Air flow, measure/determine.

Air velocity, measure and determine.

This is the only controllable option in some of the newer kilns through frequency controllers. The velocity is normally measure using a small instrument readily available from several vendors. On a rare occasion, you can see actual air velocity meters tied into the controller, although such a arrangement seems excessive in cost and equipment.

Normally speaking, when referring to the fan speed in kiln with frequency controllers, we cite the fan speed as a percentage of the full rpm. For example, a 1200 rpm motor reduced to 900 rpm would be operating a 75%.