Level probe

Level probes are special pressure transmitters for measuring the level of liquids in open containers and tanks . Level probes are drained directly into the liquid to be measured and remain permanently suspended above the bottom of the tank. The measurement is based on the hydrostatic principle. The gravitational pressure of the liquid column causes an expansion of the pressure-sensitive sensor element, which converts the detected pressure into an electrical signal. The connection cable of level probes has to fulfill several tasks. In addition to the power supply and signal transmission, the level probe is held in place by the cable. The cable also contains a thin air hose with which the ambient air pressure is conducted to the level probe. Accordingly, level probes are usually designed as relative pressure sensors, which use the current ambient pressure as the zero point of their measuring range.

Without this so-called relative pressure compensation, level probes would also measure the air pressure and its changes in addition to the hydrostatic pressure , which is placed on the column of liquid.

The sealed gauge measuring principle is also used for deep well designs . From a depth of approx. 20 m, relative pressure compensation through the thin hose is only possible to a limited extent. The level probe is then designed as an absolute pressure transmitter, the zero point of which is adjusted to the desired mean air pressure depending on the place of use. This means that the level probe no longer has any connection to the atmosphere. Any fluctuations in air pressure may have an effect on the measurement result, but play a subordinate role in deep well applications.

Measuring principle

The hydrostatic pressure, also gravitational pressure or gravity pressure, occurs within a fluid at rest. It is caused by gravity and depends on the density and the height of the liquid column. The mass of the fluid does not play a role - see also the hydrostatic paradox - i.e. not the total weight of the liquid in the container, but the filling level is decisive.

${\displaystyle p(h)=\rho gh+p_{0}}$

${\displaystyle \rho }$= Density [for water : ≈ 1000 kg / m³] ${\displaystyle \rho }$

${\displaystyle g}$= Acceleration due to gravity [for Germany: ≈ 9.81 m / s²] ${\displaystyle g}$

${\displaystyle h}$= Height of the liquid level above the point under consideration

${\displaystyle p_{0}}$= Air pressure on the surface of the liquid

${\displaystyle p(h)}$ = hydrostatic pressure as a function of the height of the liquid level.

The minimum level to be measured begins when the measuring element is completely covered near the head end of the level probe. Levels below the level probe are not recorded. Depending on the application and the mounting height, it is therefore necessary to adjust the level in the evaluation unit to the respective mounting height with an offset setting.

Executions

Depending on the requirements of the place of use, level probes offer various features:

- Protective cap

               Size and number of openings / holes

- Housing material

               Stainless steel, titanium, PTFE

- Cable material

               PE, FEP, PUR EPR, PA

- measuring principle

               relative or sealed gauge

- sensor technology

               Piezoresistive silicon sensor, ceramic thick film sensor, ceramic capacitive

Individual evidence

1. ↑ JUMO: JUMO Campus - Electronic pressure measurement technology. Retrieved June 21, 2018 .