Blog #3 Wheatstone Bridge & Sampling

Blog 3: What is a Wheatstone bridge and how is it used for sampling?

Originally developed by Charles Wheatstone, The Wheatstone Bridge is an electrical circuit usually found in many ‘direct-reading’ instruments used to search for combustible gas. It can be used to calculate an unknown resistance by using its bridge circuit. So, two legs of the circuit are kept balanced and another used to represent the unknown resistance. It then measures the resistance and compares it to a known resistor value. Today the Wheatstone bridge is still be used to measure very low values of resistances down in the milli-Ohms range.

 -    http://www.circuitstoday.com/wheatstone-bridge-circuit

 -    http://www.electronics-tutorials.ws/blog/wheatstone-bridge.html

    Combustible gases like carbon monoxide can be detected using a sensor device with the Wheatstone bridge circuit. A catalytic sensor for example, has a filament that is coated with a catalyst. This catalyst then reacts to a combustible gas and generates heat. The change in electrical resistance of the heated filament causes an imbalance in the circuit which is recorded. 

    The readout of the device if combustible gas is found presents the concentration as in imbalance within the circuit that is proportional to the amount of combustible gas present and is expressed as a percentage. Another sensor used in combustible gas meters, also uses the measurement of thermal conductivity. Thermal conductivity is the ability of the tested air to conduct heat. Just like the catalytic combustion sensor, this sensor has a filament that is a part of the Wheatstone bridge circuit.

    Combustible gases like methane are shown as a percent of the lower explosive limit/level (LEL) also known as the lower flammable limit (LFL). Usually gas-air mixtures that encourage combustion will only do so with a certain range of concentration, with its own upper and lower limits. The LEL is the lowest gas-air mixture capable of allowing combustion to occur. Thus there is also The UEL (the upper limit of combustibility) also known as the UFL (the upper flammable limit).