Pressure switches vs pressure transducers for gas alarms

HTM 02-01 and the New York State master specification below name pressure switches and pressure transducers as alternatives for initiating medical gas alarms. Of the documents on this page, only HTM 02-01 states a preference between them.

What each document says

• HTM 02-01 (2026), NHS England: plant alarm systems comprise relays and pressure switches or transducers to initiate the alarms (14.58); an area valve service unit (AVSU) may incorporate provision for pressure gauges, pressure switches or transducers on separate bosses with minimum bleed points (15.151); and transducer-type sensors are preferable to mechanical switches (15.176).
• HTM 02-01, area alarms: the alarms should display the pressure and vacuum readings on the alarm fascia (14.43), and transducers should not be capable of being isolated by a separate shut-off valve and should be connected to the pipeline using a minimum-leak connection device (14.44).
• SHTM 02-01 Part A (Scotland, Version 2, August 2026), area alarm systems: for each gas service there should be local pressure transducers for low pressure, and high pressure transducers are also required where there is any combination of oxygen, nitrous oxide and medical or surgical air installed together (10.11).
• NFPA 99, 2021 edition text, shown struck through under committee input CI-1071 in a 2021 NFPA committee report: all master, area and local alarm systems shall include alarm switches and sensors installed so as to be removable and accessible for service and testing, and power for master alarms, area alarms, sensors and switches from the life safety branch of the essential electrical system; area alarm panels for medical gas systems shall provide visual and audible indication if a mismatch occurs between the transducers and their associated circuit boards. NFPA lists the 2027 edition as current.
• New York State Office of General Services master specification (2024), a specification template edited for each project, Section 226400 Medical Gas Alarms: pressure switches or transducer sensors are specified as one item, with continuous line monitoring and electrical connections for the alarm system; vacuum switches or pressure transducer sensors are specified the same way.

Where HTM 02-01 places the sensors

HTM 02-01 says pressure sensors to provide the alarm function should be fitted to pipeline distribution systems, in adequately ventilated locations with appropriate access for maintenance. They should be located downstream of the AVSU isolation and prior to any terminal units, and no valves, apart from MSU isolation valves, should be installed downstream of an AVSU or after the pressure switches. They may be incorporated within the AVSU enclosure on the downstream side; otherwise the pressure sensor should be above the local alarm, accessible for maintenance, and the pressure switch location should be clearly identified at the local alarm. All pressure sensors should be connected to the pipeline by means of a minimum leak connector.

Testing and maintenance

• HTM 02-01 says alarm transducer testing should be undertaken annually, both for area alarms and for pressure-reducing station alarms (14.46, 14.79).
• In the HTM 02-01 Part B list of engineering tests, the local alarm function test checks that the set points of pressure switches or transducers are correct using digital gauges (Table 2).
• Pressure switches should be excluded from the carcass pressure test, which should include only first-fix carcass components (Part A, 19.105).
• Part B gives changing pressure switches as an example of work that may involve isolating a ward's gas supply (7.126), and notes that MGPS alarms will often be triggered by necessary work such as replacing transducers (11.155).

Ultra Controlo pressure switches

Ultra Controlo lists a low pressure switch that activates when pressure drops below the set threshold, and a medical gas pressure switch that monitors pipeline supply pressure and triggers alarm or changeover actions, both for oxygen, air, vacuum and nitrous oxide. See low pressure switch.