This means that four strain gages are interconnected as a loop circuit and the measuring grid of the pressure being measured is aligned accordingly. The signal generated by the strain gage bridge is a low strength signal and may not work with other components of the system, such as PLC, data acquisition modules DAQ or computers.
Thus, pressure sensor signal conditioner functions include excitation voltage, noise filtering or attenuation, signal amplification, and output signal conversion.
Furthermore, the change in the amplifier output is calibrated to be proportional to the pressure applied to the flexure, which can be calculated via the pressure sensor circuit equation. Check out our Pressure Transducer Store. Talk to an Engineer today! Pressure sensors can be classified in terms of the type of pressure measurements they gauge as well as the pressuring-sensing technology the transducer operates. In that regard, there are three methods to measure pressure: differential, absolute, and gauge.
Differential Pressure Transducer : Differential pressure is a measurement of the pressure difference between two pressure values or two pressure points in the system , thus measuring by how much the two points differ from each other, not their magnitude relative to atmospheric pressure or to another reference pressure such as absolute vacuum.
This is different from a static or absolute pressure sensor that would measure pressure using just one port and typically differential pressure sensors are packaged with two ports to which pipes can be attached and connected to the system in two distinct pressure points from where the differential pressure can be measured and calculated.
This pressure measuring approach is typically used to measure the flow of a liquid or a gas in pipes or ducts. Absolute or Vacuum Pressure Transducer: This sensor measures the absolute pressure, which is defined as the pressure measured relative to a perfect sealed vacuum. Absolute pressure sensors are used in applications where a constant reference is required. These applications require reference to a fixed pressure as they cannot be simply referenced to the surrounding ambient pressure.
For example, high-performance industrial applications such as monitoring vacuum pumps, liquid pressure measurement, industrial packaging, industrial process control and aerospace and aviation inspection use this technique. When it comes to measuring air pressure, specifically for applications such as barometric measurements for weather or in altimeters, an absolute pressure sensor is the device of choice.
Talk to our Application Specialist today! Gauge or Relative Pressure Transducer : Gauge pressure is simply a special case of differential pressure with pressures measured differentially but always relative to the local ambient pressure.
In the same respect, absolute pressure can also be considered a differential pressure where the measured pressure is compared to a perfect vacuum. Changes of the atmospheric pressure due to weather conditions or altitude directly influence the output of a gage pressure sensor. A gauge pressure higher than ambient pressure is referred to as positive pressure. If the measured pressure is below atmospheric pressure it is called negative or vacuum gage pressure.
There are a variety of pressure-sensing technologies or sensing principles capable of transducing pressure into a measurable and standardized electrical signal. This article will focus on the force collector types, which are the ones that use a force gauge i. Resistive or piezoresistive effect: Resistive pressure measurement sensors utilize the change in electrical resistance of a strain gauge bonded to the diaphragm also known as a flexure element that is exposed to the pressure medium.
The strain gauges often comprise of a metal resistive element on a flexible backing bonded to the diaphragm i. Normally, the strain gauges are connected to form a Wheatstone bridge circuit to maximize the output of the sensor and to reduce sensitivity to errors. This is the most commonly employed sensing technology for general-purpose pressure measurement and uses the same principle of how a load cell works.
Capacitive: Capacitive pressure sensors use a diaphragm that is deflected by the applied pressure to create a variable capacitor to detect strain due to applied pressure. As pressure is applied, the external pressure compresses the diaphragm, and the capacitance value decreases. As the pressure is released, the diaphragm returns to its original shape and capacitance follows. Common technologies use metal, ceramic, and silicon diaphragms.
The capacitance can be calibrated to provide accurate pressure reading. Capacitive sensors, which display a capacitance change as one plate deflects under applied pressure, can be highly sensitive and withstand large overloads.
Constraints on materials, and joining and sealing requirements, however, can restrict applications. Piezoelectric effect: Piezoelectric pressure sensors utilize the property of piezoelectric materials like ceramic or metalized quartz, to generate an electrical potential on the surface when the material is subjected to mechanical stress and strain is generated.
The charge magnitude is proportional to the pressure applied, and the polarity is defined by the pressure direction. The electrical potential accumulates and dissipates quickly as pressure changes, allowing measurement of fast-changing dynamic pressures. In physical science the symbol for pressure is p and the SI unit for measuring pressure is pascal symbol: Pa.
One pascal is the force of one Newton per square meter acting perpendicular on a surface. Other commonly used pressure units for stating the pressure level are psi pounds per square inch , and bar. Use of pressure units have regional and application preference: psi is commonly used in the United States, while bar the preferred unit of measure in Europe.
Need a complete load measuring system? We supply sensors with instrumentation ready to use straight from the box. All pressure measurements are made with respect to a reference pressure and are expressed in those main terms. Gauge pressure sensors measure the input pressure of your media with reference to ambient atmospheric pressure vented to atmosphere. Gauge is used to measure pressure relative to ambient conditions, such as with car tyre pressure.
As the sensors are open to the atmosphere, they are susceptible to humidity. Care must be taken that units are installed in dry areas otherwise internal circuitry can fail. Measures the input pressure of your media with reference to a sealed chamber closed with atmospheric pressure approximately 1bar. This protects the internal circuitry of the sensor from humidity. This range is normally restricted to minimum 7bar and above.
Outside installations or where the equipment may be washed are good application examples. Figure 6 — Bernoulli equation — The pressure applied by the moving fluids on the tube transversal section. Supposing an ideal fluid without viscosity, it is displaced with friction and so without energy loss.
