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[Instrumentation Table of Contents]
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The illustration below shows a basic bridge
circuit which consists of three known resistances, R1,
R2, and R3 (variable), an unknown
variable resistor RX (RTD), a source of voltage,
and a sensitive ammeter.
Resistors R1 and R2 are
the ratio arms of the bridge. They ratio the two variable
resistances for current flow through the ammeter. R3
is a variable resistor known as the standard arm that is
adjusted to match the unknown resistor. The sensing ammeter
visually displays the current that is flowing through the
bridge circuit. Analysis of the circuit shows that when R3
is adjusted so that the ammeter reads zero current, the
resistance of both arms of the bridge circuit is the same.
Equation A-1 shows the relationship of the resistance between
the two arms of the bridge.
Equation A-1
Since the values of R1, R2,
and R3 are known values, the only unkown is Rx.
The value of Rx can be calulated for the bridge
during an ammeter zero current condition. Knowing this
resistance value provides a baseline point for calibration of
the instrument attached to the bridge circuit. The unknown
resistance, Rx, is given by Equation A-2.
Equation A-2
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