Why the fuel setting matters and how carbon dioxide is calculated, the standard analysers meet, cold storage and CO over-range protection.
Select the fuel before every test
Many analysers measure oxygen and calculate carbon dioxide from it:
- carbon dioxide = CO2max × (21 − O2) ÷ 21
- CO2max is the most carbon dioxide the fuel can produce: about 11.9% for natural gas and 13.9% for LPG
Worked example: natural gas at 2.1% oxygen. 21 − 2.1 = 18.9, and 18.9 ÷ 21 = 0.9, so the carbon dioxide is 11.9 × 0.9 = 10.7%. CO2max is the theoretical value for complete combustion without excess air. Normal burner adjustment supplies excess air, so the measured CO2 is lower; faulty air-starved operation is a different condition. The same reading gives an air ratio of 21 ÷ 18.9 = 1.11, or about 11% excess air.
Oxygen, carbon monoxide and the flue temperature are measured directly, so the fuel setting does not change them. The calculated carbon dioxide, the ratio built on it and any efficiency figure do depend on it. Left on LPG at a natural gas boiler, the analyser overstates the carbon dioxide, so the ratio reads lower, and looks better, than it really is.
The standard
Portable analysers that measure the combustion flue gases of heating appliances are made to BS EN 50379. Boiler instructions ask for an analyser that conforms to it and is in calibration. BS 7967 is the guide to using such analysers for CO and combustion checks in homes.
Keep it out of the cold overnight
A cold analyser carried into a warm house can collect condensation inside. Water affects the sensors and can damage them permanently. Follow its storage instructions and avoid leaving it in a cold van overnight. For the KANE458s, the drying advice is to run the pump on fresh air in a warm place for a few hours and contact the service centre if problems remain.
CO over-range protection
On the KANE458s, CO above its measuring range triggers a protective sequence: the sample pump stops, the separate CO purge pump runs and the CO display shows dashes until the level falls. Stop the test and appliance safely and resolve the warning before continuing. Read the whole display: dashes can also mean a sensor problem. Other analysers may use different protection thresholds or sample-dilution systems, so use the instructions for the instrument in hand.