General Measurement System: Intermediate Modifying Stage and Terminating Stage

Let us continue with the second stage of general measurement system. The second stage or the intermediate modifying stage converts the input signal in the form that can be used easily by the final stage to indicate the value of the input physical quantity. The modifying stage may change the type of the input signal so that the output value can be measured easily. Or it may increase the amplitude and/or the power of the input signal to the level so as to drive the final terminating devices.

The intermediate modifying stage should be designed such that there is proper matching of characteristics between the first stage and second stage and the second stage and the final stage of measurement so that the output obtained is analogous to the input. The intermediate may also have to perform the functions of filtering the unnecessary input, and also integration, differentiation, telemetering etc, wherever required.

Here are some examples. In the bellows used for pressure measurement, the pressure is converted into the translation displacement that can be measured easily by the final stage. In the glass thermometer, the level of the mercury in the capillary increases or decreases depending on the bulb temperature.

Let us come back to our previous example of measuring the pressure by using Bourdon tube. Here, the coiled oval shaped tube acts as the intermediate stage. When the pressure is applied at the inlet the oval tube tends to become circular, but inner and the outer diameters of the coil tend to remain the same. Due to this the coil tends to uncoil producing the angular motion of the tip of the coil, which is connected to the final stage of the pointer that indicates the value of the applied pressure.

The final stage or the terminating stage provides the information about the input physical quantity in the form that can be easily read by the human beings or the controller. If the final output is intended to be recognized by the human beings it can be obtained in the form of the pointer movement on the predefined scale, in the digital format, by the graph etc. If the output is meant to perform some automatic function, it can be given to the controller that controls the operation of the next unit.

The output obtained at the final stage is calibrated against the input quantity so that the value shown by the pointer or the digital indicator is analogous to the input value. Thus the physical quantities like temperature, pressure, force etc which cannot be measured directly are measured with the help of the transducer in the form of pointer movement on the scale or the digital output.

In our example of Bourdon tube, the sealed tip of the coil is connected to the pointer via linkages and the gear arrangement. The pointer moves over the predefined scale that indicates the value of the pressure. When the pressure is applied the tip of the Bourdon tube uncoils, which moves the linkages and the gear that finally produce the rotary motion of the pointer on the scale indicating the actual value of the applied pressure.

1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck

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