Among the various types of pressure measurement instruments for industries, two stand out: pressure transmitters. They are useful in multiple applications and allow measurement value readings to be carried out on site for transmission to a command post or process control system. The variety of types and their optimal performance has made them a highly demanded option.
WHAT ARE PRESSURE TRANSMITTERS?
It is a device that measures the expansive force of liquids or gases. It is a sensor that is made up of a surface area and sensitive to pressure (made of silicon, steel or other materials that stand out for their resistance and for being deformable. Components are implemented on the back of the surface that convert into an electrical signal the force that is exerted on it.
With respect to pressure, it is measured in the amount of force per unit area necessary to stop the expansion applied by liquids and gases. It is basically expressed in P = F / A. The most common units that quantify pressure are PSI (pound per square inch) used in Anglo-Saxon countries and Bar belonging to the International System. The transmitters are used in pressure sensitive environments, boilers, air compressors, flow rates in pipelines, pump controls, fluid transfer systems at different pressures, and so on.
They are designed to determine absolute pressure and to support monitoring in extreme environments and processes. They have steel diaphragms that record the deformations and display them on easy-to-read analog or digital Pressure Gauge in Shanghai. There are several industries that depend on its use to measure gases or liquids at high pressures. The most commons are:
· Mining, natural gas and petrochemicals, where they allow to know the values between the exterior and interior of the equipment.
· Those dedicated to drilling offshore.
· Those depend on hydraulic and pneumatic machinery.
· Die cutting industries.
Some know these pressure measuring instruments as sensors. Another name that is often accepted is pressure transducers, however, you must be careful since some manufacturers use it to designate equipment that emits signals in volts instead of MA. Standard values on transmitters are 4 to 20 MA.
They must have the following basic characteristics:
· Digital communication interface.
· Exit sign.
· Temperature and pressure compensation.
· Easy calibration.
· Low installation time and cost.
· Self-diagnostics.
· Decreased penetration / intrusion.
· Protector of transients.
· Various interface resources.
HOW DO THEY WORK?
The objective is to achieve that the pressure value of any hydraulic or pneumatic system is translated into an electrical one so that it can be used in control equipment. To achieve this, the following sensors have been developed with specific functions:
Resistive
They are based on a principle of measurement of the variations of the resistances that induce the deformations according to the pressure. They incorporate a membrane whose deformation can be controlled and which has equally deformable electrical conductors. This causes a decrease or increase in resistance that is measured by a Wheatstone bridge.
Piezoresistives
Its measurement principle is similar to that of the previous point, although it differs in the use of semiconductors and not metal and in that the deformation causes a variation in the specific resistance. Piezoresistivity refers to the change in electrical resistance with contraction or deformation resulting from applied pressure. They commonly integrate crystalline elements connected in a Wheatstone bridge and resistors. Currently there is a transducer film that generates a higher level electronic signal, although they are not very efficient in environments with high temperatures. Silicon sensors of this type are also emerging, although their signals can also degrade with temperature and require complex circuitry to minimize errors.
Capacitive
They allow measuring the capacitance of the capacitors that varies depending on the proximity to the active surface. Capacitors are made of a membrane with two layers of metal that deform with the application of pressure. The deformation decreases the distance between the plates, thus maintaining the dielectric constant and the surface and increasing the capacity.
These pressure measuring instruments are the most reliable and adaptable in many applications. Its variation in capacitance is used so that the frequency of an oscillator can vary. It does not involve an A / D conversion, which provides precision and the elimination of drifts in digital and analog conversions. The advantages they offer are:
· They are excellent in flow applications.
· Perfect in high and low pressure systems.
· They are flexible and accurate thanks to their linear response.
· They reduce the probable total error and therefore the variability.
Piezoelectric
Its principle is based on an effect that occurs in non-conductive crystals. When the latter are compressed, an electrical polarization is generated on opposite surfaces. When the electrically charged crystal structure is delocalized, a dipole instant is produced, which is reflected in an apparent surface charge. The direction influences the polarity and the intensity of the load is proportional to the force used by the pressure and can be picked up and amplified.
WHAT TYPES ARE THERE?
There are two types, the first is the gauge pressure. They are used to read pressure directly in pneumatic or hydraulic systems. They compare a relative pressure against atmospheric measurement. Its production can be altered by changes in the pressure of the environment.
The second is differential pressure. They are useful for determining the pressure difference between two points. It has differentiated measurement, easy assembly and automatic zero point adjustment. They can be used in ventilation systems, in-line filters, adsorption dryers and filter installations in general. Also to monitor the filling of pressure vessels. It should be added that they allow the measurement and normalization of relative pressure.
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