# Light Bulb Pirani Gage Project

Posted on Feb 5, 2014

The principle is that the resistance of metals increases at higher temperatures and a heated wire suspended in a gas will lose heat by conduction and convection in proportion to the number of gas molecules present; the heat loss is thus proportinal to pressure. One of the best ways to measure the heat loss is to measure the voltage required to mai

ntain the filament at a fixed temperature. The wire has a positive coefficient of temperature and, given the value of resistance at ambient temperature, one can calculate the resistance of the wire at a chosen elevated temperature. If a Wheatstone Bridge is configured so that a reference resistor, in value equal to the resistance that the filament will reach at an elevated temperature, is paired with the sensor filament, the bridge will be out of balance at ambient temperature since these resistances are not equal (as are the insensitive resistors in the other bridge leg - as shown in the schematic). If this imbalance is used to control a voltage applied to the Wheatstone bridge, an increased voltage will heat the filament and its resistance will increase, while the reference resistor (far less temperature sensitive) remains stable. The bridge and feedback circuit stabilize when the filament resistance reaches the value of the paired reference; this assumes that the other leg of the bridge uses equal value resistors as shown in the schematic. The voltage being applied to the bridge at balance at atmospheric pressure becomes the "ATM" calibration point. As the pressure surronding the filament is reduced, less heat is lost than at atmospheric pressure, and less voltage is needed by the bridge circuit to maintain the filament at the correct resistance (and temperature); the bridge excitation voltage is then an indication of...

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