VQ500 Datasheet | SGX

FEATURES - Optimised for bustible gases and vapours - Thermal conductivity versions available - Certified explosion proof ponent - Integral pellistors - Low power - Standard miniature body size - Poison resistant - silicones and hydrogen sulfide - Shock resistant - ATEX certified - IECEx certified - CSA certified - UL recognised - Temperature Rated between -40°C and +55°C OPERATING PRINCIPLES The detector consists of a fine coil of platinum wire embedded in a ceramic bead incorporating a noble metal catalyst. The coil acts both as an electrical heater and as a resistance thermometer. The bead is mounted on a header with connecting leads and is surrounded by a metal can with the end open to the atmosphere. If a flammable gas is present when the bead is heated to about 400 to 500 °C, the gas will oxidise and the resultant release of energy will heat the bead still further. This increase in temperature is detected as an increase in resistance of the coil. The temperature of the coil is also affected by ambient temperature and by variations in thermal conductivity of the air caused by the possible presence of inert gases such as carbon dioxide. To pensate for temperature changes not caused by the oxidation of the flammable gas a second, inert bead is used. This pensator is made in the same way as a detector bead except that instead of incorporating a catalyst in the ceramic bead, the bead is treated so that oxidation cannot take place. The two beads are then used in a circuit that detects the difference in their resistances. Since the two beads are generally of a different colour, they have different emissivity and hence different slope resistances. Therefore, to obtain the best temperature performance, it is necessary to connect a fixed resistor in...