Reference Air Test (Leakage Test): While the oxygen probe is operating within the furnace, clean reference air should be continuously supplied at a specific flow rate, approximately 5 to 2 CFH. To verify that the supply of reference air is normal, you can initially change the control instrument's mode to manual, then set the display to O2 mV (if the meter supports it). Turn off the reference air for one minute and observe the decrease in the millivolt signal; if the value is less than 5mv, that's acceptable. If your atmosphere controller does not support O2 mV display, we recommend using a temporary millivolt signal meter.