Wheatstone Bridge Combustible Gas Sensors

Last Updated: September 3, 2020

Definition - What does Wheatstone Bridge Combustible Gas Sensors mean?

A Wheatstone bridge combustible gas sensor is a type of combustible gas sensor that features a Wheatstone bridge circuit as part of its sensing mechanism. A “Wheatstone bridge” is a type of circuit used to measure the electrical resistance of a substance. They are commonly used to monitor changes in an environment (e.g., the presence of an explosive gas) by monitoring changes in the electrical resistance of that environment.

Wheatstone bridge sensors are more commonly referred to as catalytic bead sensors. They are used to monitor areas (such as mines) where there is a risk that the working area may be exposed to dangerous levels of explosive gas (e.g., Methane), and are part of a family of sensors called “pellistors” (pellet resistors).

Safeopedia explains Wheatstone Bridge Combustible Gas Sensors

The Wheatstone bridge combustible gas sensor is composed of two coils of platinum wire which are connected electrically into a Wheatstone bridge circuit. On the sensing end of each wire is a bead (or pellet) of conductive material. One of these beads is impregnated with a conductive catalyst that makes it prone to oxidation, while the other bead acts as a reference.

When activated, the sensor passes current through the wires, which increases their temperature to a point that would allow oxidation to occur if the catalyzed bead were to come into contact with a combustible gas. When this occurs, the oxidation process causes the catalyzed wire to heat up further, increasing its level of electrical resistance above that of the non-oxidized wire. It is this imbalance of resistances that the Wheatstone bridge circuit is able to detect.

Wheatstone bridge sensors can detect increases in gas concentration linearly, which means that they can precisely report whether the amount of gas concentration in the air is 20%, 40%, or 100% of the lower explosive limit (LEL) for combustible gas concentration. They are also relatively low-cost, and can be used in harsh, high-temperature environments.

Wheatstone bridge sensors are one of two types of combustible gas sensors that are commonly used, the other being infrared (IR) sensors. Although there is a significant amount of overlap between each sensor-type’s use cases, they also have distinct advantages and disadvantages. For instance, IR sensors can detect higher concentrations of gas, and unlike Wheatstone sensors, they do not need oxygen to work. In contrast, Wheatstone sensors are easier to calibrate in situations where flexibility is necessary or if the specific gas of interest may not be known. Both types of sensor can detect certain gases that the other can not.

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