Introduction to measuring instrumentation
- 2. 2 Bridge AC Bridge DC Bridge Bridge Measurement Wheatstone Bridge Kelvin Bridge Maxwell- Wein Bridge Hay Bridge Schering Bridge Wein Bridge
- 3. Bridge Measurement Bridges are commonly used to measure unknown quantity (i.e. resistance, inductance, capacitance, admittance , conductance and any of the impedance parameters. The simplest form of the bridge is for measuring resistance and is known as Wheatstone bridge 3
- 4. Wheatstone Bridge The Wheatstone bridge is normally used for the comparison and measurement of resistances in the range of 1Ω to 1 MΩ. The bridge consists of the four resistances together with a source and a null detector usually Galvanometer. R2 and R3 are normally known resistors, R1 is a variable resistance, and Rx is the unknown resistance 4
- 5. Wheatstone Bridge Balanced bridges means that the potential between points B and D equal Zero when the voltage E is applied to the circuit by closing switch S1. To examine the balanced bridge condition, we can close switch S2 and if the sensing device (G) reads no current then, the bridge is balanced and if it reads some current then the bridge is unbalance We can balance the bridge by varying R1 until (G) reads zero current. 5
- 6. Wheatstone Bridge 6 If the bridge is balanced bridge, then: Also and Rearrange Eqs (1) and (2) Now we can determine the value of the unknown resistance Rx if 1. We guarantee that the bridge is balanced (G reading is zero) 2. The values of R1, R2 and R3 are known 1 2 2 1 1 2 1 R i R i V V 3 2 3 2 R R E i i 2 1 1 x x R R E i i 3 2 3 1 1 3 2 R R R R R R R R x x
- 7. Wheatstone Bridge The resistance R1 is called standard arm of the bridge. The resistance R2. and R3 are called ratio arms 7
- 10. Example 10
- 11. Solution 11
- 13. Example 13
- 14. Solution 14