Posted in | Gas Sensors

MP-7217-Miniature MEMS Pellistor for Flammable Gas Sensing

The MP-7217 Pellistor, a sturdy device with low power requirements, is often, but not solely, utilized in mining applications. With a certified fire-retardant enclosure, it is inherently safe and resistant to poison.

This device has been built to offer the essential sensor performance required to allow an appropriate instrument to be compliant with the numerous Group 1 mining performance standards. Although they are designed with methane response (up to 5% volume in air) in mind, the MP7217 sensors are also able to detect other combustible gases and vapors.

Features

  • Small in size (Ø14 mm), and designed for low power, battery operation
  • Determined to be inherently safe, with strong resistance to mechanical shocks
  • Low orientation effect
  • Certified according to ATEX and IECEx, and UL recognized (File E186043)

Principle of Operation

The structure of the silicon pellistor comprises a pair of precisely micro-machined diaphragms with two implanted planar heater meanders covered with a coating which incorporates a noble metal catalyst for the detector device and an inert layer for the compensator device.

The meander can be used as both a resistance thermometer and as an electrical heater. The device is attached to a PCB using wire bonding and is enclosed by a plastic can with an open end. If a combustible gas is detectable when the device’s temperature is increased to approximately 400 – 500 °C, the gas will oxidize and the ensuing relief of energy will cause the device to become even hotter.

This surge in temperature is perceived as an increase in resistance of the meander. Ambient temperature, as well as changes in thermal conductivity of the air as a result of the potential present of inert gases (e.g. carbon dioxide), can also have an impact on the temperature of the meander.

A second inert device is therefore used to offset temperature variation which is not a result of the oxidation of the flammable gas. This compensator is constructed similarly to a detector device, but rather than including a catalyst within the coating layer, the device is treated to ensure that oxidization is not possible.

The two devices are then applied within a circuit that senses the variance in their resistance levels. As the two devices are usually different in color, they have varying emissivity and thus, varying slope resistances. As a result, to achieve optimum temperature performance, it is sometimes a requirement to connect a fixed resistor alongside the compensator to account for its greater slope resistance.

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