# How are CO, CO2 and moisture tested at medical gas outlets?

**How are impurities such as carbon monoxide, carbon dioxide, moisture and oil tested at medical gas outlets?**

Under HTM 02-01 (2026), the NHS England guidance, the Quality Controller (MGPS) samples gas at terminal units with electronic analysers calibrated on site, or detector tubes, for impurities such as carbon monoxide, carbon dioxide and water vapour; oil is tested at the compressor plant's test point and at a sample of outlets.

Impurity testing is part of the quality control (QC) testing described in chapter 20 of HTM 02-01 Part A (2026 edition). It is the responsibility of a Quality Controller (MGPS) and is carried out on new installations, after repairs and modifications, and periodically: gases made on site by medical and surgical air compressor plants, synthetic air plants and oxygen generator plants should be tested quarterly. On new installations the supply source should be tested before it is brought online, to avoid introducing contamination into the pipeline. Gas identity is covered in [identity checks at outlets](https://library.ultracontrolo.com/answers/how-is-gas-identity-checked-at-outlets/), and the full acceptance of a new air plant in [medical air commissioning](https://library.ultracontrolo.com/answers/how-is-medical-air-tested-at-commissioning/).

## Instruments

- Electronic analysers are preferred for repeatability, resolution and accuracy, and should be calibrated on site before testing with certificated calibration gases.
- Detector tubes can still be used for the pharmacopoeial tests for oil, carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen oxides and water vapour, but HTM 02-01 notes that they are single-use, can be affected by high humidity or low temperature, and may show cross-sensitivities.
- Polytest tubes, which respond to a range of compounds such as volatile hydrocarbons, may be used as an optional general contamination test on a representative sample of terminal units.
- Water vapour at terminal units should be measured with an electronic test device; oil with an appropriate test device, for example an oil detection tube or an oil impactor.

## Where samples are taken

- QC testing should start at the terminal units furthest from the area valve service unit.
- Water vapour at the terminal unit is tested to find contamination of the pipeline; the same test at the compressor plant confirms dryer performance. Hoses should be purged first, because they often read high at first after absorbing water vapour while there is no flow.
- Oil is tested at the plant test point of newly installed compressed air plant before it supplies any pipeline, and on existing plant at the routine frequency; a representative sample of terminal units on new and modified compressor-fed systems is also tested.
- For ad hoc investigations, sampling is considered at end-of-line and random terminal units and at the source.

## Limits in HTM 02-01

| Impurity | Limit | Applies to |
|---|---|---|
| Carbon monoxide | 5 ppm (v/v) | Terminal units |
| Carbon dioxide | 500 ppm (v/v) | Medical, surgical and dental air |
| Carbon dioxide | 300 ppm (v/v) | Oxygen, nitrous oxide and oxygen/nitrous oxide pipelines |
| Oxides of nitrogen (NO and NO2) | 2 ppm (v/v) | Gases for which Table 34 lists the test |
| Water vapour | 67 vpm, about -46 °C atmospheric dew point | Terminal units; dental air is accepted at a -20 °C dew point |
| Oil | 0.1 mg/m³ | Compressed air plant test point and sampled terminal units |

Terminal units are also tested for sulphur dioxide, but HTM 02-01 gives different limits for some gases in its paragraph 20.77 and its Table 34, so this page does not quote one. Increasing or fluctuating carbon dioxide readings in air or PSA-generated oxygen can be an early sign of dryer failure or poor compressor maintenance. An odour check during purging and particulate testing covers many impurities qualitatively, but nitrous oxide, oxygen/nitrous oxide mixture and carbon dioxide are left out of it.

## Recording

Results are recorded as numbers for each terminal unit and test point where possible, and as pass or fail for particle and odour tests and for detector tube tests.

### Can detector tubes replace electronic analysers?

HTM 02-01 (2026) prefers electronic equipment but accepts detector tubes for the pharmacopoeial impurity tests, noting that they are single-use and can be affected by humidity, low temperature and cross-sensitivities.

### Why test water vapour at the plant and at the outlet?

Under HTM 02-01, the plant test confirms dryer performance, while the terminal unit test looks for water contamination of the pipeline itself.

## Sources

- [NHS England, HTM 02-01 Medical gas pipeline systems, Part A (2026), chapter 20, paras 20.3 to 20.86, Tables 33 and 34](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [NHS England, HTM 02-01 publication page (2026 edition, Parts A and B)](https://www.england.nhs.uk/publication/health-technical-memorandum-02-01-medical-gas-pipeline-systems/)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/how-are-impurities-tested-at-medical-gas-outlets/