# What causes a high dew point alarm on a medical air dryer?

**What can cause a high dew point alarm on a medical air dryer, and what happens when it activates?**

Under HTM 02-01 (2026), the NHS England guidance, dryness above −46 °C atmospheric dew point is a plant emergency, and a duty dryer failing on dryness should change over automatically to the standby dryer; the 2016 US DOE sourcebook adds that hotter or lower-pressure inlet air reduces a dryer's rated capacity.

## What the alarm means

HTM 02-01 (2026) sets the hygrometers on the dryer control unit to −46 °C atmospheric dew point, which its testing chapter treats as equivalent to about 67 vpm of water vapour, and lists dryness above −46 °C at atmospheric pressure as a plant emergency. When the duty dryer fails on dryness or pressure, the control unit should isolate it, bring the standby dryer online, activate the appropriate fault indicator and its volt-free contacts, and stay in that mode until the fault has been rectified. The failed indicator should remain until it is reset manually.

## Causes named in the sources

- Dryer failure. HTM 02-01 lists dryer failure as a plant fault for each dryer unit, and notes that increasing or fluctuating carbon dioxide readings can be an early indication of dryer failure or poor compressor maintenance.
- Demand above dryer capacity. In its guidance on using compressors for pipeline tests, the HTM warns that system demand should not exceed the maximum flow capacity of the dryers, otherwise wet air will result.
- Inlet conditions. The US Department of Energy sourcebook (third edition, 2016) explains that dryer ratings are usually based on standard inlet conditions and that an increase in inlet temperature or a decrease in inlet pressure reduces a dryer's rated capacity, with correction factors from most manufacturers. HTM 02-01 asks for manufacturers to be consulted on the operating temperature range, and says refrigerator cooling may be needed between the compressors and the dryer and filter set in extreme cases.
- Fouling and maintenance. The Compressed Air Challenge fact sheets (May 2026 revision) recommend a coalescing filter before a desiccant dryer to prevent fouling of the desiccant bed, note that inadequate maintenance can lead to poor moisture control, and say a drain trap stuck closed lets condensate back up and be carried downstream.

## Checking the reading

HTM 02-01 says the water vapour test at the compressor plant confirms dryer performance, while the test at terminal units looks for water in the pipeline. The effect of flow rate through the dryer and of sampling time on the instrument reading should be taken into account. Terminal units fed by flexible hoses often exceed the limit on first testing because of absorption of water vapour in the absence of flow; the hoses should be purged before testing, and results can be compared with nearby terminal units on rigid pipework. Failure to achieve the minimum dew point standard should not be accepted, and remedial action should be discussed with the Authorised Person (MGPS).

## Ultra Controlo dryers

The ULTRADRY adsorption dryer accepts compressed air inlet temperatures from +5 °C to +50 °C and has a coalescing filter at the inlet and a dust filter at the outlet. ULTRAMED treatment units change columns according to dew point and have a low dew point indicator alarm, and the ULTRAAR ME uses two integrated ULTRAGEST systems to monitor and control the dew point continuously.

### What happens to supply when a dryer fails on dryness?

Under HTM 02-01 (2026), the dryer control unit should take the duty dryer offline and bring the standby dryer online automatically, and dryness above −46 °C is reported as a plant emergency.

### Can carbon dioxide readings warn of dryer problems?

Yes. HTM 02-01 notes that increasing or fluctuating carbon dioxide readings in air or PSA-generated oxygen can be an early indication of dryer failure or poor compressor maintenance.

## Sources

- [NHS England, HTM 02-01 Medical gas pipeline systems, Part A (2026), paras 7.17, 7.87, 7.89, 19.113, 20.70 to 20.74 and 20.76](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)
- [US DOE and Compressed Air Challenge, Improving Compressed Air System Performance: A Sourcebook for Industry, third edition (2016), Section 1, Dryers](https://www.energy.gov/sites/prod/files/2016/03/f30/Improving%20Compressed%20Air%20Sourcebook%20version%203.pdf)
- [Compressed Air Challenge, Fact Sheet #11 Proven Opportunities at the Component Level (May 2026, Rev. 01)](https://www.compressedairchallenge.org/data/sites/1/media/library/factsheets2026/fact-sheet-11-proven-opportunities-at-the-component-level_updated2026.pdf)
- [Compressed Air Challenge, Fact Sheet #5 Maintenance of Compressed Air Systems for Peak Performance (May 2026, Rev. 1)](https://www.compressedairchallenge.org/data/sites/1/media/library/factsheets2026/fact-sheet-5-maintenance-of-compressed-air-systems-for-peak-performance_updated2026.pdf)
- [ULTRADRY datasheet, pp. 2 and 3](https://www.ultracontrolo.com/products/ultradry)
- [ULTRAMED datasheet, p. 3](https://www.ultracontrolo.com/products/ultramed)
- [ULTRAAR ME datasheet, p. 2](https://www.ultracontrolo.com/products/ultraar-me)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/what-causes-a-high-dew-point-alarm-on-a-medical-air-dryer/