Pressure and its units
Millibar, pascal and gauge pressure, and the pressures found in a domestic installation.
This topic supports
- M03.1Convert between common pressure units and explain gauge pressure.
Draft: this lesson hasn't yet been technically reviewed.
Almost every check a gas engineer makes involves pressure: tightness tests, the meter outlet check, burner pressures. Before any of that, you need to know what the numbers mean.
Gauge pressure
A gauge reads pressure above the surrounding air. That's why you zero it at atmospheric pressure before a test. Pressures in gas standards are gauge pressures unless they say otherwise, and so are all the pressures on this site.
Units
Domestic gas pressures are given in millibar ().
You may see the pascal (Pa) or kilopascal (kPa) on electronic gauges and appliance data, so check which unit the display is set to before you read it.
The pressures in a domestic installation
| Where | Typical figure | Where it's from |
|---|---|---|
| Low pressure supply at the ECV | Up to 75 mbar counts as "low pressure". A working pressure of 25 mbar or more at the ECV outlet means the supply is fine | IGEM/G/13 Table 1; IGEM/UP/1B |
| Meter outlet (operating pressure) | Nominally 21 mbar. With all appliances at high load, 18.5–23 mbar after a minute | IGEM/G/13 4.1, 5.2 |
| Loss across the meter | No more than 4 mbar | IGEM/G/13 Table 1 note 1 |
| Loss in the pipework, meter to appliance | No more than 1 mbar | BS 6891 5.3.2 |
| Tightness test | Let-by test at 7–10 mbar, test at 20–21 mbar | IGEM/UP/1B Ed. 4 5.3.2.1 |
What each of these tells you, and when to measure it, is covered in 3.3 and 3.4.
How much gas a domestic supply can deliver
IGEM/G/13 caps the gas load of a home on a low pressure supply at 65 kW gross heat input, about what a U6 meter (up to 6 m³/h) can deliver. But don't assume an existing service was designed for that much. It was sized for the property and appliances when it was laid. Where the gas transporter has fitted an ECV label showing the service capacity, check it before adding appliances. A bigger load may need a larger service, which may be chargeable.
Sources: IGEM/G/13 2.3, 3.1gauge pressure, 65 kW BS 6891:2015 Table 1 noteunits IGEM/G/13 4.1, Table 1pressure limits BS 6891:2015 5.3.21 mbar IGEM/UP/1B Ed. 4 2.6, 5.3.2.1test pressures CCN1 Issue 16 §7, §10assessed
Tutor notes
- Ask learners to convert 21 mbar to pascals and kilopascals, and 2.1 kPa back to mbar.
- Ask why a gauge is zeroed before use, and what a reading of "0" means.
- Use the table to ask: if the ECV outlet reads 26 mbar and the meter outlet 17 mbar, where would you look first? (See 3.3.)