Using Gas Analyzers
How to Take Gas Samples
The first step in taking a gas sample is to make a small hole in the flue pipe that is sized to fit the probe closely, so air leaks are minimized. To get the most accurate measurement, the gas-sampling probe must be placed prior to any draft damper or diverter, so that the gases are not diluted, and as close to the equipment breach as possible so the gases have not cooled in the flue. If there is a stack economizer or similar device, the measurement should be taken just downstream of the installed device. It is important to note that in order to have the most accurate, representative measurements, there should be no gas leaks.
Making Temperature Measurements
In order to make temperature measurements, the thermocouple probe is placed at the point of highest exhaust gas temperature at the base of the flue and toward the center for small ducts. If the stack gas temperature is underestimated, the operating efficiency will be overstated.
Taking O2 Measurements
After calibrating the instrument used to measure O2 levels, it is necessary to put the analyzer where the emissions are released, usually the stack. Start the combustion process and look for stability in the oxygen readings for anywhere between one to three minutes.
Soot is most commonly measured during equipment tune-up and maintenance by extracting a sample of the exhaust gases using a manual sampling pump or a special soot probe. The sample is taken from the same location as the stack temperature measurements. These measurements may be made with smoke pumps or with probes that have filters.
Documenting your measurements is fundamental to insure short and long term understanding of your combustion process. As the boiler changes with time, it is important to compare today’s measurements to past data to identify components in the combustion system that justify replacing to maximize the combustion efficiency of the process.
Gas analyzers with built-in printers and/or data logging capabilities provide for the best methods of data storing and record keeping.