Level Measurement & Control


Capacitance overfill alarm devices for aggressive media

August 2004 Level Measurement & Control

Application CNS 20 capacitance level sensors for electrically conducting and non-conducting media can be used with the CapFox ENT 20 Ex unit as an overfill alarm device.

It is especially suitable for use in extremely aggressive media due to the (partial) PTFE insulation of the probes. The unit can also be used in hazardous areas belonging to zone 0 in conjunction with the CNS 20 Ex high-temperature probe.

The system consists of a switching amplifier and a voltage free output relay. The control circuit is intrinsically safe (EEx ia II C). CNS 20 and CNS 20 Ex sensors can be used.

A green LED indicates the operating voltage, a red LED indicates that the relay has responded. In case of cable breaks or short circuits, the relay in the output circuit de-energises, the red LED lights up and the system switches to 'Error'. (The switching amplifier must be installed outside of the hazardous area.)

The level electrode and the preamplifier of the CNS 20 and CNS 20 Ex probes are integrated in a sturdy weatherproof housing and comply with NAMUR DIN 19234. With the electrode partially PTFE insulated, in conjunction with a conducting tank wall or an earthed counter electrode, it forms a capacitor whose capacity depends on the dielectric characteristics of the environment.

As soon as the dielectric is no longer formed by a gas (free electrode), there is a capacity jump that is converted into a switching signal by the connected CapFox ENT 20 Ex switching amplifier. The constant direct current supplied by the pre-amplifier to the switching amplifier lies within a given range when there are no disturbances. In case of a defective probe or connection cable, this range is exceeded and the relay responds and a signal (visible) is generated.



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