Wheatstone Bridge

What is a Wheatstone Bridge?

  The Wheatstone Bridge is a tool used to measure the resistance through the use of an electrical circuit.  It was first invented in 1833 by Samuel Christie, but was later improved by Charles Wheatstone.  The device was named after Wheatstone later in 1834.  The original purpose of this device was to compare and analyze soil, while today it is used for many other purposes than soil analysis.

How does it work?

  The Wheatstone Bridge is powered by a battery.  The whole premise of the device is through comparison.  A comparison of the change in the amount of conductivity between that of a known substance to that of an unknown sample of air.  There are sensors in the device that are coated with a catalyst.  The catalyst then reacts with the combustible gas to generate heat. This change in temperature causes a difference in the circuit conductivity. This difference in conductivity is then proportional to the amount of gas present.

How is it used in air sampling?

  Even though the methods were explained above, a more specific use for this tool is for air sampling. One specific situation is for the measurement of oxygen present in the air.  The device is not limited to only measuring oxygen, but this is one of the many common uses.  A magnet surrounds the filament, with the air sample in the device.  Due to the nature of oxygen, it is attracted to the magnetic field in the apparatus.  The air sample is then heated which makes oxygen loose its charge.  This forces the oxygen out of the previous magnetic field.  After that filament is cooled down, there is a voltage that is the output of the Wheatstone Bridge.  This voltage is proportional to the level of oxygen that was in the measured sample.  Oxygen is just one of the many gases that can be detected in a sample through the use of a Wheatstone Bridge.  Other gases that can be identified are mercury vapor, NO2, NO, carbon monoxide, and other combustable gases. Overall, this is a very effective way to measure levels of gases in the air. 

Sources:

  Basics of Industrial Hygiene 

     page 132-133

  Bright Hub Engineering 

     http://www.brighthubengineering.com/commercial-electrical-applications/53492-how-to-find-unknown-resistance-using-wheatstone-circuit/

  Indoor Air Quality Engineering: Environmental Health and Control of Indoor Pollutants 

     page 190-191