What flow test is done on a medical gas outlet?

A medical gas outlet is called a terminal unit in HTM 02-01 and ISO 9170-1. Chapter 19 of HTM 02-01 Part A (2026 edition), published by NHS England, lists the tests every terminal unit should undergo, recorded on form B24:

• Gas specificity: calibrated solid test probes for each gas are tried in turn, and only the correct probe should enter the collar and lock.
• Probe retention and fixings: the probe locks in and passes a pull test, and the terminal unit is checked for secure fastening.
• Collar concentricity: the collar moves freely in the fascia plate, with a gap of no more than 2 mm.
• Anti-swivel pin: the probe rotates freely in a vertical-facing terminal unit and does not rotate in a forward-facing one.
• Probe seal leakage: no leaks are found while the probe is moved gently in all directions.
• Static pressure: the system pressure with no flow, read on a test gun fitted with a gas-specific probe, with the trigger not pressed.
• Flow pressure and flow rate: the pressure at the terminal unit under a prescribed flow, selected by fitting the appropriate flow jet and pressing the trigger.
• Labelling and identification to BS EN ISO 9170-1, a visual check.

Test flows in HTM 02-01 Table 2

The test flow is there to establish that the terminal unit works correctly and is not obstructed. Table 2 gives indicative values:

HTM 02-01 says the overall pipeline design should be based on a 5% pressure drop from the source of supply to the terminal unit outlet at these test flows.

System flow test after additions (form B26)

Where a system has been added to or modified, HTM 02-01 also requires a design flow performance test, recorded on form B26. Calibrated metered leaks, marked for example at 10, 20, 100 and 275 L/min on 400 kPa systems and 350 L/min on 700 kPa systems, represent the total diversified design flow less the flow of the test device, and a representative number of terminal units is then tested for pressure and flow. In a 28-bed ward, the sample includes the terminal units furthest from the AVSU and nearest the entrance, including the treatment room; in an operating department, a sample in each suite and 20% of the terminal units in recovery, with one oxygen terminal unit tested at 100 L/min to represent oxygen flush.

In service

Part B of HTM 02-01 (2026) asks for all terminal units to be tested quarterly: a blank probe checks gas specificity, mechanical function and leaks, and a test instrument checks flow and pressure drop. Blank test probes are only for leak tests; flow and pressure-drop tests need a calibrated flowmeter, pressure gauge and calibrated jets, and test equipment should be calibrated at least once a year. A schedule of the results for each terminal unit should be kept, with static and flow pressures, issues found and repairs made. See how often outlets are tested.

In Canada

A 2025 British Columbia training textbook (SkilledTradesBC) describes the inspection tests of the Canadian standard, citing CSA Z7396.1-12 (CSA Group now lists CSA Z7396.1:22 as the newest edition). Each terminal unit is tested for flow resistance: a test assembly with a 1.98 mm flow regulation orifice measures the static and then the flow pressure, and the pressure drop across the terminal unit and its inlet connector shall not exceed 70 kPa for nitrogen, 30 kPa for other medical gases and 13 kPa for medical vacuum. During the gas identity check, the pressure at each terminal unit is recorded at a flow of 15 to 25 L/min and must be the same as the static pressure.