Domestic gas meter types and components

What a meter measures

A domestic gas meter measures the volume of gas passing to the installation, so the supplier can bill for what is used. Its primary job is measurement: the regulator controls pressure and the ECV provides emergency isolation. Some smart meters also contain a shut-off valve the supplier can operate remotely, but that does not replace the ECV.

The conventional domestic credit meter is a positive-displacement diaphragm meter. It repeatedly fills and empties internal chambers of known volume and counts the cycles, so its index is a running total of volume. Some models compensate internally for gas temperature; the index still shows volume. Engineers also use the index to time a test dial and work out an appliance's gas rate. A smart display inside the home is not the primary meter and must not be used as the test index.

Size designations

• U6: the conventional domestic diaphragm meter size, rated for a maximum of about 6 m³ of gas per hour. The 6 is a flow rating, not an age, an appliance count or a distance.
• E6: the ultrasonic (electronic) meter of similar capacity.
• U16 and U25: larger diaphragm meters.
• G-sizes such as G16 and G65: European size designations for larger meters of several types, including rotary and turbine meters on commercial and industrial loads.
• DN15 and DN20: nominal pipe sizes, not meter sizes.

The rating matters when appliances are added: their combined maximum demand must not exceed the meter's capacity. Above it, more pressure is lost through the meter and some appliances may not get the gas they need. The rating is a limit for choosing the meter; the meter does not cut off at the badge figure.

What the markings tell you

A size code is not always the maximum flow. The Elster BK-G4, for example, has a stated measuring range of 0.04 to 6 m³/h. Its G4 label must not be read as a 4 m³/h maximum. Qmin and Qmax identify the ends of the specified range: a reference flow below Qmin is outside that range, even though gas may still be passing.

Keep the other plate figures separate:

• pmax is the meter's maximum allowable operating pressure. It does not set the regulator's normal delivery pressure or the pressure for a test.
• Cyclic volume is the displacement of one complete measuring cycle. It is neither the running total on the index nor the gas-holding volume of the installation for a purge calculation.
• On the BK index, V identifies volume at metering conditions, Vtb volume converted to base gas temperature, and Vb volume converted to base pressure and temperature. Read the suffix before interpreting the figure.

A pulse output also has its own scale. With an interface marked 0.01 m³ per pulse, 23 recorded pulses represent 0.23 m³, or 230 litres. You also need the time to turn that volume into a flow rate. Use the pulse value marked on the actual interface: they differ.

For the BK meter, removing or damaging the calibration seal ends its approval for measurements that are subject to statutory testing. A readable index or a good gas-tightness result does not bring that approval back. The regulator seal protects a different part.

The Landis+Gyr G470 illustrates a different measuring principle: it uses ultrasonic travel time, with no moving measuring parts. A smart display or remote reading does not tell you whether the meter itself is ultrasonic or a diaphragm meter.

Reading the index

• Metric meters read in cubic metres (m³).
• Older imperial meters read in hundreds of cubic feet. One cubic foot is 0.0283 m³, so multiply a reading in hundreds of cubic feet by 2.83 to get m³.
• Read the unit first: mixing up the two, or misreading the dials, is a common error.

For a billing reading sent to the supplier, use the whole-unit digits and leave out the red fractional digits. For a timed gas-rate check, read the test dial or fractional index changes required by the method instead. Ignoring those small changes could hide the volume used during the test.

The bill is for energy, so the supplier converts volume to kilowatt-hours using the calorific value and a correction for temperature and pressure:

kWh = volume (m³) × 1.02264 × calorific value (MJ/m³) ÷ 3.6

The 1.02264 corrects the metered volume to standard temperature and pressure; dividing by 3.6 turns megajoules into kilowatt-hours.

Worked example

Over a month, the later reading minus the earlier one comes to 120 m³ on a metric index, and the supplier’s calorific value is 39.4 MJ/m³.

1. Corrected volume: 120 × 1.02264 = 122.7168 m³.
2. Energy: 122.7168 × 39.4 = 4835.04 MJ, and 4835.04 ÷ 3.6 = 1343.07, so about 1343 kWh.

On an imperial meter, multiply the reading difference by 2.83 first, then carry on from step 1.

The other parts at the meter

• ECV: shuts off and isolates the supply in an emergency. It needs a securely fitted handle that turns without tools, and a label that identifies it, shows on and off and gives the emergency number. On a lever valve, in line with the pipe is open and across the pipe is closed. In a suspected escape, operate it only if it is safe and readily accessible.
• Regulator: controls the outlet pressure. It is not a user adjustment.
• Test point (test nipple): a small sealed tapping on the meter outlet or regulator. Loosen the screw and connect a manometer to read standing pressure (no appliances running), working pressure (with the specified appliances running) and let-by and tightness readings. Afterwards, retighten the screw and check the point with leak detection fluid.
• External meter box: ventilated so escaped gas disperses outside, with a key-operated door so the user keeps access to the meter and ECV.