To achieve success, all manufacturing processes depend on operational consistency and repeatability within carefully defined parameters. This is especially true of compressed air systems, where effective control relies on the accuracy and dependability of measurement systems and sensors. If one of these devices fails, or functions incorrectly, the integrity of part or all of the compressed air system – and the associated manufacturing process - can be called into question.
Compressed air quality is governed by the ISO standard 8573. This is divided into nine sections, with ISO 8573-1 defining six classes of air contamination and purity classes, covering oil, water and particulate matter. The remaining eight sections – ISO 8573-2 to 8573-9 – focus on test methods for each contaminant, ranging from the detection of liquids and solids to gases and aerosols, and including organic solvents and microbial organisms. For more information, see our blog ISO 8573-1 Moisture Measurement in Compressed Air.
Clearly, compliance with ISO 8573 depends on the accuracy and reliability of sensors, either deployed on-line or in spot-checking instruments.
Sensor drift is a common phenomenon in devices used for measuring process conditions such as temperature, pressure, flow or dew point. The accuracy of the device can be impacted by a variety of factors, including aging of mechanical and electrical components, fluctuations in the operating environment and repeated contact with contaminants in the process media. These changes can result in a gradual shift in the output of the sensor, which can lead to errors in the measurement and may impact the reliability of the device.
To minimize the impact of sensor drift, it is important to perform regular calibration and maintenance procedures to ensure the device is functioning correctly and producing accurate measurements. In addition, using sensors that are designed to withstand harsh process conditions – such as high temperatures, corrosive materials and extreme environments – can help to minimize the effects of drift and improve the overall accuracy and reliability of the measurement.
In much the same way that other mechanical production systems – compressors, dryers, power transmission units, conveyors etc. – undergo routine maintenance to ensure their operational efficiency, so too should the sensors that are used for process monitoring.
For dew-point sensors, spot checks can be carried out using a portable dew-point hygrometer, such as Michell Instruments’ Easidew Portable and MDM50, to verify the sensor measurements. This type of device provides a fast response and, if used regularly, will quickly pinpoint sensors that are out of tolerance.
As a general rule, sensors in safety-critical areas should always be checked first, and most frequently, followed by those devices that are vital to the efficiency of the process. With new systems or devices, it also makes sense to capture data regularly until sufficient information has been analyzed, and then to use the results to calculate future checking intervals.
Regular calibration is particularly important if the sensor has been in operation for an extended period of time. Most manufacturers will specify a recommended calibration schedule, which is typically at least once a year. However, this may vary depending on the specific device, the operating environment and the application for which it is being used.
It is important to emphasize that the need for recalibration may be more frequent for sensors that are operating in harsh or extreme conditions, or for those that are being used in critical applications where accurate measurements are essential.
Calibration is the process of comparing the output of each sensor against a precision reference instrument, such as one of Michell Instruments’ S1000 Chilled Mirror Hygrometers used with an ADG400 Dew-Point Generator, or the fully integrated HG10 Humidity Probe Calibrator. These instruments will be considerably more accurate – typically at least ten times – than the device being assessed, to avoid the tolerances of the reference instrument influencing the readings being gathered from the sensor under test. Generally, a calibration program will compare multiple readings that are repeated across the measurement range of the test sensor. This determines the degree of drift and enables correction factors to be applied to bring the output readings of the sensor into line with the calibration reference.
Dew-point sensor calibration can be carried out in house, especially if the number of sensors involved warrants the investment in precision reference instruments. Alternatively, sensors can be returned to the manufacturer for testing or be sent to a suitable accredited calibration laboratory. At Michell Instruments, for example, we operate an extensive UKAS-approved laboratory and can calibrate most moisture sensors and instruments. This work is carried out against test equipment that is traceable to NPL (National Physical Laboratory) and NIST (National Institute of Standards & Technology) standards.
For many customers, especially those with a relatively small number of dew-point sensors, our Sensor Service Exchange Program presents a better option. Under this arrangement, we supply a calibrated sensor in advance, which the customer swaps for their existing device; they then return the old sensor to us for recalibration or repair. This minimizes downtime for the customer and ensures that the quality and performance of their process or production operation is maintained.
It’s important to realize that process sensors are not ‘fit and forget’ devices. As we’ve explained above, they are prone to ageing and affected by environmental conditions. Regularly checking their condition and operating parameters, preferably as part of a documented and carefully managed maintenance regime, is therefore essential for both production efficiency and compliance with ISO 8573.
If you would like to know more about sensor calibration, talk to one of our technical experts.
Or visit our Dew-Point Calibration Services page.
We are the world’s leading experts in moisture monitoring and measurement. We have eight different technologies covering all moisture applications, backed by unrivalled technical and customer support. To learn more, talk to one of our application specialists today.
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