A method of generating and maintaining sulfur trioxide gas for calibrating a sulfur trioxide detection device within a combustion gas probe, the method comprising: receiving a known concentration of sulfur dioxide gas; receiving an oxidizing agent gas; exposing the known concentration of sulfur dioxide gas and the oxidizing agent gas to a catalyst, the catalyst being heated, to react the known concentration of sulfur dioxide gas and the oxidizing agent gas into a known concentration of sulfur trioxide gas; and maintaining the known concentration of sulfur trioxide gas in a gaseous state, wherein maintaining the known concentration of sulfur trioxide gas in the gaseous state includes: diluting the sulfur trioxide gas with a dilution gas; and maintaining the diluted sulfur trioxide gas above a conversion point temperature of sulfuric acid, passing a portion of the diluted sulfur trioxide gas, having a known concentration of sulfur trioxide gas, through a sulfur trioxide detection device; calibrating the sulfur trioxide detection device by measuring the known concentration of sulfur trioxide gas passing through the sulfur trioxide detection device; adjusting the measurements of the sulfur trioxide detection device based on calibration using the known concentration of sulfur trioxide gas; and wherein receiving the known concentration of sulfur dioxide gas includes pre-heating the sulfur dioxide gas before exposing the sulfur dioxide gas to the catalyst, wherein exposing the known concentration of sulfur dioxide gas and the oxidizing agent gas to a catalyst includes heating the catalyst to a temperature between 225 degrees Celsius and 600 degrees Celsius, and the method further comprising heating the catalyst to above about 550 degrees Celsius to restore a degraded condition of the catalyst, the degraded condition causing reduced efficiency of converting the sulfur dioxide into the sulfur trioxide. receiving a known concentration of sulfur dioxide gas; receiving an oxidizing agent gas; exposing the known concentration of sulfur dioxide gas and the oxidizing agent gas to a catalyst, the catalyst being heated, to react the known concentration of sulfur dioxide gas and the oxidizing agent gas into a known concentration of sulfur trioxide gas; and maintaining the known concentration of sulfur trioxide gas in a gaseous state, wherein maintaining the known concentration of sulfur trioxide gas in the gaseous state includes: diluting the sulfur trioxide gas with a dilution gas; and maintaining the diluted sulfur trioxide gas above a conversion point temperature of sulfuric acid, diluting the sulfur trioxide gas with a dilution gas; and maintaining the diluted sulfur trioxide gas above a conversion point temperature of sulfuric acid, passing a portion of the diluted sulfur trioxide gas, having a known concentration of sulfur trioxide gas, through a sulfur trioxide detection device; calibrating the sulfur trioxide detection device by measuring the known concentration of sulfur trioxide gas passing through the sulfur trioxide detection device; adjusting the measurements of the sulfur trioxide detection device based on calibration using the known concentration of sulfur trioxide gas; and wherein receiving the known concentration of sulfur dioxide gas includes pre-heating the sulfur dioxide gas before exposing the sulfur dioxide gas to the catalyst, wherein exposing the known concentration of sulfur dioxide gas and the oxidizing agent gas to a catalyst includes heating the catalyst to a temperature between 225 degrees Celsius and 600 degrees Celsius, and the method further comprising heating the catalyst to above about 550 degrees Celsius to restore a degraded condition of the catalyst, the degraded condition causing reduced efficiency of converting the sulfur dioxide into the sulfur trioxide.