It is vitally important that the compressed air and gases used in medical applications for patient care are clean and nearly sterile. Contaminants such as water vapor, carbon monoxide, carbon dioxide, nitrogen oxides, solid particulate and oil aerosols are common in compressed air and other gases, and, if left uncontrolled, are harmful to patient health.
In the case of water vapor, there are two key concerns:
Not surprisingly, there is legislation in place to set standards for moisture levels in medical gases. The 2002 European Pharmacopoeia, published by the European Department for the Quality of Medicines in Strasbourg, contains a monograph describing the method recommended for the determination of moisture content of medical gases. The recommended maximum allowable moisture concentration is 67 parts per million by volume (-46 °C dew point).
In the UK, the correct use and control of medical gases is regulated by the Health Technical Memorandum. This document was updated to HTM02 and includes the recommendation that “All medical gas supplies should be continuously monitored for dew point or moisture content.”
The HTM regulations give specific information on the specification and measurement of dew point or moisture content.
The relevant section states:
15.146 “The plant test point and a representative sample of terminal units distributed throughout the pipeline systems should be tested for total water content. The water content must not exceed 67 ppm(v) (equivalent to an atmospheric pressure dew point of approximately -46 °C). The typical water content of medical gas cylinders is normally below 5 ppm(v). Water vapor content may be measured using the appropriate test device described in Appendix E (see also paragraph 15.117).”
Whilst paragraph 15.117 tells us:
15.117 “An electronic dew-point meter should be used in preference to water content measurements”.
More details are given in paragraph 7.41:
7.41 “The dryer control system should ensure that regeneration is operated in proportion to the compressed air usage. The effectiveness of the control system will become apparent when the efficiency of the compressor system is tested at 10 % and 0 % of the system design flow. Evidence of the reliability and performance of a dryer system should be sought from manufacturers, since these items are critical to the overall performance of the compressor system. The dryer control system should include a dew-point hygrometer and display with a minimum accuracy of ±3 °C in a range from -20 °C to -60 °C atmospheric dew point, with an alarm set point of -46 °C. (Note that this supersedes the HTM 2022 requirement for an alarm trigger point of -40 °C). It should be arranged that in the event of open circuit, a “plant emergency” alarm is initiated.”
Our dew point meter ranges from Michell Instruments are suitable for both portable, fixed-point measurements, and spot checking.
The Michell MDM300 high-speed portable dew-point hygrometer uses the latest ceramic metal oxide trace moisture sensor for rapid spot-check humidity measurements. The MDM300 can provide measurement in both dew-point and moisture content reaching 67ppmV in less than 10 minutes.
The Michell Easidew Advanced Online Hygrometer permanently monitors moisture in the gas line. It is available with an optional pressure sensor for live pressure compensation. When combined with the Easidew Sampler it provides a complete, easily installed system for continuous monitoring.
As mentioned above, moisture is just one of many contaminants that need to be controlled in medical gases and breathing air. Our related article What Are The Key Measurements to Ensure Safety in Breathing Air? has a section focused on ensuring safety of hospital gases.
Find out more about our impedance dew-point transmitters, or contact us with your question.
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