Reliable monitoring of humidity (“Dew point”) in compressed air systems
OBJECTIVE
Qrometric were asked by Ipsen Biopharm to supply a system that delivered reliable, accurate and continuous monitoring of the humidity (moisture) content of their class 2 compressed air supply system.
TECHNICAL NOTE
In this application the measured humidity parameter is ‘dew point’. The colder the dew point (°C dp ) temperature of a gas, the drier it is. Class 1 systems need to be maintained below -70°C dp and class 2 systems below -40°C dp*
*when expanded to atmospheric pressure.
**technically this is actually a frost point temperature, but the term is used loosely in the ISO 8573 compressed air quality standard (see forthcoming discussion paper on metrology in compressed air applications for clarification).
ISSUE
Compressed air systems typically employ drying systems that are measured and controlled with humidity sensors that use an Aluminium Oxide substrate whose capacitance varies with the dew point. However, these sensors often provide unreliable measurements, especially when exposed to continuous dry conditions. The substrates dry out, take a long time to react to increasing humidity and effectively read far too low for variable time periods.
As discussed in an NPL paper from 2004 drift can be very significant. As this is a time dependent drift effect, it is very difficult to compensate for or calibrate out. Manufacturers tend to replace sensors during ‘calibration’ at the end of the service interval. This caused the customer audit trail, compliance and change control issues.
Having understood the fundamental nature of the problem, the customer decided a different approach was required.
Note the difference from the dew point shown on the MBW 973L to the temperature shown on the Class 1 dryer display panel.
Qrometric specialise in the accurate measurement of humidity and the calibration of both dew point and relative humidity sensors. We suggested the permanent installation of an MBW chilled mirror reference hygrometer to continuously validate the measurement data from the dew point sensors: effectively, a ‘live’ calibration system. As discussed in the 2004 NPL paper, drift of dew point chilled mirrors is negligible over time and have accuracy specifications of better than ±0.1°Cdp over the calibrated range of the instrument.
SOLUTION
Qrometric supplied and installed three MBW Calibration 973L chilled mirror dew point hygrometers. Two 973L were installed as live monitors on each of the driers and one used as a transfer standard (ISO17025 accredited calibration).
973L Key Specifications and Features:
Frost/dew point range: -80 to + 20 °C
Calibrated frost/dew point range: -70 to +20 °C
Optional Temperature: -50 to +100 °C
Fundamental drift free humidity/moisture measurement.
Integral frost/dew point measuring head
Optimal Response Injection System for fast response spot checks.
Pressures up to 20 bar, with integrated sensor
Intuitive, easy to use LCD touch screen user interface.
RESULT
- ±0.1°C dew point measurement accuracy with continuous monitoring on the class 2 system.
- Dependable and traceable historical data is now available.
- The improved accuracy and reliability means driers can be used as required, resulting in cost and energy savings.
This upgrade helped form part of the continuous improvement process resulting in Ipsen Biopharm (Wrexham) achieving Shingo Bronze Medallion status in April 2019.
Subsequently an equivalent solution has been planned for installation in Q1 2020 on their Class 1 compressed air system at the same site.
References: NPL 2004 AN EVALUATION OF PERFORMANCE OF TRACE MOISTURE MEASUREMENT METHODS
S.A. Bell et Al National Physical Laboratory, Teddington, United Kingdom, BOC Edwards, Crawley, United Kingdom
Contact Qrometric to discuss your Compressed Air Dryer Monitoring requirements on 0207 099 5807 or use the contact page to send an enquiry.