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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Principles of Measurement
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