EXPLOSIMETER CONSIST OF A POISON RESISTANT FLAMMABLE PELLISTOR AS A SENSING ELEMENT. PELLISTOR RELY ON THE PRESENCE OF OXYGEN ( MINIMUM 11% BY VOLUME) TO OPERATE EFFECTIVELY AND FOR THIS REASON EXPLOSIMETER MUST NOT BE USED FOR MEASURING HYDROCARBON GAS IN INERT ATMOSPHERE.
OPERATING PRINCIPLE:-
IN THE PIC ABOVE WE CAN SEE THE ELECTRICAL CIRCUIT INCORPORATING A PELLISTOR IN A WHEATSTONE BRIDGE.
THE PELLISTOR UNIT BALANCES THE VOLTAGE AND ZERO THE DISPLAY AITOMATICALLY WHEN THE INSTRUMENT IS SWITCHED ON IN FRESH AIR. IT NORMALLY TAKES 30 SECONDS FOR A PELLISTOR TO EEACH ITS OPERATING TEMPERATURE.
GAS SAMPLE MAYNE TAKEN IN FOLLOWING WAYS:-
- DIFFUSION
- HOSE AND ASPIRATOR BULB
- MOTORISED PUMP
FLAMMABLE VAPOURS ARE DRAWN THROUGH A SINTERED FILTER INTO THE PELLISTOR COMBUSTION CHAMBER. WITHIN THE CHAMBER ARE 2 ELEMENTS, THE DETECTOR AND THE COMPENSATOR. THIS PAIR OF ELEMENTS IS HEATED TO ABOUT 400_600 °C. WHEN NO GAS IS PRESENT, THE RESISTANCE OF THE TWO ELEMENTS ARE BALANCED AND THE BRIDGE WILL PRODUCE A STABLE BASELINE SIGNAL. COMBUSTIBLE GASES WILL CATALYTICALLY OXIDISE ON THE DETECTOR HEAD CAUSING ITS TEMPERATURE TO RISE. THIS OXIDATION CAN ONLY TAKE PLACE IF THERE IS SUFFICIENT OXYGEN PRESENT. THE READING IS TAKEN WHEN DISPLAY IS STABLE.