Deadweight tester

A pressure balance, also referred to as a pressure balance, is an instrument by which a well-defined pressure in a liquid or a gas is generated by a defined force is exerted on a piston of known cross section. In practice, feel like it weights on the piston (the piston is vertical ).

Pressure balances serve as a standard instrument for calibration ( calibration) in the pressure measuring technique and also be used for precision measurements of the pressure dependence of various properties of materials.

Since the air pressure acting on the outside of the piston, pressure compensators usually measure only the positive pressure, ie the difference between the pressure in the gas / liquid in the and in the surrounding air. To measure the absolute pressure of the outside of the piston has with the weights placed in a vacuum chamber (absolute pressure scale, also called " absolute pressure piston vacuum gauge ").

The pressure P is then given by P = F / A, where F is the force and A is the cross sectional area of the piston. Since the pressure in a liquid ( to a lesser extent also in gases ) from the amount of which depends (hydrostatic pressure ), the formula is only valid in the height of the piston end. For precision measurements is to continue to take into account that the weight depends on a mass of the gravitational acceleration and this is location-dependent. For extreme accuracy requirements, the deformations of the piston and cylinder are to be considered under pressure and the buoyancy of the weights in air.

To eliminate measurement errors due to the friction between piston and cylinder, the piston, together with the weight thereon is rotated about its axis. When pressure balances for gas, the piston is usually sealed with oil. But hydraulic oil can also be used directly as a medium for transmitting pressure. Pressure compensators usually have a spindle, can be pressed with more oil to adjust the height of the piston.

Pressure scales are characterized by good accuracy. Ordinary, commercially available devices achieve accuracies up to 0.005 % of measuring range, with specially designed equipment for calibration purposes can be achieved accuracies up to about 10-6.

Pressure balances can be made ​​for very different measuring ranges, because both the piston diameter and the force can be changed.