VQ35 Series
Thermal Conductivity Gas Detector Elements
4.25 V
1kO
ZERO SET
27 O
27 O
OUTPUT
REFERENCE
ELEMENT
SENSING
ELEMENT
6821A
To be read in conjunction with "Introduction to Pellistor Gas
Sensors" and Pellistor Application Note 5 – Thermal
Conductivity Sensors.
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
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 ............seenote 3
Mechanical
Mounting ..............seepage 3
Outlines ..............seepage 2
BRIDGE CIRCUIT
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.
SGX Sensortech(IS) Ltd Registered in England No.08067077
A1A-VQ35_Ser Issue 6, March 2007
