Sensor Times
Monday, February 24, 2025
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Pressure Sensor

 

A pressure sensor measures the pressure of gases and liquids. A pressure is one type of the force required to stop a gas or fluid from expanding. When pressure sensor senses pressure, it gives output in terms of optic signals, visual signals and/or auditory signals. Pressure sensors can be used in systems to measure other variables such as fluid/gas flow, water level. The pressure is coupled to sensor through tube, pipe or atmosphere.The pressure sensor will not respond to force directly applied to the sensing area.

 

Mechanical Pressure Gauges

In mechanical gauges, the motion generated by the force-summing device is converted by mechanical linkage into dial or pointer movement. The better gauges provide adjustments for zero, span, linearity, and (sometimes) temperature compensation for mechanical calibration. High-accuracy mechanical gauges take advantage of special materials, balanced movements, compensation techniques, mirror scales, knife-edge pointers, and expanded scales to improve the precision and accuracy of readings. The most accurate mechanical gauges, test gauges, are used as transfer standards for pressure calibration, but for applications requiring remote sensing, monitoring, or recording they are impractical. Their mechanical linkages also limit their frequency response for dynamic pressure measurements.

 

Electromechanical Pressure Sensors

Electromechanical pressure sensors, or pressure transducers, convert motion generated by a force-summing device into an electrical signal. These sensors are much more useful and adaptable than mechanical gauges, especially when applied in data acquisition and control systems. In well-designed transducers, the electrical output is directly proportional to the applied pressure over a wide pressure range. For rapidly changing—dynamic—pressure measurement, frequency characteristics of the transducer are an important consideration.

 

Pressure sensor classification

 

Pressure sensors can be classified by three ways.

• Pressure range
• Temperature range
• Type of pressure

 

In terms of pressure type, pressure sensors can be divided into five categories.

 

Pressure sensors are available with a variety of reference pressure options: gauge ,absolute differential, and sealed. All use a force-summing device to convert the pressure to a displacement, but that displacement is then converted to an electrical output by any of several transduction methods. The most common are strain gauges, variable capacitance, and piezoelectric.

 

Absolute pressure sensor
This sensor measures the pressure relative to perfect vacuum pressure (0 PSI or no pressure). Atmospheric pressure is measured using an absolute pressure sensor.
Gauge pressure sensor
This sensor measures the pressure relative to a given atmospheric pressure. Example: A tire pressure gauge. Blood pressure measurements are taken using a gauge pressure sensor.
Differential pressure sensor
This sensor measures the difference between two or more pressures introduced as inputs to the sensing unit. Differential pressure is also used to measure flow or level in pressurized vessels.
Sealed pressure sensor
This sensor is the same as the gauge pressure sensor except that it is previously calibrated by manufacturers to measure pressure relative to sea level pressure (14.7 PSI).

 

Strain Gauge Transducers

 

Strain gauge transducers are based on metal or silicon semiconductor strain gauges. The gauges can be discrete units attached to the surface of the strained element or unbonded gauges. The gauge material can be sputtered onto a diaphragm or diffused into a silicon diaphragm structure. The most common force-summing device for strain gauge transducers is the diaphragm, which may be flat or sculptured. Strain gauges are also used on Bourdon tubes and bellows assemblies.
Metal strain gauges are networks of wire or patterns of thin metal foil fabricated onto or into a backing material and covered with a protective film.
Their design permits the use of a large active length (= large R) in a small area. They are made of specially formulated alloys with relatively large piezoresistive effects. Silicon strain gauges are doped to resistivity levels that produce the optimum combination of piezoresistive and thermoresistive effects. Strain gauge materials are characterized by their strain sensitivity, but when fabricated into strain gauges they are characterized by their "gauge factor," defined as relative resistance change divided by strain.

