Purpose of Measuring Equipments

About the purpose of measuring equipments

A measuring instrument exists to provide information about the physical value of some variable being measured. In simple cases, an instrument consists of a single unit which gives an output reading or signal according to the magnitude of the unknown variable applied to it. However, in more complex measurement situations, a measuring instrument may consist of several separate elements. These components might be contained within one or more boxes, and the boxes holding individual measurement elements might be either close together or physically separate. Because of the modular nature of the elements within it, a measuring instrument is commonly referred to as a measurement system, and this term is used extensively to emphasize this modular nature.

Common to any measuring instrument is the primary transducer: this gives an output which is a function of the measurand (the input applied to it). For most but not all transducers, this function is at least approximately linear. Some examples of primary transducers are a liquid-in-glass thermometer, a thermocouple and a strain gauge. In the case of a mercury-in-glass thermometer, the output reading is given in terms of the level of the mercury, and so this particular primary transducer is also a complete measurement system in itself. In general, however, the primary transducer is only part of a measurement system. The types of primary transducers available for measuring a wide range of physical quantities widely available.

The output variable of a primary transducer is often in an inconvenient form and has to be converted to a more convenient one. For instance, the displacement-measuring strain gauge has an output in the form of a varying resistance. This is converted to a change in voltage by a bridge circuit, which is a typical example of the variable conversion element.

Signal processing elements exist to improve the quality of the output of a measurement system in some way. A very common type of signal processing element is the electronic amplifier, which amplifies the output of the primary transducer or variable conversion element, thus improving the sensitivity and resolution of measurement. This element of a measuring system is particularly important where the primary transducer has a low output. For example, thermocouples have a typical output of only a few millivolts. Other types of signal processing element are those which filter out induced noise and remove mean levels, etc.

The observation or application point of the output of a measurement system is often some physical distance away from the site of the primary transducer which is measuring a physical quantity, and some mechanism of transmitting the measured signal between these points is necessary. Sometimes, this separation is made solely for purposes of convenience, but more often it follows from the physical inaccessibility or environmental unsuitability of the site of the primary transducer for mounting the signal presentation/recording unit. The signal transmission element has traditionally consisted of single- or multi-cored cable, which is often screened to minimize signal corruption by induced electrical noise. Now, optical fiber cables are being used in ever increasing numbers in modem installations, in part because of their low transmission loss and imperviousness to the effects of electrical and magnetic fields.

The final element in a measurement system is the point where the measured signal is utilized. In some cases, this element is omitted altogether because the measurement is used as part of an automatic control scheme, and the transmitted signal is fed directly into the control system. In other cases, this element takes the form of either a signal presentation unit or a signal recording unit. These take many forms according to the requirements of the particular measurement application.

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