Domestic gas meters

The gas meter measures how much gas a home uses, so the supplier can bill for it. For you, it's also a test instrument: you read it to gas-rate appliances, and you need to know its type to work out installation volumes for tightness testing.

What a meter is

IGEM/G/1 describes a gas meter as an instrument that measures, stores and displays the quantity of gas that has passed through it. A smart meter is a meter with extra functions, such as two-way data communication with the supplier.

Primary and secondary meters

• The primary meter is the meter nearest to, and downstream of, the service pipe. It measures the gas the supplier provides, and its reading is the basis of the gas bill.
• A secondary meter is any other meter, used to measure gas that one person provides for another, for example a landlord recharging tenants. It's installed in the installation pipework, after the primary meter.

A secondary meter doesn't have to be legally stamped, and doesn't normally have its own regulator. Otherwise it's installed to the same requirements as a primary meter.

Meter types

Two kinds of domestic meter are in service:

• Diaphragm meters. IGEM/G/4 describes these as positive displacement meters whose measuring chambers have flexible walls (diaphragms). The chambers fill and empty in turn, passing a set volume each cycle (the cyclic volume), and the index counts the cycles. New ones must meet BS EN 1359.
• Electronic meters measure gas without moving chambers, for example ultrasonic or thermal mass meters. New ultrasonic meters must meet BS EN 14236. Smart meters can be either type.

As a billing arrangement, a domestic primary meter is a credit, prepayment or smart meter.

Identifying a meter

The designation tells you the meter's type and size:

You may also meet the larger U16 (G10) meter.

A 6 m³/h meter meets the needs of most homes. IGEM/G/13 caps the load of a home on a low pressure supply at about what a U6 can deliver: 65 kW gross (see 3.1). If a calculation shows a bigger meter is needed, the design follows IGEM/GM/6.

The meter's serial number follows IGEM/GM/PRS/50. On a domestic meter it is 14 characters long:

• G4 (diaphragm) or E6 (electronic), then
• a letter for the manufacturer, then
• a sequence number. On a meter made for a semi-concealed meter box, the first digit is a 5. Then
• two digits for the year of manufacture, and a version code.

So G4K00010031901 is a diaphragm meter from manufacturer K, made in 2019.

The meter type matters for tightness testing: each type has its own internal volume, which goes into the installation volume calculation (see 6.3).

Approved and stamped

Every primary meter must be approved by the National Measurement and Regulation Office (or an authority it accepts) and stamped before delivery. This is the meter's metrological seal, required by the Gas Act. Only an authorised meter examiner may break and remake that seal.

Reading a meter to gas-rate an appliance, including electronic and smart meters, is covered in 3.5.

Sources: IGEM/G/1 Ed. 3 Section 5meter, smart meter GSIUR 1998 reg. 2(1); IGEM/G/4 Ed. 2primary, secondary meter GSIUR 1998 regs 16(1), 17; BS 6400-1:2016 6.4.2, 11.3secondary meter rules IGEM/G/4 Ed. 2diaphragm meter, cyclic volume, meter examiner BS 6400-1:2016 6.4.1meter requirements Gas Safe Register TB 112 (2017)smart meter types IGEM/GM/PRS/50 4.2serial numbers IGEM/UP/1B Ed. 4 Table 7designations IGEM/G/13 3.165 kW IGEM/G/11 Ed. 2 Table 1situations 2.5, 2.6

▸Tutor notes

• Show learners several meters (or photos of data plates) and have them read the designation and serial number: type, manufacturer letter, year, and whether it's made for a semi-concealed box.
• Ask why a prepayment meter can't feed a secondary meter, and what's needed where a secondary meter is fitted.
• The sources reviewed so far don't describe how ultrasonic and thermal mass meters measure gas, so this lesson doesn't either.