The Drager Polytron SE Ex sensing heads are gas detectors for the continuous monitoring of flammable gases and vapors in the ambient air. The measurement is based on the heat of reaction, where a chemical reaction takes place in a catalytic bead (so-called pellistor) inside the sensor.
By this concentrations of flammable gases can be detected long before they tend to be ignitable, or: before they reach the lower explosive limit (LEL). The sensing heads are intended to be used in the harsh industrial environment and connected to a suitable central controller by means of a 3-core cable.
For different applications Drager offers three versions:
Drager Polytron SE Ex PR M and Drager Polytron SE Ex HT M to monitor for concentrations up to 100 %LEL, where the version HT is suitable for temperatures up to 150 °C, and a Drager Polytron SE Ex LC M to detect very low concentrations up to 10 %LEL for leak detection.
TWO VARIANTS – Drager Polytron SE Ex PR M and SE Ex LC M are available in two variants differing in the size of the junction box. The big junction box with the cable entry on the left side is preferably used for out-door applications. The cable gland can also be inserted at the bottom side.
COMPREHENSIVE EXPLOSION PROTECTION – The Drager Polytron SE Ex sensing heads are certified acc. to the directive 94/9/EG (ATEX 95) and marked by II 2 GD, indicating that they are suitable to be operated in potentially explosive atmospheres (zone 1 and 2 as well as zone 21 and 22).
MEASURING SIGNAL – The flame-proof encapsulated gas sensor produces a mV-signal which is proportional to the gas concentration and can be evaluated by a suitable central controller (e.g. Drager REGARD Ex or Drager Polytron SE Ex). Connected via a shielded 3-core cable of several hundreds of meters length to the sensing head, the central controller is intended to be used to activate alarms if dangerous gas concentrations occur.
PELLISTOR SENSORS – As they are used as precisely temperaturedependent resistors, the measuring beads housed in the Ex-sensor are called pellistors (from engl. pellet and resistor). A pellistor is made of very porous ceramic material embedding a small platinum wire coil. By means of an electrical current of approx. 270 mA on the one hand the platinum coil heats up the ceramic bead to approx. 450 °C, on the other hand the platinum coil acts as a measuring resistor dependent on the bead’s temperature.
When molecules of a flammable gas penetrate into a catalytic bead they react with the activated airborne oxygen which is adsorbed in the porous ceramic and release heat of reaction causing the pellistor’s temperature rising e.g. by about 2 °C for 10 %LEL octane.
The resulting increase of the pellistor’s resistance is in the magnitude of some milli-Ohms and is proportional to the gas concentration.