Question

1) A thermocouple element when taken from a liquid at 50 degrees Celsius and plunged into a liquid at 100 degrees celsius at time equal to zero gave the following e.m.f. values. Determine the 95% response time. Time 0 20 40 60 80 100 120 e.m.f. (mV) 2.5 3.8 4.5 4.8 4.9 5.0 5.0

2) What are the advantages and disadvantages of a plastic film type of potentiometer and when compared with the wired-wound potentiometer.

3) A shaft encoder is to be used with a 50 mm radius tracking wheel to monitor the linear displacement. If the encoder produces 256 pluses per revolution, what will be the number of pulses produced by a linear displacement of 300 mm?.

4) Suggest a sensor that could be used, as part of a control system, to determine the difference in level between two containers. The output is to provide an electrical signal for the control system.

Answer 1

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1) Wire wound potentiometers:

This potentiometer comprises of several rounds of wire wound around the shaft of the non-conducting material. The turns of the coil are bonded together by an adhesive. In this case the slider, connected to the body whose displacement is to be measured, moves on the potentiometer track and it makes contacts with successive turns of the coil. In this case the wire between the two successive turns is not covered by the slider, which limits the resolution of the wire wound potentiometers. However, the larger the number of turns of the coil, more is the resolution of the coil. The resolution is measured as the reciprocal of the number of turns of the coil. This devise has low noise and is mechanically rough and tough.

Plastic film pots:

              These pots comprise of the specially impregnated plastic material having resistance characteristics that are controlled properly. These can be used for rotary as well as translational slider movements. They have very good life, resolution better than the wire wound pots, and low noise.

                    In carbon film and the plastic film pots the resolution is limited by the grain size of the particles, hence their accuracy is very high. Their resolution can be as high as 10 rest to -4 and is usually limited only by the spring connected between the slider and the body whose motion is to be measured.

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