General Measurement System: The Detector-Transducer Stage

In the direct method of measurement the physical quantity like length or mass are measured directly by the measuring instruments. The indirect method of measurement comprises of various stages for the measurement of the physical quantity like temperature, pressure, force etc, since they cannot be measured by the direct instruments. In this method the transducer is used which is connected to a host of other instruments to convert one form of energy that cannot be measured into the other form that can be measured easily. The input and the output values are calibrated so that for all the value of output the value of the input can be calculated.

The General System of Measurement comprises of three stages (see the fig), these are:

I) First stage - the detector-transducer stage.

II) Second stage – Intermediate modifying stage

III) Final stage – Terminating stage comprising of: indicator, recorder, some controller as individuals or in combination.

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The main function of the first stage is to detect or sense the input signal. The input signal is usually the physical quantity that has to be measured. It can be temperature, pressure, force, velocity, or any other quantity that has to be measured. The devise most commonly used for detecting the input signal is the transducer or sensor. The transducer can detect the input quantity that has to be measured but cannot be measured directly by the instruments. It has the ability to convert these signals into analogous output that can be measured easily.

The detector, which is transducer, should be able to sense the input signals that are to be measured, but at the same time it should be insensitive to the other possible types of inputs. For example if the pressure measuring transducer like bellow is being used, it should be sensitive to the pressure, but insensitive to the acceleration or other disturbances at the input. Similarly, if strain gauges are being used, they should be sensitive to temperature only. In actual practice it is very rare that the transducer is sensitive only to the signals that are to be measured.

Let us consider an example here for the measurement of pressure using Bourdon tube. The pressure of fluid cannot be measured directly, hence Bourdon tube is used as the transducer to convert the property or signal of pressure into other property or signal that can be measured easily. The Bourdon tube is a thin tube with oval cross section and coiled into an arc with included angle less than 360 degree (see the fig). One end of this tube is connected to the inlet pressure and the other end, which is sealed, is connected to the pointer that moves on the angular scale. When the pressure is applied to the Bourdon tube the oval section tends to become circular, due to which the tube tends to uncoil and move the end connected to the pointer.

Here the Bourdon tube senses the pressure, and it acts as the transducer that detects the quantity to be measured.

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

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