VQ41TSB Combustible Gas Detector Elements 7217 COMPENSATOR To be read in conjunction withIntroduction to Pellistor Gas (BLACK NUMBER) Sensor and Pellistor Application Notes 1, 2, 3, 4, 6 and 7. R INTRODUCTION (SEE 1 kO NOTE 4) The VQ41TSB is a flammable gas sensor optimised for the 50 kO detection of ammonia in air. It operates by detecting the heat ZERO SET produced by the catalytic oxidation of ammonia on a pair of 1 kO matched elements. It may be used for the detection of ammonia in air mixtures with concentrations of 600 ppm upwards and is particularly suitable for monitoring over the range zero to 1.5% by volume (zero to 10% LEL). This makes DETECTOR (RED NUMBER) the sensor ideal for use in ammonia refrigerant detection systems intended to comply with BS4434:1989. Interference from water vapour or carbon dioxide is eliminated OUTPUT by using a matched, inert compensating element. The use of an improved poison resistant construction provides protection Recommended Bridge Circuit (See note 3) against the common substances that can progressively degrade the performance of a sensor and specifically provides OPERATION a high degree of protection against silicone poisoning agents. In The output of the sensor is dependent on a complex this latter case the protection is very much higher than that combustion process established around the sensing beads. It is provided by standard poison resistant catalytic flammable gas imperative that the layers of gas around the beads are not detectors. Tests show no effect on zero or sensitivity to disturbed, otherwise the bead temperature and hence device ammonia when exposed to 100 ppm HMDS for greater than output will change. The devices must therefore be mounted in one hour. a draught free environment. Ideally, the elements should be The VQ41TSB employs new catalyst chemistry and device mounted symmetrically side by side with the open ends structure to optimise the ammonia response. There is negligible exposed to the gas being monitored. The elements are sensitivity to xylene and very little sensitivity to methane under supplied with O rings so that gas tight seals may be made normal operating conditions, but the device can be used to between the elements and the mounting. As for all sensors detect hydrogen and butane. operating on the catalytic oxidation principle, the sensor should be separated from the gas being monitored by a suitable flame GENERAL DATA arrestor. In use, the operating voltage should not be allowed to vary by Electrical more than the specified +0.1 V, or the output in clean air may This information relates to the sensor operating in the change in sympathy. A typical plot of sensitivity against supply recommended bridge circuit shown below. voltage is shown in Figure 1. Operation continuous Bridge supply . 2.0 + 0.1 V NOTES Sensor power consumption 250 mW max 1. The sensitivity is partially influenced by the flow of gas to Typical sensor current 100 mA the elements. Consequently, it is affected by the geometry Sensitivity in SGX Sensortech test block (see note 1): of the mounting arrangements. The quoted figures minimum . 15 mV/% NH 3 in SGX Sensortechs standard test block, were obtained typical 20 mV/% NH 3 details of which are available on request. Typical sensitivity to hydrogen . 45 mV/% H 2 2. Shock and vibration measurements are strongly dependent Mechanical on the mounting arrangements of the sensor. 3 The values of the resistors in the bridge circuit are suitable Outline . see page 2 for circuits where negligible current is drawn from the Shock (see note 2) . 250 g, 5 blows in each plane output terminals, such as when a high input impedance Vibration (see note 2) 20 g, 24 cycles output amplifier is used. from 100 to 3200 Hz 4. The elements are provided as a matched pair with a trimming resistor R, which must be connected across the MARKING compensator as shown. Occasionally a trimming resistor is Each element carries a unique serial number written on the side not required and in such cases the slip packed with the of the can. On the detector the number is red and on the sensor will be marked N/R instead of quoting the value of compensator it is black. the resistor supplied. SGX Sensortech(IS) Ltd Registered in England No.08067077 A1A-VQ41TSB Issue 4, March 2007 Figure 1. Typical Response to 20% LEL 7216 NH 3 60 40 H 2 C H 4 10 CH 4 20 AROMATIC C8H10 0 0 1 2 3 4 BRIDGE VOLTAGE (V) OUTLINE OF DETECTOR ELEMENT 7218 D F Ref Millimetres Inches E 11.3 max 0.445 max 1B A 10.8 min 0.425 min C 6.35 max 0.250 max B 5.85 min 0.230 min 3.69 max 0.145 max 1A 1G C 3.43 min 0.135 min D 40.0 min 1.575 min E 1.0 nom 0.039 nom 6.35 max 0.250 max F* H O-RING INSULATOR METAL 5.33 min 0.210 min FLANGE G 8.2 max 0.323 max H 1.5 nom 0.059 nom Inch dimensions have been derived from millimetres. * For the compensator element, dimension F is 7.88 mm (0.310 inches) max, 6.86 mm (0.270 inches) min. WARNING To satisfy the requirements of the recognised approval authorities, SGX Sensortech recommends that a suitable sintered metal flame arrestor be used with the device to ensure safe operation. SGX Sensortech(IS) Ltd Registered in England No.08067077 A1A-VQ41TSB Issue 4, March 2007 SENSITIVITY (mV)