The work carried out by the resultant of the forces acting on a system is equal to the variation of the kinetic energy, the work-energy theorem. With this, we have:. This is the Bernoulli equation that proves that the pressures total along a tube is always constant on an ideal system. The interesting thing on this equation is that the following pressures can be recognized:. This ratio is very useful to calculate de fluid speed, given the impact pressure and the static pressure.
From this ration, there may be calculated, for example, the fluid flow:. The C values are experimental results and for each type of primary measurement element and impulse-take system, C varies in function of the piping diameter D , the number of Reynolds Rd and the ratio of the diameters related to section A1 and A2. In function of the reference, the pressure measurement can be classified as: gauge, absolute and differential or relative.
Figure 7 — References for Pressure and most usual Pressure types. Note that the gauge pressure is given by the difference between the absolute and the atmospheric pressure. Generally, sensors are classified according to the technique used on the mechanical pressure over a proportional electronic signal.
All techniques have a single purpose: turningthe pressure applied on a sensor into an electronic signal proportional to it:. They are mostly formed by crystal elements strain gauge interconnected in bridge wheatstone with other resistors that provide zero adjustment, sensitivity and temperature compensation.
The construction material varies according to the manufacturer and today solid state sensors are easy to find. Disadvantages: limited operation temperature range, applicable on low pressure ranges because they generate a very low, unstable, excitation signal.
Currently there is the so-called film transducer , which is made from steam deposition or the injection of strain-gauge elements directly on a diaphragm, minimizing the instability due to the use of adhesives on the alloy of bonded wire models. The great edge is that it produces a higher level electric signal, however totally vulnerable at high temperatures, because the temperature affects the adhesive material used when sticking the silicon to the diaphragm. Several techniques based on the production of piezoresistive silicon sensors silicon substrate are emerging, however susceptible to signal degradation in function of the temperature and require compensation from complicated circuits, error minimization and zero sensitivity.
They are totally unviable in applications subject to long high temperature periods, as the diffusion degrades the substrates in these conditions. The piezo-electric material is a crystal that produces a differential tension proportional to the pressure applied on its faces: quartz, Rochelle salt, barium titanium, tourmaline, etc.
This material cumulates electric loads in certain areas of its crystal structure, when they suffer physical deformation by the action of a pressure. The piezo-electricity was discovered by Pierre and Jacques Curie in Their disadvantage is that they require a high impedance circuit and a high gain amplifier and are susceptible to noises.
Furthermore, due to their dynamic nature they do not perform solid state pressure measurement. However, their advantage is a quick response. The relation between electric load and the pressure applied to the crystal is practically linear:.
They generally follow the technology principle known as vibrating wire. A magnetic wire coil is attached to the diaphragm, which oscillates when subject to a magnetic field that will conduct an electric current. The sensor is formed by a silicon capsule set on a diaphragm that vibrates when a pressure differential is applied and the vibration frequency depends on the applied pressure.
These sensors are the most reliable and have been used on million of applications. They are based on transducers whose pressure applied to diaphragm sensors produces a variation of capacitance between them and a central diaphragm, for instance. This variation is typically used to vary an oscillator frequency, used as a capacitor bridge element and also to vary an oscillator frequency.
This frequency can be measured directly by the CPU and converted into Pressure. It is worth remembering that this reading principle is totally digital and is used by Smar since the middle 80s. Smar is the only Brazilian company and one of the few in the world to make this type of sensor. They have linear and practically insensitive to temperature variations, being the favorite for instrumentation and process control, as they have excellent performance on stability, temperature and static pressure.
Some of their best features:. Figure 10 — Example of a capacitive sensor construction. The sensitivity of fiber sensors, namely, the less intense measurable disturbance, may depend on:. Optical fiber sensors are compact and show sensitivity comparable to similar conventional devices. Pressure sensors are built using a moving membrane on one of the fiber ends. The advantages of these sensors are: high sensitivity, small size, flexibility and resistance, low weight, long life span, long transmission distance, low material chemical reactivity, ideal to operate on intrinsically safe, high voltage and hazardous ambient , electric isolation, electromagnetic immunity, multiplexing of signals, i.
A technique used on optical sensor construction is the Fabry-Perot Interferometer, a device generally utilized to measure high precision wave lengths, whose two partially reflecting mirrors of glass or quartz are aligned and provide a maximum fringe contrast and the distance between them through mechanical variation.
The distance variation could be generated by pressure and it would work as a pressure sensor. Figure 11 — Pressure Sensor with the Fabry-Perot principle. In the industry, among several equipment used for pressure measurement two of them can be enhanced: the manometer and the pressure transmitter. The gauge meter is used for local pressure reading and normally has a connection to the process and a display when electronic or a pointer when mechanical for local pressure reading.
They are usually inexpensive devices to be used when the pressure does not have to be transmitted to a control system and no exactness is required. For example, static pressures, pump pressures, etc. There are also differential, sanitary models, vacuometers, etc.
Figure 12 — Types of manometers. An intelligent pressure transmitter combines the sensor technology and its electronic. Users must be cautious about some points to avoid paying higher prices for something they will not use or is not required by their application:. Figure 13 — LD - HART mA Pressure Transmitter with capacitive sensor, single electronic plate, high performance the transmitter with the best response time in the market.
The microprocessed pressure transmitters have the great advantage of allowing better interaction with the user, with friendly interfaces.
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