VQ548MP & MP-72170 pages

103-31-394-031-EH-0714.pdf

This datasheet describes the use of the MP-7217 Pellistor.
This is commonly used, but not exclusively, in mining
applications. It is a low power, intrinsically safe, extremely
robust and poison-resistant device in a certified flameproof
enclosure.
The MP-7217 has been designed to provide the basic
sensor performance that will enable a suitable instrument to
meet the various (Group 1) mining performance standards.
The MP-7217, whilst being optimised for its methane
response (up to 5% volume in air), will also detect other
flammable gases and vapours. For further information,
contact SGX Sensortech.

MP-7217 and VQ548MP
MEMS Pellistor MP Series

FEATURES
•

Low power – designed for battery operation

•

Small size (14.4 x 6.7 mm)

•

Compatible with VQ500 head instrument

•

High resistance to mechanical shocks

•

Low orientation effect

•

ATEX/IECEX certified package

•

The certification markings are:
II 1G Ex ia IIC Ga and

I M1 Ex ia l Ma

II 2G Ex d IIC Gb and

I M2 Ex d l Mb

OPERATION
It is recommended that the detector and compensator be
run in a Wheatstone bridge circuit.
A suitable circuit is shown below. In use, the bridge supply
voltage should be stable to within + 0.05 V, or the output in
clean air may change in sympathy. Although it is generally
recommended that pellistors should be run with a constant
voltage supply to the bridge, it is possible to use a constant
current supply provided that the voltage across the bridge in
clean air remains within the recommended limits.

OPERATING PRINCIPLE
The silicon pellistor structure consists of a pair of accurately
micro machined diaphragms with two embedded planar
heater meanders coated with a layer incorporating a noble
metal catalyst for Detector device and with inert layer for
Compensator device.
The meander acts both as an electrical heater and as a
resistance thermometer. The device is mounted on a PCB
with wire bonding and is surrounded by a plastic can with
the end open to the atmosphere. If a flammable gas is
present when the device is heated to about 400 – 500 °
C,
the gas will oxidise and the resultant release of energy will
heat the device still further. This increase in temperature is
detected as an increase in resistance of the meander. The
temperature of the meander 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 compensate for temperature changes
not caused by the oxidation of the flammable gas a second,
inert device is used. This compensator is made in the same
way as a detector device except that instead of
incorporating a catalyst in the coating layer, the device is
treated so that oxidation cannot take place. The two devices
are then used in a circuit that detects the difference in their
resistances. Since the two devices are generally of a
different colour, they have different emissivities and hence
different slope resistances. Therefore, to obtain the best
temperature performance, it is necessary on occasion to
connect a fixed resistor in parallel with the compensator to
correct for its higher slope resistance.

Recommended circuit diagram

Whilst SGX Sensortech has taken care to ensure the accuracy of the information contained herein it accepts no responsibility for the consequences of any use thereof
and also reserves the right to change the specification of goods without notice. SGX technologies accepts no liability beyond the set out in its standard conditions of sale
in respect of infringement of third party patents arising from the use of tubes or other devices in accordance with information contained herein.
In case of modification of the product, SGX disclaims all liability.

DS-0140, Issue 1, 08-Jan-2014