Engineering:Level probe

A level probe is a special pressure transmitter for level measurement of liquids in open vessels and tanks. Level probes are submerged directly into the liquid and remain permanently floating above the tank bottom.  The measurement is carried out according to the hydrostatic principle. The gravity pressure of the liquid column causes an expansion of the pressure-sensitive sensor element, which converts the measured pressure into an electrical standard signal. The connecting cable of level probes has several tasks to fulfil. In addition to the power supply and signal forwarding, the level sensor is held in place by the cable. The cable also includes a thin air tube that directs the ambient air pressure to the level probe. Level probes are therefore 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 not only measure the hydrostatic pressure but also the air pressure on the liquid column. At sea level, this is about 1013 mbar - which would correspond to the pressure exerted by a water column ten meters high.  In addition, a variable air pressure would affect the measurement result. Typical air pressure fluctuations of about +/- 20 mbar, corresponding to +/- 20 cmWs (water column).

For deep well designs, the Sealed Gauge measuring principle is also used. From a depth of approx. 20 m, the relative pressure can only be compensated to a limited extent by the thin hose. The level sensor is then designed as an absolute pressure transmitter whose zero point is adjusted to the desired mean air pressure depending on the location of use. This means that the level sensor no longer has any connection to the atmosphere. Possible fluctuations in air pressure may have an impact on the measurement result, but they play a rather minor role in deep well wells.

Measuring principle

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

[math]\displaystyle{ p(h) = \rho g h + p_0 }[/math]

[math]\displaystyle{ \rho }[/math] = Density [for water: [math]\displaystyle{ \rho }[/math] ≈ 1.000 kg/m³]

[math]\displaystyle{ g }[/math] = Gravitational constant [: [math]\displaystyle{ g }[/math] ≈ 9,81 m/s²]

[math]\displaystyle{ h }[/math] = Height of the liquid column

[math]\displaystyle{ p_0 }[/math] = Ambient air pressure

[math]\displaystyle{ p(h) }[/math] = hydrostatic pressure

The measuring minimum level starts from a complete covering of the measuring element near the head end of the level sensor. Filling levels below the level probe are not detected. Depending on the application and mounting height, it is therefore necessary to adjust the level in the evaluation unit to the respective mounting height with an offset setting. 

Design types

Depending on the requirements of the site, level probes offer different features:

Protective cap

Size and number of openings / holes

Housing material

Stainless steel, Ttitanium, PTFE

Cable material

PE, FEP, PURE EPR, PA

Measuring principle

Relative or sealed gauge

Sensor technology

Piezoresistive silicon sensor, ceramic thick-film sensor, ceramic capacitive