checking interval. 

For less critical measurements that have, for instance, only a 0.01% influence on the control of the process, the checking interval can be extended or, in some cases, stopped altogether based on collected data.

Methods for checking and calibrating sensors

There are two primary methods for checking and calibrating sensors in a compressed air system: routine in-situ checking and laboratory calibration. Both methods require accurate recording and documentation of the check/calibration procedure.

Routine in-situ checking: This involves using a portable checking device to verify the accuracy of the installed sensor without removing it from the process. The portable device is brought to the sensor's measuring point, and readings are compared with the live online sensor. 

To ensure reliability, a minimum of three measuring points should be checked across the sensor's likely working range. This is known as an ‘as found’ check as opposed to a calibration or recalibration.  It is quick and non-disruptive to the process, but less accurate than laboratory calibration.

Laboratory calibration: This is a more thorough and accurate method, which involves removing the sensor from the compressed air system and calibrating it in a controlled environment, such as an approved laboratory or workshop. 

The sensor is calibrated using a reference standard with an inaccuracy factor lower than the device being checked. It is a controlled process, using a trained operator to calibrate the sensor, across multiple points, ensuring accuracy throughout its range. 

This method typically includes both an ‘as found’ check and, if necessary, an ‘as left’ (adjusted) calibration to bring the sensor back into specification. While laboratory calibration is more accurate, it requires more time and a spare calibrated sensor to be installed temporarily to avoid process interruption.

Examples of routine checks

These two examples of routine checks in a compressed air treatment system illustrate the importance of regular sensor maintenance and calibration:

• Dew-point transmitter on dryer outlets: Regular spot checks of the dew-point transmitter are necessary to verify moisture content at the dryer outlet, ensuring air quality standards are met. Accurate dew-point measurements prevent moisture-related issues like corrosion in downstream equipment
• Temperature PT100 sensor on dryer inlet: The temperature of compressed air entering the dryer impacts the efficiency and life cycle of the dryer columns. As the air temperature increases, so does the water vapour content, requiring the dryer to work harder. Regular spot checks of the PT100 sensor at the dryer inlet ensure optimal dryer operation, preventing excessive wear and maintaining consistent air quality.

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