Gas Rate Calculation Formula Metric

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Metric Gas Rate Calculator

Volume for 1 revolution (e.g., 0.010, 0.100)

Time elapsed for the volume above

Standard UK approx. 38.0 – 40.0

Please enter valid positive numbers for Volume and Time.

Understanding the Metric Gas Rate Calculation Formula

For gas engineers and heating professionals, calculating the gas rate (or heat input) is a critical step in commissioning and servicing gas appliances. Unlike older imperial meters, modern metric gas meters measure volume in cubic meters ($m^3$). To convert this volumetric flow into a power rating (kilowatts), specific calculations must be applied.

The Metric Gas Rate Formula

The calculation determines the amount of energy being supplied to the appliance. The standard formula used in metric gas rating is:

Heat Input (kW) = (Volume (m³) × CV × 3600) ÷ (Time (s) × 3.6)

However, this can be simplified mathematically to:

Heat Input (kW) = (Volume (m³) × CV × 1000) ÷ Time (s)

Where:

• Volume ($m^3$): The amount of gas used during your test (often one revolution of the test dial).
• CV (Calorific Value): The energy content of the gas, usually measured in Megajoules per cubic meter ($MJ/m^3$). In the UK, this averages around 38.0 to 40.0.
• Time (s): The time in seconds it took to consume the measured volume.

How to Perform the Test

1. Turn off other appliances: Ensure no other gas appliances (cookers, fires, other boilers) are running to isolate the appliance you are testing.
2. Run the appliance at max: Turn the boiler or appliance to its maximum setting (e.g., hot water mode or chimney sweep mode).
3. Observe the Meter: Locate the decimal test dial or the last digit on the digital display.
4. Measure: Time exactly how many seconds it takes for the meter to record a specific volume (e.g., 0.010 $m^3$ or 0.100 $m^3$).
5. Calculate: Input your values into the calculator above to get the Gross Heat Input in kW.

Why is this important?

Calculating the gas rate ensures the appliance is burning the correct amount of fuel as specified by the manufacturer.
If the calculated kW is significantly lower than the manufacturer’s rating, the appliance may be under-gassed (blocked jet, low pressure).
If it is higher, it may be over-gassed, which can be dangerous and lead to incomplete combustion.

Common Metric Test Volumes

• E6/U6 Meters (Electronic): Often measure in increments of 0.001 $m^3$. A common test is 0.010 $m^3$.
• Older Metric Mechanical: May have a test dial where one revolution equals 0.002 $m^3$ or 0.01 $m^3$. Always check the markings on the dial face.

function calculateGasRate() {
// 1. Get Input Values
var volInput = document.getElementById(‘grcVolume’);
var timeInput = document.getElementById(‘grcTime’);
var cvInput = document.getElementById(‘grcCV’);
var volume = parseFloat(volInput.value);
var time = parseFloat(timeInput.value);
var cv = parseFloat(cvInput.value);
// 2. Validation
var errorBox = document.getElementById(‘grcError’);
var resultsBox = document.getElementById(‘grcResults’);
if (isNaN(volume) || isNaN(time) || isNaN(cv) || time <= 0 || volume <= 0 || cv <= 0) {
errorBox.style.display = "block";
resultsBox.style.display = "none";
return;
}
errorBox.style.display = "none";
// 3. Calculation Logic
// Flow Rate in m3/hr = (Volume / Time in Seconds) * 3600
var flowRateM3Hr = (volume / time) * 3600;
// Energy Logic:
// Energy (MJ) = Volume (m3) * CV (MJ/m3)
// Power (MW) = Energy (MJ) / Time (s)
// Power (kW) = Power (MW) * 1000
// Simplified: kW = (Volume * CV * 1000) / Time
var grossKW = (volume * cv * 1000) / time;
// Net Heat Input (approx 90% of gross for natural gas)
var netKW = grossKW * 0.90;
// BTU Conversion (1 kW = 3412.142 BTU/hr)
var btu = grossKW * 3412.142;
// 4. Update UI
document.getElementById('resKW').innerHTML = grossKW.toFixed(2) + " kW";
document.getElementById('resFlow').innerHTML = flowRateM3Hr.toFixed(3) + " m³/hr";
document.getElementById('resBTU').innerHTML = Math.round(btu).toLocaleString() + " BTU/hr";
document.getElementById('resNet').innerHTML = netKW.toFixed(2) + " kW";
resultsBox.style.display = "block";
}