I believe what makes the difference in the mechanical method of tuning is consistency of air flow and pressure. If you apply equal amounts of air consistently to the reed your reading will be more accurate. example: An air pump with a regulator will provide a consistent 1/4 LB of pressure per square inch every time when set to a specific setting of 1/4 Lb, for a human to blow a 1/4Lb of pressure per square inch consistently without a regulator is not possible (accurately) without the volume of air moving up and down. The air chamber in sorts is a kind of regulator that encapsulation the air in a secondary vessel before releasing the air through. You still build up pressure and can blow harder and louder but not if as you would directly into the reed. (it will still vary but not as much as if there was no chamber) The example I am stating is for reeds inside an air chamber. In the Clavietta where the air is stored in an air chamber and then released out to the reed that is outside the air chamber. This is much easier, in my opinion to tune a reed. When tuning an accordion reed block, they remove the block from the accordion and tune usually on a jig made with a bellow attached to a board with a hole and place the reed block over the hole. sort of a bellow you would use to in a fire place.