Waterguide - Pressure measurement
What is pressure measurement?
Pressure is a force that impacts on an area. Mathematically, this relationship is expressed with equation:
It contains the unit Newton for force (F) and m² for the unit of area (A). So pressure is a force per unit of area that can also be described with the unit Pascal (Pa).
What types of pressure measurement are there?
In addition to absolute and relative pressure measurement, there is also hydrostatic pressure measurement and pressure difference measurement. Furthermore, different pressure measurement processes are used for different requirements and fluids.
What is absolute or relative pressure measurement?
Absolute pressure refers to pressure in an empty space (vacuum). This value is unequivocal, as there is no pressure in a vacuum, so p equals 0 Pa. Relative pressure, on the other hand, refers to the atmospheric pressure on Earth, which can depend on the altitude. At sea level, the average air pressure is 1013.25 hPa, which equates to approx. 1 bar when rounded up.
When measuring absolute pressure, you are measuring a pressure change in a vacuum. This involves constructing a vacuum, for example enclosed by two ceramic plates. An electrically conductive material is applied to the ceramic substrate. The vacuum (dialectricum) separates the electrically conductive plates (electrodes), which produces a capacitor that can statically store electrical charge in a DC circuit in an electrical field. This stored voltage per charge (electrical capacity) is measured with the unit Farad.
If pressure is subsequently exerted on to the ceramic area, it will deform and cause the electrical capacity of the cell to change. This change in capacity can be expressed in a mathematical equation with the exerted pressure. An absolute pressure measurement will display a pressure value even when the system is not pressurised, namely the atmospheric pressure that the environment exerts on the closed cell system.
When measuring relative pressure, on the other hand, the setup is the same, but a channel to the surroundings is left open so that the atmospheric pressure in the cell can become the zero point. This will cause pressure changes to be measured relatively to the atmospheric pressure. When the system is not pressurised, the pressure indicator will display 0 Pa or bar.
As well as digital pressure measurement, there is also the classic, analogue measurement method using a manometer. One such device is a Bourdon-tube gauge, which pushes the fluid against e.g. a circular coiled spring, which deforms. The contraction of the spring causes a connecting rod to move a gear, which shifts the pointer axis of the pressure indicator.
What is a hydrostatic pressure measurement?
In a hydrostatic pressure measurement, you use a pressure gauge that is installed e.g. at the lower end of a tank. When you fill the tank with water, the water column will press against the cell of the pressure sensor. As the water level increases, more water will press against the conductivity cell, and so the display will show that the pressure is increasing.
The fluid column, the density of the medium and the gravitational constant are proportional to one another and produce the exerted hydrostatic pressure.
As a result of this relationship, you can use the height of the water column to directly determine the exerted pressure and vice versa.
But it is important to remember that, in addition to the pressure exerted from the water column, the atmospheric pressure also presses against the water column if the tank is open. This is the reason why it is called a “relative hydrostatic pressure measurement” when the atmospheric pressure in the measurement equates to the zero point. This is because the pressure is continually compensated, meaning that the gas in the tank does not affect the pressure or fill level measurement.
What is a differential pressure measurement?
In a differential pressure measurement, a transmitter is used to subtract two different pressures from one another. In a closed tank, the pressure of the fluid column is offset with the head pressure of the gas above the fill level. Both values are transmitted through oil-filled pipes to the transmitter with its differential pressure conductivity cell, which converts the pressure values into an electrical signal and thus determines the differential pressure.