VQ35MB

VQ35 Series Thermal Conductivity Gas Detector Elements To be read in conjunction with "Introduction to Pellistor Gas Sensors" and Pellistor Application Note 5 – Thermal Conductivity Sensors. BRIDGE CIRCUIT REFERENCE ELEMENT 6821A INTRODUCTION The VQ35 series devices consist of two matched elements which are used for the detection of gases in 0 – 100% by volume concentrations using the recommended bridge circuit (see below) and the mounting arrangement shown on page 3. The VQ35M is supplied with braided leads and the VQ35MB has extended pins for fitting directly to printed circuit boards. The elements operate on the thermal conductivity principle. The sensing element is open to the atmosphere under test and the reference element is supplied sealed in reference air. The response of the devices is dependent upon the difference between the thermal conductivity of the atmosphere under test and the reference air. When the atmosphere under test has a thermal conductivity higher than the reference air, the sensing element loses more heat to the surroundings than the reference element. This increased heat loss causes a cooling of the sensing element and a subsequent reduction in the resistance of the sensing element compared to the reference element. The thermal conductivity of a gas mixture is dependent on temperature and the individual thermal conductivity of the constituents of the mixture. The temperature of importance is the operating temperature of the sensing element (typically a maximum of 500 8C at the recommended bridge supply voltage). The sensor can be operated at any bridge voltage up to and including the recommended bridge supply, with the sensing element therefore running at temperatures up to and including the typical maximum. The thermal conductivity of some of the more common gases relative to air is tabulated on page 2 at various temperatures. Note that when operating the sensor in a constant voltage mode the response will be non-linear to increasing concentrations of a gas. GENERAL DATA 27 O 4.25 V 1 kO ZERO SET 27 O SENSING ELEMENT OUTPUT MARKING Each element is identified by a unique serial number written on the can of both the sensing and reference elements. The serial number is written in red on the sensing element and black on the reference element. In addition, the sensing element carries a red circular label on the base identifying the device type. NOTES 1. Operation may be under either direct flow or diffusion conditions in appropriate mountings (see page 3). 2. With open-circuit conditions at the bridge output. 3. The response time is a function of the type of mounting used. ORDERING INFORMATION Order Code Description VQ35 Flexi-leg sensor with open compensator can. This device needs to have the compensator mounted in a sealed chamber filled with air or nitrogen. VQ35B PCB legged sensor with open compensator can. This device needs to have the compensator mounted in a sealed chamber filled with air or nitrogen. VQ35M Flexi-leg sensor with sealed compensator can. VQ35MB PCB legged sensor with sealed compensator can. Electrical The information given below relates to the VQ35 series operating in the recommended circuit shown. Operation (see note 1) . . . . . . . . . continuous Bridge supply . . . . . . . . . . . 4.25 V Bridge power consumption . . . . . . . 0.26 W max Response to methane in air (see note 2) . . 1.0 mV/% min Response time . . . . . . . . . . . . see note 3 Mechanical Mounting . . . . . . . . . . . . . . see page 3 Outlines . . . . . . . . . . . . . . see page 2 SGX Sensortech(IS) Ltd Registered in England No.08067077 A1A-VQ35_Ser Issue 6, March 2007 THERMAL CONDUCTIVITY OF GASES (Relative to air) Element temperature Air Nitrogen Oxygen Argon Carbon dioxide Water vapour Methane Ethane Propane Carbon monoxide Ethylene Acetylene Hydrogen Helium Ammonia 0 8C 100 8C 200 8C 300 8C 400 8C 500 8C 600 8C 1.000 0.996 0.987 0.686 0.621 0.969 1.244 0.742 0.619 0.961 0.703 0.886 7.371 5.972 1.082 1.000 0.993 1.026 0.687 0.745 1.076 1.500 1.027 0.874 0.962 0.970 1.040 6.918 5.681 1.295 1.000 0.997 1.049 0.682 0.832 1.192 1.723 1.271 1.092 0.970 1.195 1.155 6.692 5.492 1.493 1.000 0.999 1.062 0.674 0.893 1.303 1.911 1.474 1.271 0.975 1.378 1.240 6.548 5.338 1.670 1.000 0.998 1.065 0.663 0.933 1.404 2.066 1.638 1.415 0.976 1.524 1.301 6.435 5.197 1.824 1.000 0.994 1.062 0.650 0.959 1.493 2.192 1.769 1.529 0.974 1.637 1.344 6.336 5.062 1.955 1.000 0.988 1.056 0.636 0.975 1.574 2.296 1.874 1.619 0.970 1.727 1.376 6.252 4.939 2.069 OUTLINE OF VQ35M (All dimensions without limits are nominal) 2311B F D E 1B C 1G 1A J O-RING INSULATOR METAL FLANGE Ref Millimetres A B C D E F G H J 11.05 + 0.25 6.10 + 0.25 3.56 + 0.13 63.5 min 1.0 * 8.2 max 9.53 1.5 * sensing element 6.25 + 0.50 reference element 7.75 + 0.38 H SEE NOTE Note No bends may be made in this length. OUTLINE OF VQ35MB (All dimensions without limits are nominal) 7218 F D E 1B C 1A 1G INSULATOR METAL FLANGE SGX Sensortech(IS) Ltd Registered in England No.08067077 H O-RING Ref Millimetres A B C D E F G H 11.05 + 0.25 6.10 + 0.25 3.56 + 0.13 40.0 min 1.0 * 8.2 max 1.5 * sensing element 6.25 + 0.50 reference element 7.75 + 0.38 A1A-VQ35_Ser Issue 6, March 2007 RECOMMENDED MOUNTING ARRANGEMENTS 4689A SENSING ELEMENT EXPOSED TO GAS UNDER TEST REFERENCE ELEMENT SEALED OFF FROM GAS UNDER TEST LEAKPROOF JOINTS AA AB Ref Millimetres AA AB AC AD AE 9.53 + 0.13 8.33 + 0.13 8.20 min 9.53 + 0.13 8.20 min 1AC 60 MESH STAINLESS STEEL GAUZE FLOW RATE 500 cm3/min AD 1AE FILTER PAPER AND 60 MESH STAINLESS STEEL GAUZE SGX Sensortech(IS) Ltd Registered in England No.08067077 A1A-VQ35_Ser Issue 6, March 2007