What are Transducers? Working of the Thermometer

We have seen that there are two methods of measurement of any physical quantity: the direct method and the indirect method. In the direct method any physical quantity like length, mass, etc are measured directly by some instruments like the measuring tape, weighing scale etc. In the indirect method of measurements some transducing devise, called transducer, is used, which is coupled to a chain of the connecting apparatus that forms the part of the measuring system. In this system the quantity which is to be measured (input) is converted into some other measurable quantity (output) by the transducer.

Transducers are the mechanical, electrical, electronic, or electromechanical devices that convert one form of the energy or the property that cannot be measured directly into the other form of energy or property that can be measured easily. The signal given to the transducers is called as input, this is the parameter that is to be measured but cannot be measured directly. The signal obtained from the transducer is called as output, which can be measured easily.

The transducer selected for the measuring system is such that the output obtained is proportional to the input. Since the output can be measured easily by the available instruments, the scale can be calibrated between the values of the output and the input. From this calibration, for all the values of the output the input value i.e. the parameter or physical quantity to be measured can be obtained easily.

The transducers are used for various applications, but here we are considering transducers used for the measurement of the physical quantities like temperature, pressure, flow etc.

There are numerous types of transducers and they have different types of applications, let us see the example of use of transducer for measurement of temperature. In our day-to-day life we have to measure the temperature many times and the most common devise used for this purpose is the thermometer. The temperature of the surroundings or body cannot be measured directly, but we know that liquids tend to get expanded when heated and get contracted when cooled. This property of the liquids is used to measure the temperature in thermometers, which is type of transducer.

In thermometers there is thin capillary tubing and small bulb at the bottom, which is filled with highly temperature sensitive liquid called mercury. When the temperature of the bulb of the thermometer increases, the mercury tends to expand and fill the capillary tube to certain level depending on the temperature. The thermal expansion of mercury is proportional to the temperature of the mercury, so more is the bulb temperature more is expansion of the fluid. Thus if the bulb temperature is higher, mercury will expand to higher levels in the capillary and if its temperature is lesser, the rise in level will also be lesser.

Now, outside the capillary tubing, the scale is marked that indicates the temperature of the body. This scale is marked from the standard scale obtained by considering the extent of expansion of mercury at various temperatures. Thus the level of the mercury in the capillary indicates the temperature of the body.

Thermometer is a sort of the transducer in which the property of the thermal expansion of the fluids is used for the measurement of the temperature of any body or space. Here the property of thermal expansion is calibrated to the scale in the capillary to enable the temperature measurement. The value of the temperature cannot be measured directly, but the rise in level of the mercury can be measured easily. The rise in level of mercury is proportional to the temperature of the body and this scale is developed considering the relation between the two.

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