Most air hand pumps that are designed to be used as pressure calibrators include a coarse pressure generator and a fine pressure adjuster. The effectiveness of both is dependent on the test volume connected to the pump.
The coarse or main pressure generator is typically a piston and cylinder arrangement which is actuated either by a set of calipers, a plunger, or a lever. The coarse change in pressure is applied by compressing the air to a high level inside a small cylinder which is then released to the lower pressure test volume increasing the pressure upon each cycle of the pump.
The fine pressure adjuster is a thumb-screw mechanism that incrementallly reduces or increases the closed test volume which in turn changes the stored pressure by an equivalent proportion in accordance with Boyle’s Law.
The number of coarse pump cycles or turns of the fine volume adjuster required to reach a desired pressure, depends on the test volume connected to the pump. For example If the test volume is doubled
the number of cycles or turns will be doubled to achieve the same pressure change.
When testing a high pressure with a pneumatic hand pump it is essential to keep the test volume as small as possible. Also, winding out the fine adjustment screw as far as possible before applying any pressure will provide you with a convenient pressure booster when you or the pump runs out of steam at high pressures.
It is very difficult to set very low pressures with a small test volume due to the relatively large capacity of most calibration hand pumps. Increasing the test volume can help reduce the sensitivity of the coarse pressure pump, but alternatively using the fine volume adjuster for applying and incrementing the test pressure may produce the same results.