Measuring Combustible Gases with Dual Mode MEMS Pellistors

SGX Sensortech has introduced the new, advanced MP-7217-TC sensor that is designed to measure combustible gases across the total volume range (Figure 1). The company manufactures a wide range of industrial gas sensing products. These high reliability, high-performance sensors protect workers from the hazards of asphyxiant, toxic, and flammable gases for a wide range of applications.

MP-7217-TC

Figure 1. MP-7217-TC

The MP-7217-TC, a high performance, MEMS-based sensing device, works in the following modes:

  • Above the flammable limit and up to 100% by volume, the sensor works in thermal conductivity mode
  • Below the flammable limit, the sensor works in pellistor or catalytic mode

The sensor module includes a fully controllable microswitch that enables users to shift between the two operating modes quickly and easily. Now, the MP-7217-TC sensor makes it simple for instrument designers to improve the functions of their gas detectors. Earlier, it was only possible to determine across two different ranges using two individual sensors.

The latest SGX product offers this facility in a single sensor and thus may reduce the size of the end product. Moreover, the MP-7217-TC sensing device is driven by a MEMS pellistor/thermal conductivity element, which not only has improved resistance to contamination, but also has excellent mechanical robustness. It requires less than half the power of a typical flammable device, usually 110MW or less.

MP-7217 Data

This article describes the application of the MP-7217 Pellistor, which is generally used in mining applications. It is an intrinsically safe, rugged, low power and poison resistant device that comes in a certified flameproof enclosure.

The MP-7217 sensor has been specifically designed to provide the basic sensor performance that will allow a suitable instrument to fulfill the various (Group 1) mining performance standards. The MP-7217 device, while optimized for its methane response (up to 5% volume in air), is also capable of detecting certain flammable vapours and gases.

Features of MP-7217

  • Low orientation effect
  • Low power, designed for battery operation
  • Compact size (∅14mm)
  • Excellent resistance to mechanical shocks
  • Intrinsically safe
  • ATEX/IECEx certified package
  • The certification markings are: ATEX: II 1 G Ex ia IIC T4 Ga ATEX: I M1 Ex ia l Ma IECEx: Ex ia IIC T4 Ga IECEx: Ex ia l Ma

Operating Principle

The structure of the silicon pellistor includes a couple of precisely micro-machined diaphragms with two integrated planar heater meanders.

The meander serves both as a resistance thermometer and an electrical heater. The device is positioned on a PCB with wire bonding and is enclosed by a plastic can with the end exposed to the atmosphere. If a flammable gas is present when the device is heated to 400-500 °C, the gas will oxidize and the energy thus released will heat the device further.

This rise in temperature is perceived as a rise in meander resistance. The meander’s temperature is also impacted by ambient temperature and by differences in thermal conductivity of the air induced by the potential presence of carbon dioxide gas.

In order to offset the temperature variations not caused by the oxidation of the flammable gas, another inert device is used. This compensator is built in the same manner as a detector device, except that instead of integrating a catalyst in the coating layer the device is treated so that oxidation does not occur.

Both devices are then used in a circuit that detects the variation in their resistances. Given that both devices are of a different colour, they exhibit different emissivities and so have different slope resistances. To achieve the best temperature performance, a fixed resistor can sometimes be connected in parallel with the compensator to correct its higher slope resistance.

Operation

The detector and compensator can be run in a Wheatstone bridge circuit. Figure 2 shows a suitable circuit.

Recommended circuit diagram

Figure 2. Recommended circuit diagram

While in use, the bridge supply voltage must be within + 0.05V, or else the output in clean air can vary. Though it is usually recommended that pellistors are operated with a constant voltage supply to the bridge, a constant current supply can also be used if the voltage across the bridge in clean air stays within the recommended limits.

Conclusion

SGX Sensortech’s dual mode MEMS pellistors are high-performance sensing devices, which make it easy for instrument designers to improve the functionality of their gas detectors. The devices provide superior robustness, have improved resistance to contamination, and require less than half the power of a typical flammable device.

About SGX Sensortech (IS) Ltd

SGX Sensortech is a market leader in innovative sensor and detector devices that offer unrivalled performance, robustness and cost-effectiveness.

SGX have been designing and manufacturing gas sensors for use in industrial applications for over 50 years, offering excellent applications support for an extensive range of gas sensors and the expert capability for custom design or own label.

As an independent OEM supplier of gas sensors, we pride ourselves on providing customers with unrivalled product reliability and personal product support via specialist engineers.

SGX gas sensors are built to the highest standards with all pellistor and infrared gas sensors achieving ATEX and IECEx certification, SGX gas sensors are also UL and CSA approved.

Our product portfolio has continued to expand in technology and detectable gases used in a wide range of applications including:-

  • Mining
  • Oil and gas
  • Confined space entry
  • Indoor air quality
  • Industrial area protection
  • Leak detection

This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.

For more information on this source, please visit SGX Sensortech (IS) Ltd.

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