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Copyright © 2001, MCB UP Limited
Capacitive level probes with a difference
Capacitive level probes with a differenceKeywords: Capacitance, Sensors, Level detection
The PER level probe works on the patented "three electrode principle". Each measuring point has two electrodes, a measuring electrode and a screen electrode that surrounds the measuring electrode. The walls of the container, assuming the vessel is metal, become the third electrode. If the vessel is non-metallic then a metal strip must be placed down the side of the vessel. The output is balanced when the container is empty by using a potentiometer on the remote evaluation unit. The output is only switched when there is a direct line of contact from the measuring electrode to the container wall. As soon as the level falls below the set measuring point there will be no output, even if the media is a paste or cream and sticks to the probe. There will be no output because there is no direct line to the container wall and the field lines cannot pass by the screen electrode (see Figure 1).
The remote evaluation unit can either be a single for a single point switching probe, or else three channels for handling up to three independent outputs, the position of which can be specified along with the length and the housing material when ordering the probe. PTFE is an option that is acceptable to the food industry. The evaluation units can either be AC with change over relay outputs or 18 to 36V DC with three PNP antivalent outputs.
Figure 1 The PER level probe works on the patented "three electrode principle"
The probes are suitable for any media, such as powders, granules, pastes or liquids of greater r > 1.2
The permitted temperature range is from –40°C to +180°C (200°C for a short time). With three different measuring electrodes in one probe the PER level can be used for a low level, high level and alarm output all in the single probe.
There is also an analogue version of the Rechner level probe giving an analogue output across the customer selected measuring range. These analogue probes are referred to as true level probes.
For further information on the Rechner PER and TRUE level probes contact Ian Frais, UK. Tel: +44 (0)118 976 6450; E-mail: Info@rechner-sensors.co.uk