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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)

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bridge methods

Bridge methods

ABE425 Engineering Measurement Systems

agenda
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
Null-type DC Wheatstone bridges are used for accurate resistance measurement

The bridge is balanced when the voltage Vdis 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

measurement procedure using galvanometer and decade resistor box
Measurement procedure using Galvanometer and decade resistor box

Decade Box

Galvanometer

Known, constant

At balance:

Unknown resistor

thompson mirror type galvanometer 1880 note the antenna to compensate for electric magnetic fields
Thompson mirror type galvanometer (1880). Note the ‘antenna’ to compensate for electric/magnetic fields
null type ac wheatstone bridge for impedance measurement
Null-type AC Wheatstone bridge for impedance measurement

The bridge is balanced when the voltage Vdis adjusted to zero by tuning

capacitor and inductance coil models
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
Null-type Parallel-Resistance-Capacitance bridge for capacitance and dissipation factor measurement
slide13

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
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

output deflection for r 2 r 3 1 000 ohm showing significant non linearity
Output (deflection) for R2, R3 = 1,000 Ohm showing significant non-linearity

Bridge balance

measuring the drag coefficient of a sphere using a compensation method
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
Links

Schneider, N. 1904. Electrical instruments and testing. Spon and Chamberlain, New York.