A calibration technique for a measurement device that produces a spark in an interpane space of an insulating glass unit to determine the content of an inert gas, in particular, argon. Standard calibration units are created which have the same construction as an IGU produced an assembly line. The standard calibration units are filled with varying percentages of argon/oxygen mixture. The calibration technique can be performed on an assembly line by aiming the measurement device at a particular insulating glass unit produced on the assembly line. The measurement device is activated to take a reading of the unit on the line. One of a plurality of calibration standard units of insulating glass units is selected that has substantially an identical construction as the unit on the line except that the calibration standard unit selected has a known, specific amount of certified argon gas that should be the same as the unit on the line. The measurement device is aimed at the calibration standard unit. The measurement device is activated to take a reading of the calibration standard unit. The measurement taken of the calibration standard unit is compared to the known amount of argon gas in the standard calibration unit. And, finally adjusting the calibration of the measurement device if the comparison shows a discrepancy.