A worked metric gas-rate calculation, converted to gross heat input and checked against the data plate.
Checking the gas rate tells you whether an appliance is burning roughly the right amount of gas. Here is the method with worked numbers you can reuse.
Step 1 - measure. With other gas appliances off, run the appliance at full rate and read the meter over a timed period. For this example the meter advances 0.040 cubic metres (m3) over 120 seconds.
Step 2 - convert to gas rate. There are 3600 seconds in an hour, so the rate is 0.040 m3 x (3600 / 120) = 0.040 x 30 = 1.2 m3 per hour.
Step 3 - find gross heat input. Use gross heat input (kW) = gas rate (m3/h) x calorific value (MJ/m3) / 3.6. The division by 3.6 converts megajoules per hour into kilowatts. Taking a calorific value of 39.0 MJ/m3 for this example: 1.2 x 39.0 / 3.6 = 46.8 / 3.6 = 13.0 kW gross.
Step 4 - gross versus net. Data plates may quote a net figure instead. A commonly used conversion for natural gas is to divide the gross figure by about 1.11, which gives roughly 13.0 / 1.11 = 11.7 kW net for this example. Always check which basis the data plate uses.
Step 5 - interpret. Compare your figure with the appliance data plate. A reading well below the rated input can mean under-gassing, a partial blockage or a governor or injector fault; a reading well above it is also a fault to investigate. For real work, use the actual metered volume and the declared calorific value, not the example figures.