Meters · for gas engineers in training
How a gas meter counts
Nearly every UK home has a G4 meter, the modern name for the old U6. It is a box of four small compartments. Gas from the case fills one while the one beside it empties into the house pipework, then valves swap them over, and every swap moves the index on. Switch some appliances on and watch the drums, then lift the case to see the parts that do the counting.
What you are looking at
- Case
- Two pressed-steel halves, powder-coated, joined by a folded flange round the middle and held by four crimp clips. The whole case is full of gas at the inlet pressure. On the front of the lower half is the yellow gas emergency label: what to do if you smell gas, the emergency number 0800 111 999, and boxes for the meter exchange details.
- Inlet and outlet
- 1 in BS 746 threaded bosses on top, 152.4 mm apart, as on every UK U6 and G4 meter. Gas goes in on the left and out on the right; the arrow on the smell-gas label says so.
- Outlet test point
- A brass test nipple on the outlet boss. Loosen the screw half a turn, push on the gauge tube, and read the working pressure: about 21 mbar from the meter regulator (BS 6400-1).
- Index
- Eight drums: five black for whole cubic metres, three red for litres. You bill and record the black figures. The last red drum turns once every 10 litres; a magnet in it gives the pulse output, 1 pulse = 0.01 m³.
- Measuring unit
- Four compartments behind two diaphragms. Gas from the case fills one compartment while the one on the other side of its diaphragm is pushed out to the outlet, then the slide valves swap them over.
- Crank and valves
- Each diaphragm rocks a flag rod; links from the two flag arms turn one crank, which works the slide valves and drives the index through a gear train and a magnetic coupling. Every turn of the crank is 1.2 litres of gas.
Reading the index
The index has eight drums. The five black ones count whole cubic metres; the three red ones count litres (thousandths of a cubic metre). The reading is the black figures. The last red drum turns smoothly, one figure for every litre, and goes round once every 10 litres, which is what you watch on a test.
Where you'll meet it
Supply checks, meter exchanges and customer readings: record the black figures and the meter serial number.
From cubic metres to the bill
The meter counts volume, but gas is sold by energy. The supplier converts with the volume correction factor and the calorific value (CV) of the gas that day, about 39.5 MJ/m³:
kWh = m³ × 1.02264 × CV ÷ 3.6
So one cubic metre is about 11.2 kWh on the bill.
Where you'll meet it
Explaining a bill to a customer, or checking a high-consumption complaint against the index.
Gas rating an appliance
With only the appliance under test running, and it has been running for 10 minutes, read the index, time exactly 2 minutes, and read it again. Then:
m³/h = litres used × 30 ÷ 1000 · kW gross = m³/h × CV ÷ 3.6
Compare it with the heat input on the data plate (gross or net: net ≈ gross ÷ 1.11 for natural gas). Try “Time 2 minutes” above with one appliance on; “Speed ×10” runs the meter ten times faster so you don't have to wait.
Where you'll meet it
Commissioning and servicing a boiler, fire or cooker: one of the checks in your ACS assessments.
Why it is called a G4
G4 is the size in BS EN 1359: a nominal flow of 4 m³/h and a maximum (Qmax) of 6 m³/h, about 66 kW of gas. Older UK meters of the same size were called U6. Every UK domestic meter of this size has the same 1 in BS 746 connections at 152.4 mm centres, so a G4 replaces a U6 without changing the pipework.
Down to the transitional flow Qt (0.6 m³/h) it must be within ±1.5 %; down to Qmin (0.04 m³/h, a single small burner) within ±3 %.
Where you'll meet it
Adding a large appliance: add up the heat inputs to check the meter (and the pipework) can supply them all at once.
When nothing is on, nothing should move
The meter only turns when gas flows through it. With every appliance off, a creeping red drum means gas is going somewhere: a leak, or an appliance left on.
The brass test point on the outlet takes a gauge: about 21 mbar working pressure from the meter regulator. Close it fully afterwards and test it for leaks.
Where you'll meet it
Tightness testing to IGEM/UP/1B, and the working-pressure check at the meter outlet test point on every job.
Sources
- George Wilson Industries, Meter G4 1.2L TVI Mechanical, general arrangement drawing (issue A, 10/07/17): dimensions, 1 in BS 746 threads at 152.4 mm centres, fascia plate (Qmax 6 m³/h, Qmin 0.04 m³/h, V 1.2 dm³, pmax 75 mbar, 1 imp = 0.01 m³, BS EN 1359 class 1.5).
- BS EN 1359:1999+A1:2006, Gas meters. Diaphragm gas meters: G designations, Qmin, Qt and Qmax, maximum permissible errors, pressure absorption.
- Measuring Instruments Regulations 2016 (UK, from Directive 2014/32/EU), Schedule MI-002 gas meters: class 1.5, ±3 % from Qmin to Qt, ±1.5 % from Qt to Qmax.
- Gas (Calculation of Thermal Energy) Regulations 1996: energy = volume × 1.02264 × calorific value ÷ 3.6.
- BS 6400-1, installation of domestic-size gas meters (2nd family gases, up to 6 m³/h): the meter regulator gives 21 mbar nominal at the meter outlet.
- IGEM/UP/1B, tightness testing and direct purging of small low-pressure natural gas installations: the meter is part of the installation being tested; watch the index for movement during a let-by test.
- Elster BK-G4 datasheet (A13.11.2012): pressure loss curve, about 0.9 mbar at 6 m³/h with natural gas; used here as a typical curve.
Drawn after the GWi G4 1.2L TVI drawing; the case and index follow it, the measuring unit inside is a simplified four-compartment unit sized so one crank turn passes the plate's 1.2 dm³. Not to scale with the other figures in the set.