Piston pressure gauge
A piston manometer , also known as a pressure balance , is an instrument with which a precisely defined pressure is displayed in a liquid or gas by exerting a defined force on a piston with a known cross-section . In practice, weights are placed on the piston (the piston is vertical).
Pressure scales serve as a standard instrument for calibration ( calibration ) in the pressure measurement , and various properties are used materials for precision measurements of the pressure dependence.
Since the air pressure acts on the outside of the piston , pressure compensators normally only measure the overpressure , i.e. the difference between the pressure in the gas / liquid and the surrounding air. To measure the absolute pressure, the outside of the piston with the weights must be placed in a vacuum chamber (absolute pressure balance, also known as “absolute pressure piston vacuum meter”).
The pressure p then results from 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) depends on the height ( hydrostatic pressure ), the formula only applies at the height of the piston end. For precision measurements it must also be taken into account that the weight on a mass depends on the acceleration due to gravity and this is location-dependent. In the case of extreme accuracy requirements, the deformations of the piston and cylinder under pressure and the static buoyancy of the weights in air must be taken into account.
In order to exclude measurement errors due to static friction between the piston and cylinder, the piston and the weights on it are rotated around its axis. In the case of pressure compensators for gas, the piston is usually sealed with oil. Hydraulic oil can also be used directly as a medium for pressure transmission. Pressure compensators usually have a spindle with which more oil can be pressed in to adjust the height of the piston.
Pressure balances are characterized by their good accuracy. Ordinary, commercially available devices achieve accuracies of up to 0.005% of the measured value; with specially designed devices for calibration purposes, accuracies of up to approx. 10 −6 can be achieved.
Pressure balances can be manufactured for very different measuring ranges because both the piston diameter and the force can be changed.
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See also
- hydraulic pressure compensator