The first task of an automated system is to obtain information about the process being controlled. The devices and instruments have to be responsible then accurately measure the variables, of these processes is essential for optimal control of them.
There are several ways of measuring variables which are also very varied. But mainly we describe what are considered the most important in most industrial processes: pressure, temperature, level and flow.
PRESSURE MEASUREMENT
There are several ways of classifying the elements or pressure measuring devices. One of them, divides them into those so-called balance of gravity, using elastic deformation devices and those that use elements that deliver an electrical response representative of the measured pressure. The type of pressure is usually measured in industrial processes is called gauge pressure which is what has icom reference or starting point at atmospheric pressure. To this end use meters whose scales are called pressure gauges graduated in various units. In the following table we compare the most important units:
Table I Comparison of pressure units
BALANCE OF GRAVITY DEVICES
Measured pressures unknown, balance against the gravitational force of liquids though they are commonly used in laboratory, find applications in industrial plants.
Among the most used are the gauge-tube "U", the inclined tube, the bucket and the bucket tipped tube. In these cases, the manometric liquid used is mercury, although the water is also used when it comes to measuring low pressures. In addition to these devices, we have type float and those with liquid seal that are slowly being pushed aside by other types of meters.
ELASTIC DEFORMATION ELEMENTS
These are devices that alter their shape when subjected to pressure. Within these, there are Bourdon tubes (in "C", spiral and helical), where the shape and type of material defining the range of application, also used the so-called diaphragms, capsules and bellows, usually common in the measurement of relatively lower pressure in the case of Bourdon tubes). These elements have been most used in the manufacture of gauges.
Fig 1 - Manometers
Electrical pressure transducer
Any of the elastic deformation devices can be attached to an electrical device to form an electrical pressure transducer. These produce changes in resistance, inductance or capacitance. Within the first mention should be made to the gauges extesisiométricas (strain GAGAS). A strain gauge is simply a very fine wire to form a grid which is attached to a special role. When the grid is affected by pressure, a change of resistance according to the following formula:
Where p is the resistivity of the wire, the length of the L and A, the cross-sectional area.
This type of transducer can be used to detect small movements and therefore small changes in pressure. Strain gauges used Wheatstone bridge circuits to obtain an electrical outlet. Another similar type of transducer used an elastic disc in which the deformation element is silicon and is sealed with silicone fluid and protected by a diaphragm.
The capacitive sensor consists of two conductive and a dielectric placas1. As pressure increases, las1 plates tend to move away, changing its capacitance. The fluid that is: measuring, serves as a dielectric.
Because the level measurement and flow measurement in some cases require differential pressure, such pressure is of great importance in industrial processes. Among the most used dispositivosj whose output is used by some standard, makes transmitters, are pneumatic or electronic. In both cases they are instruments in which a diaphragm capsule or capacitive sensor filing serves as primary sensor of course the main difference in the output structure (transmission).
