Dewpoint Measurement in Breathing Gases

The Importance of Measuring Trace Moisture

Breathing gases are essential in a wide range of applications, from medical life support to industrial safety and diving operations. Ensuring the purity and safety of these gases is therefore critical, with trace moisture measurement playing a vital role in maintaining gas quality and protecting users.

What Are Breathing Gases?
Breathing gases refer to any gas mixture intended for human inhalation and include medical air, oxygen (O₂), nitrous oxide (N₂O), and mixtures such as heliox. These gases are delivered via pipelines or cylinders for applications such as:
• Supplementary oxygen in hospitals, ambulances, and aviation
• General anaesthesia and respiratory support via ventilators
• Hyperbaric/SCUBA diving and life-support systems

These gases are supplied via cylinders, pipelines, or centralised systems, and their composition must meet strict safety and purity standards.

Suppliers deliver these gases under stringent control, following standards such as ISO 7396 (medical gas pipeline systems), ISO 18562 (biocompatibility of gas paths), the European Pharmacopoeia, CGA/ANSI G 7.1, and OSHA 1910.134.

How Moisture Causes Problems
Trace moisture, even at exceptionally low levels, can cause significant issues in breathing gas supply systems:
• Moisture promotes rust and degradation in cylinders, pipelines, and valves, compromising equipment integrity and causing corrosion of equipment and pipework.
• Elevated moisture levels within gas lines enable bacterial and mould growth, posing health risks and increasing the likelihood of microbial contamination.
• Water vapour can react with other gas components, leading to impurities and degradation of gas quality.
• In cold environments, moisture in the gas supply can freeze, blocking regulators and delivery systems.
• Excess moisture may irritate airways or worsen respiratory conditions, thereby increasing respiratory health risks.

Solutions
Dewpoint measurement ensures dryness and helps prevent microbial growth or corrosion in pipelines. Common solutions include:
• Online dewpoint sensors/transmitters for continuous measurement. Installed directly in the medical air pipeline or sampling line, these are often used with real-time alarms. A typical measurement range of –40 °C to –70 °C dew point is acceptable for medical air compliance.
• Portable dewpoint meters or handheld dewpoint hygrometers for spot checks at various sample points and for verifying pipeline dryness during commissioning or audits. Key features include fast response, battery power and data logging functionality.

Applicable Standards
The 2002 European Pharmacopoeia, published by the European Department for the Quality of Medicines in Strasbourg, describes the recommended method for determining the moisture content of medical gases. The maximum allowable moisture concentration is 67 parts per million by volume (–46 °C dew point).

The Health Technical Memorandum regulates the correct use and control of medical gases in the UK and was recently updated to HTM02. This includes the recommendation that “all medical gas supplies should be continuously monitored for dew point or moisture content.”

Key standards governing breathing gas purity and moisture content include:
• EU Pharmacopoeia: Medical oxygen ≤ 67 ppm H₂O (≈ –46 °C dew point)
• ISO 7396-1 / BS EN 7396-1: Governs pipelines for compressed medical gases, requiring safe, moisture-free installation
• UK HTM02: Continuous monitoring, max 67 ppm / –46 °C dew point; typical cylinder content ~5 ppm
• OSHA & CGA G 7.1: Grade D breathing air, max dew point –50 °F (~–45 °C)
• NFPA 99: Health care facilities

Conclusion
Trace moisture measurement is a critical aspect of breathing gas safety and quality. By implementing reliable measurement solutions and adhering to international standards, organisations can prevent equipment damage, reduce health risks, and ensure compliance.

The measurement of moisture in breathing gases is fundamental to:
• Ensuring patient safety by preventing equipment malfunction, infections, and compromised therapy
• Maintaining regulatory compliance under ISO, CGA, HTM, and Pharmacopoeia standards
• Enhancing the reliability of gas delivery systems by reducing corrosion and moisture-induced damage