1. ABE425 Engineering Measurement Systems
Bridge methods
ABE425 Engineering Measurement Systems

2. 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

3. 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

4. This is also a balance method
Roberval design

5. Measurement procedure using a Galvanometer and a decade resistor box
Decade Box
Unknown resistor
Galvanometer
Known, constant
In balance:

6. Galvanometers are VERY SENSITIVE instruments to detect zero current
D'Arsonval galvanometer

7. Thompson mirror type galvanometer (1880)
Thompson mirror type galvanometer (1880). Note the ‘antenna’ to compensate for electric/magnetic fields

8. Null-type AC Wheatstone bridge for impedance measurement
The bridge is balanced when the voltage Vd is adjusted to zero by tuning

9. 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

10. Null-type Parallel-Resistance-Capacitance bridge for capacitance and dissipation factor measurement

11. Maxwell bridge to measure inductance, resistance and quality factor of low quality coils (Q<10)

12. Hay bridge to measure inductance, resistance and quality factor of high quality coils (Q>10)

13. Wien bridge for frequency measurement

14. 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

15. 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

16. Deflection type DC Wheatstone bridge

17. Output (deflection) for R2, R3 = 1,000 Ohm showing significant non-linearity
Bridge balance

18. Output (deflection) for R2, R3 = 10,000 Ohm showing reduced non-linearity
Bridge balance

19. 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

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

21. ABE425 Engineering Measurement Systems
The End
ABE425 Engineering Measurement Systems