 

Bonded Strain Gauges
Discrete metal or silicon strain gauges are usually bonded (glued) to the surface where strain is to be measured, and provide an output proportional to the average strain in their active area. The typical gauge factor is around 2; a strain of 1 µin./in. would produce a resistance change of 2 µ / . Unstrained resistance ranges from 120 to several hundred ohms. Because a significant length of wire or foil is necessary to provide high unstrained resistance, metal strain gauges cannot be made extremely small.


Unbonded Strain Gauges
Unbonded strain gauge transducers use relatively long strands of strain gauge wire stretched around posts attached to a linkage mechanism. The linkage is designed such that when pressure increases, half of the wire is farther stretched and the other half is less so. The primary advantage of unbonded over bonded is a higher gauge factor, on the order of 3. Because no adhesives are required, they can also be designed and fabricated for use at higher temperatures. Unbonded strain gauge transducers tend to be large.


Sputtered Strain Gauges
Strain gauge material may be sputtered onto a nonconductive diaphragm to create the strain gauges . Location and orientation are controlled by masking, and the molecular bond created by the sputtering process eliminates any problems with adhesive bonding. Gauge factors are similar to those of unbonded gauges. Surface preparation and other process controls are quite critical. The fabrication process offers some of the advantages of a silicon diaphragm, such as good linearity and high natural frequency, as well as the good temperature characteristics of metal gauges.


Semiconductor Strain Gauges
These devices are made of semiconducting silicon. Their gauge factor is dependent on the doping level—more lightly doped, higher resistivity material has a higher gauge factor. However, it also has greater thermal sensitivity, causing both resistance and gauge factor to change significantly with temperature. Discrete silicon strain gauges are used just as are metal gauges, glued to the strained surface in the desired orientation to provide maximum sensitivity for pressure measurement. In addition to their higher gauge factor (which provides higher sensitivity), they are also smaller, allowing more miniaturization.


Bonded Discrete Silicon Strain Gauges
Early silicon strain gauge transducers used discrete silicon strain gauges bonded with adhesives to the surface of a strained element. These devices were similar to bonded metal strain gauges, except that the silicon types provided much higher output and had greater temperature errors. Furthermore, the silicon gauges were smaller than metal gauges, so the sensors could be made smaller.


Diffused Diaphragm Sensors
Discrete strain gauges, metal or or silicon, require tedious microassembly for installation, but diffused diaphragm sensors can be fabricated using semiconductor masking and processing techniques. This approach provides precision location and orientation of the gauges for optimum linearity and sensitivity, allows extreme miniaturization, and reduces assembly costs. It also removes the variability of the adhesive and its application.


Sculptured-Diaphragm Sensors
Early diffused silicon diaphragm pressure transducers used a simple, flat silicon diaphragm of uniform thickness. Silicon microfabrication techniques (MEMS) allow great flexibility in the mechanical design of the diaphragm.
Anisotropic etching provides precise control of etching directions in the silicon crystal. Extremely small yet complex shapes can be fabricated, permitting the diaphragms to be shaped for optimum combinations of linearity, sensitivity, and frequency response characteristics.


Capacitive pressure sensor

In this capacitive pressure sensor two metal plates are separated by a dielectric material. This forms a capacitor. This component is connected in parallel with inductor. This LC circuit determines the oscillator frequency. If object strikes the sensor, the plate spacing momentarily decreases. This causes increase in capacitance but drop in oscillator frequency. When object move away from the transducer the foam spring back and the plate back to the original spacing. The oscillator frequency returns to normal. If the dielectric material is maintained constant, this mechanism provides a very repeatable transducer. The primary advantages are low hysteresis; good linearity, stability, and repeatability; static pressure measurement capability; and a quasi-digital output.

