Resistance thermometer circuit

Process Instrumentation

1. RESISTANCE THERMOMETER CIRCUITS VARADA SUJEETH B130345CH

2. What is resistance thermometer?

3. CONSTRUCTION Consists of a Wheatstone bridge for the measurement of resistance. A battery is connected across two junction points, to provide a potential. Other 2 junctions are connected to a galvanometer. Two fixed resistors A,B. Resistance thermometer bulb, r.

4. Adjustable slide wire, s. Slider, c. Resistors and slide wires are made of manganin , to avoid any effect due to ambient temperature changes.

5. PRINCIPLE Null method of measurement. At balance or null point ,potential across A and B is equal to potential across r and s . i.e , i a A= i b B i a r = i b s Combining both, r =(A/B)s

6. WORKING Resistance bulb is made in contact with the body ,whose temperature is to be measured. Resistance of bulb changes corresponding to the temperature. Wheatstone is balanced by the movement of slider over the slide wire resistance. Movement of slider can be measured using a calibrated scale.

7. DISADVANTAGES Contact resistance vary with condition of slide wire. Dirt, dust and wear can increase contact resistance. Resistance of connecting lead wires changes with temperature. Current through bulb causes Joule heating effect , which changes the temperature.

8. CALLENDER – GRIFFITHS BRIDGE Improved version Slide wire, s is placed at top of bridge so that it lies in both arms of bridge. Balanced condition i d D = i b (B+S-f*S ) i d r = i b ( A+f *S) f-fraction of slide-wire lying in arm A

9. Dividing r=D*(A/S +f)/(B/S +1-f) Advantages: effect of contact resistance is avoided.

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