Transducers

=What is a transducer?= toc =How are transducers used in Anesthesiology?=
 * Converts one type of energy into another
 * Usually used as a sensor or detector, but broadly many common devices can be thought of as transducers
 * microphones, guitar pickups, television antennae are all examples
 * Vibrations of guitar strings -> fluctuations in magnetic field -> electrical signal to guitar amp -> sound
 * The most obvious example is the pressure transducer used in arterial blood pressure monitors

=Wheatstone Bridge=
 * Originally invented in 1833 by Samuel Hunter Christie, but improved by Charles Wheatstone in 1843
 * Charles Wheatstone, it should be noted, invented the telegraph
 * Fascinating circuit which can be used to measure an unknown resistance
 * An excellent presentation and tutorial can be found at [|Wisconsin Technical College: Wheatstone Bridge]
 * [[image:http://www.circuitstoday.com/wp-content/uploads/2010/03/Wheatstone-bridge.png]]
 * The resistance of R1, R3 are fixed
 * R2 is variable in some designs of the circuit- most likely in the pressure transducers in use today the resistance is fixed
 * Rx is the voltage to be measured
 * Kirchhoff's first rule states that the sum of current flowing into a node is equal to the current flowing out
 * Or, perhaps better stated: the sum of currents meeting at a common point is zero
 * Therefore, current will flow across the bridge, past the galvanometer, in order to equalize the current traveling through the legs of the circuit
 * An analogy...
 * [[image:http://www.homesalesri.com/UserFiles/Image/LargeMap.gif width="800" height="472"]]

= = //THERE IS A MISSING PIECE HERE....// __How does knowing a resistance help you figure out the pressure???__ =__The Piezoresistive Effect__=
 * Boston and Worcester are connected (roughly) but the Mass Pike and route 9
 * Let us say that traffic one morning is terrible on the pike, but not so bad on route 9, and everyone on the road knows this due to the traffic report or their gps receiver
 * You could get an idea of just how bad the traffic jam is by the flow of cars of the pike onto the route 9 exit
 * // What does this do for us? //**
 * We have a way of measuring resistance very accurately, by judging the current flow past the galanometer
 * Mechanical strain on a semiconductor changes the material's resistance
 * Strain changes the spacing of atoms, and thus alters the relative ease/difficulty for electrons to conduct between them
 * By placing a semiconductor into the fluid column of the arterial line, subjecting it to the fluctuating pressures, the resistance changes dynamically with the fluctuations in pressure
 * The Semiconductor is hooked up to a Wheatstone bridge
 * The changing current through the galvanometer is the output of the system, and the software works out the mathematics to get back to the pressure.

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