 

Piezoelectric Transducers
Piezoelectric (PE) pressure transducers use stacks of piezoelectric crystal or ceramic elements to convert the motion of the force-summing device to an electrical output. Quartz, tourmaline, and several other naturally occurring crystals generate an electrical charge when strained. Specially formulated ceramics can be artificially polarized to be piezoelectric, and they have higher sensitivities than natural crystals. Unlike strain gauge transducers, PE devices require no external excitation. Because their output is very high impedance and their signal levels low, they require special signal conditioning such as charge amplifiers and noise-treated coaxial cable.
Because the PE transducers are self-generating, dependent on changes of strain to generate electrical charge, they are not usable with DC or steady-state conditions. They have an inherent low-frequency rolloff that is dependent on the signal conditioning's low-frequency time constant.
Their primary advantage is their ruggedness, and, without integral electronics, their usefulness at high temperatures. If not properly compensated, though, they are sensitive to shock and vibration and may exhibit large changes of sensitivity with temperature variations.


Elastomer pressure sensor

Elastomer conducts electricity very well. It has foam like consistency so that it can be compressed. Conductive plates are attached to the plates. When pressure appears at some point in elastomer pad than material compressed and this lowers its electrical resistance.
This is detected as in increase in current between plates. The greater the pressure becomes, the more the elastomer compressed and current increases more.

 

Different technologies used in making pressure sensors

 

Fiber optic sensors
This technology uses the properties of fiber optics to affect light propagating in a fiber such that it can be used to form sensors. Pressure sensors can be made by constructing miniaturized fiber optic interferometers to sense nanometer scale displacement of membranes. Pressure can also be made to induce loss into a fiber to form intensity based sensors.


Mechanical deflection
This technology uses the mechanical properties of a liquid to measure its pressure. Like, the effect of pressure on a spring system and the changes of compression of spring can be used to measure pressure.


• Strain gauge
Some materials experience the change in resistance when there is change in its stretch or strain. This technology makes use of the change of conductivity of material when experiencing different pressures and calculates that difference and maps it to the change of pressure.


Semiconductor piezoresistive
This technology uses the change in conductivity of semiconductors due to the change in pressure to measure the pressure.
• Microelectromechanical systems (MEMS)
This technology combines microelectronics with tiny mechanical systems such as valves, gears, and any other mechanical systems all on one semiconductor chip using nanotechnology to measure pressure.


Vibrating elements
This technology uses the change in vibration on the molecular level of the different materials elements due to change in pressure to calculate the pressure.
Variable capacitance
This technology uses the change of capacitance due to change of the distance between the plates of a capacitor because of change in pressure to calculate the pressure.

 
What are the parameters to select the right pressure sensor?

• Electrical output
Pressure sensors are available with either voltage outputs or current outputs
• Accuracy
• Operating environment
• Mechanical coupling

• Media compatibility

 

Calibration

 

There are two types of calibration devices: deadweight testers that provide base-line standards and "laboratory" or "field" standard calibration devices that are periodically recalibrated against the primary. These secondary standards are less accurate than the primary, but they provide a more convenient means of testing other instruments.

 

Application

 

Pressure sensing
This is the direct use of pressure sensors to measure pressure. This is useful in weather instrumentation, aircraft, cars, and any other machinery that has pressure functionality implemented.


• Altitude sensing
This is useful in aircraft, rockets, satellites, weather balloons, and many other applications. All these applications make use of the relationship between changes in pressure relative to the altitude. This relationship is governed by the following equation:

 

:

 

• Automation
Pressure sensor be placed in bottom of the tank to monitor how it full or empty. It is measuring or monitoring the pressure while knowing volume and liquid density. Utilizing digital display controller, PLC, computer, or data acquisition sensor, you can measure and control the weight of the amount dispensed and/or the flow of liquid during the automation process.

 

• Hydraulic system
Most cranes, earthmovers and similar equipment are operated using hydraulic systems. In order to control the movement, holding, gripping or applied force, pressure sensors are utilized to monitor and provide pressure feedback to these systems. Using pressure sensors will also allow the operator to fully control the mechanical devices and apply known high force or load effortlessly. The pressure sensor can also be used to monitor the hydraulic fluid level for preventive maintenance. If and when the fluid pressure begins to reach an unsafe level, the user would be notified.


 
 
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