These gauges are apt to get out of order for various reasons namely, there is no theoretical method of determining the motion of the pointer due to a given pressure; this is done by tests in which known pressures are employed, and accordingly the divisions on the graduated scale are usually unequal, hence these instruments are tested by attaching them either to a mercury column, or to a dead weight safety valve having for its seat an exact square inch surrounded by a knife edge, or a piston of standard area loaded with weights. This sharp edge is covered by a fibre washer of leather for moderate pressures, say 150 lbs. per square inch, or vulcanized fibre or its equivalent for higher pressures.
Fig. 451.
Fig. 451 represents a pump that can be used for pressures up to 10,000 lbs. per square inch. The Hand-Lever Pump shown at the right in cut is used for filling the Pressure Pump cylinder and connections with oil or glycerine, and may also serve for testing gauges of low pressures up to 15 or 20 lbs. The suction pipe a is connected with the reservoir containing the oil or glycerine, which after being used is discharged by valve d and returned into the reservoir by pipe c.
In filling the pump the cylinder spindle has to be screwed all the way out, and the valves b and d closed before it is put under pressure.
Fig. 452.