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Agenda Null methods (calibration and highly accurate measurements) DC bridges (Resistance measurement) Wheatstone Capacitor and inductance (coil) models AC bridges (Inductance / capacitance measurement) Null-type Parallel-Resistance-Capacitance bridge for capacitance and dissipation factor measurement Maxwell bridge for inductance (coil) and quality factor measurement Wien bridge Approximate measurement of Inductance and Capacitance Deflection methods (control systems) Deflection-type Wheatstone bridge and non-linearity
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ABE425 Engineering Measurement Systems
Bridge methods ABE425 Engineering Measurement Systems Agenda Null
methods (calibration and highly accurate measurements)
DC bridges (Resistance measurement) Wheatstone Capacitor and
inductance (coil) models AC bridges (Inductance / capacitance
measurement) Null-type Parallel-Resistance-Capacitance bridge for
capacitance and dissipation factor measurement Maxwell bridge for
inductance (coil) and quality factor measurement Wien bridge
Approximate measurement of Inductance and Capacitance Deflection
methods (control systems) Deflection-type Wheatstone bridge and
non-linearity Null-type DC Wheatstone bridges are used for accurate
resistance measurement
The bridge is balanced when the voltage Vd is adjusted to zero by
tuning R1 while R2 and R3 are known and kept constant. The
null-detector is usually some type of galvanometer The unknown
resistance value can then be computed using the values of the other
resistances Since there are no inductances (coils) or capacitances,
a DC source is sufficient This type of bridge is used for strain
gage measurements This is also a balance method
Roberval design Measurement procedure using a Galvanometer and a
decade resistor box
Decade Box Unknown resistor Galvanometer Known, constant In
balance: Galvanometers are VERY SENSITIVE instruments to detect
zero current
D'Arsonval galvanometer Thompson mirror type galvanometer
(1880)
Thompson mirror type galvanometer (1880). Note the antenna to
compensate for electric/magnetic fields Null-type AC Wheatstone
bridge for impedance measurement
The bridge is balanced when the voltage Vd is adjusted to zero by
tuning Capacitor and Inductance (coil) models
Capacitor model with Capacitance and dissipation resistance
Inductor (coil) model with Inductance and series resistance
Inductances have a quality factor Null-type
Parallel-Resistance-Capacitance bridge for capacitance and
dissipation factor measurement Maxwell bridge to measure
inductance, resistance and quality factor of low quality coils
(Q10) Wien bridge for frequency measurement The coil
characteristics inductance and series resistance can be measured by
equalizing the voltage across a variable resistor and the coil
itself Series resistance of the coil RL measured with a DVM
Approximate method of measuring capacitance
Measure the AC Voltages for a known input frequency across resistor
R and capacitor C Resistance measured with a DVM Deflection type DC
Wheatstone bridge Output (deflection) for R2, R3 = 1,000 Ohm
showing significant non-linearity
Bridge balance Output (deflection) for R2, R3 = 10,000 Ohm showing
reduced non-linearity
Bridge balance Measuring the drag coefficient of a sphere using a
compensation method
Electric current returns sphere to original position Air flow
pushes sphere to the right Drag coefficient ~ Electric current
Links Schneider, N Electrical instruments and testing. Spon and
Chamberlain, New York. ABE425 Engineering Measurement Systems
The End ABE425 Engineering Measurement Systems