Pressure gauges are tools used to measure a physical quantity called pressure. The pressure is defined as the quotient between the perpendicular component of the force and the area on which it acts, being defined through the following equation:
P = F / A
Where P is the pressure and F is the force perpendicular to the area A. The unit of pressure in the International System of Units is the pascal, abbreviated Pa. Since it is a small unit for frequently handled pressures, multiples of the pascal , as the kilo, the mega and the giga are in widespread use.
To measure pressure, a multitude of devices have been created, which can be grouped into one of these categories, according to the physical principle they make use of:
Pressure is, along with temperature, one of the most widely measured quantities in the world. This is due to the fact that fluids, be they gases or liquids, exert pressure on the walls of the containers that contain them and on the objects inside..
Pressure gauges are essential instruments in various areas, the main ones being:
In climate studies, the measurement of pressure is very important, since it is a main indicator of the climate: low pressures are associated with rains and storms, while high pressures indicate clear skies.
On the other hand, in all industrial and metallurgical processes there are numerous fluids continually exerting pressure on pipes and vessels, whether they are at rest or in motion. Hence the need to know its value at all times, in order to properly regulate these processes..
Another very familiar example is the check of automotive power, for which the level of compression in the engine cylinders is measured, which is the pressure measured when the piston is at the point of minimum volume. Likewise, the tires must be filled to an adequate pressure, which is checked with a portable gauge designed for this purpose..
Blood pressure or blood pressure is an indication of the general state of health of people and is a routine examination, which is easily performed with a device called blood pressure cuff or blood pressure monitor.
The pressure gauge to use depends on the pressure class to be measured:
Depending on the type of pressure to be measured, various instruments have been designed. These are the main ones:
Barometers measure atmospheric pressure. There are several types, depending on the sensor mechanism.
This instrument was invented by the Italian physicist Evangelista Torricelli (1608-1647). It consists of a tube filled with mercury inverted over a container that is also filled with mercury and open to the atmosphere..
By inverting the tube, the mercury descends, leaving a space at the top where it becomes almost empty, except for a small amount of mercury vapor. There the absolute pressure is quite close to 0.
The force exerted by the air on the exposed part of mercury and the weight of the mercury column in the tube are balanced, since the whole assembly is at rest.
Now, the force of the air on the surface of the mercury is exerted through pressure, and this in turn is related to the height of the column of mercury in the tube, a value close to 760 mm and equivalent to 101 325 Pa.
This barometer uses water instead of mercury, but the liquid column is longer, since the water is lighter: 10.3 m high.
This device does not use liquid, but a metal chamber partially under vacuum that contracts or expands according to the pressure to which it is subjected. The pressure difference is transmitted by a spring, the elongation of which moves a rod attached to the gauge indicator. The pressure is read on a display fitted with a scale.
When a recording system of some kind, such as paper and ink, or a digital display is added to the instrument then the instrument is a barograph and allow the visualization of atmospheric pressure variations in the form of a graph, useful for predicting the weather.
Manometers usually measure relative pressure, although they can sometimes give absolute readings.
It is used to measure small and moderate relative pressures. The simplest consists of a U-shaped glass or plastic tube, filled with fluid, such as mercury, with one of the branches sealed or open to the atmosphere and the other connected to the pressure to be measured..
When pressure is applied at one end, the mercury rises up the other branch until the weight of the column balances the pressure at the end. The difference between the original level and the new level is proportional to the pressure difference between the two points.
A variant of the U-tube manometer is the inclined tube manometer, which is used to measure small pressure differences.
This type of manometer transfers the pressure variations to a graduated scale or dial, through some mechanism. This can be, for example, the deformation of a spring or a tube shaped like a C or wound in a spiral, which tends to straighten when pressure is applied at one end or to bend more if it decreases..
The tube is called Bourdon tube, by Eugene Bourdon, the French engineer who invented it in 1849.
The deformation of the tube moves a stop provided with a mechanical system with gears that converts the linear movement of the tube into one of rotation at the other end, there an indicator needle is connected that can be moved on a dial. The position of the needle on the graduated scale on the dial indicates the pressure value.
They cause a pressure difference to cause a variation in some electrical quantity such as resistance, for example. Said variation can be converted into an electric current that is proportional to the pressure difference and the reading is made on a digital screen..
In a pressure gauge of this type, the elastic deformation of the sensor made of a conductive material produces a change in its electrical resistance. The change in resistance is related to the change in pressure. In the same way, you can vary the capacitance in a capacitor or the inductance in an inductor.
Another kind of sensor is of the piezoelectric type, such as quartz or tourmaline, in which a change in pressure on a crystal of one of these minerals is translated into a voltage.
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