Pressure Measurement

# What is a pressure sensor and what are its types?

We can define pressure as the force applied over a surface area. When the pressure is applied to a sensor then the pressure-sensitive element would determine the pressure. The pressure sensor would also have certain elements to convert this into an output signal. Mostly a pressure sensor would measure the pressure of gasses and liquids.

### What are the types of pressure?

Type of pressure

Absolute pressure sensor

We can describe this device as the sensor which would measure the pressure relative to the reference chamber which would be a vacuum.

Gauge pressure sensor

This type of pressure sensor would measure the pressure or compare the pressure relative to the presently available atmospheric pressure.

Vacuum pressure sensor

In this type of device, the pressure measurement is done regarding the vacuum

Sealed gauge pressure sensor

In this type of sensor, the pressure measurement is carried out regarding a fixed pressure

Differential pressure sensor

This type of measurement is done when we need to determine the difference between two pressures.

### Why do we need pressure measurement?

• Measurement of pressure is really important in pressure-driven equipment such as turbines, IC engines, etc.
• It can be used to determine the losses in pipes and ducts and thus we could determine the energy efficiency
• Pressure measurement can be used to determine the atmospheric conditions
• We can measure the flow rate or velocity indirectly
• Pressure measurement is inevitable for pneumatic or hydraulic mechanical elements

### What are the factors that must be considered while choosing a pressure sensor for a process?

• We must consider the character of the substance which is in the process
• Environmental conditions of the system
• Pressure range of the process
• We should choose a pressure sensor according to the level of precision and sensitivity required for that process.

### What are static and dynamic pressure?

Static pressure

If the fluid is not moving or is blocked then the pressure applied by that fluid that is not moving in space is called static pressure.

Dynamic pressure

It is the opposite of static pressure the pressure exerted by a moving fluid is called dynamic pressure.

### What are the types of pressure sensors?

Pressure sensors

Types of the pressure measuring devices

Pressure sensor(Mechanical)

This type of sensor would convert the pressure to force and then this force would be measured.

Pressure sensor (Electrical)

This pressure sensor would convert the pressure into an analog electrical signal and its magnitude corresponds to the applied pressure.

Pressure measurement (Mechanical Output)

1. Bourdon tube pressure sensor

The major part of this pressure sensor is a tube which would vary its shape when it is exposed to pressure. When pressure is applied to the tube internally then it won’t stay bend it would straighten and after releasing the pressure it would gain its actual shape. The tube tip would move according to the internal pressure variation and then it would be converted onto a scale.

Advantages of bourdon tube

• Wide operating range
• Economical
• Quick response
• Good sensitivity
• Pressure measurement is direct

Disadvantages of bourdon tube

• These sensors are sensitive to vibrations and shocks
• Temperature variation would affect its measurement
• Hysteresis on cycling.

Applications of bourdon tube

• Tank temperature measurement
• Used in the automotive industry for filling levels
• Automotive air brake pressure measurement
• It is used in high-pressure cutting

2. Helix and spiral tube

This type of sensor tubes has the shape according to their name and one end of this sensor would be sealed. So when the pressure is applied to the tube then it would straighten, the straightening amount is dependent according to the amount of pressure. So the spiral and helical tube would regain its actual form after releasing the pressure.

Advantages of helix and spiral tube

• High accuracy and sensitivity
• There won’t be any movement losses because of the links and levers

3. Bellows

This type of pressure sensor is made up of a sealed chamber and this chamber consists of multiple ridges similar to pleats. These pleats would be slightly compressed while it is in the normal position, so when the pressure is applied to the chamber then the chamber would expand and the pleat would open. We can use this type of pressure sensor for low-pressure measurements. This sensor also has a spring which is used for the accurate measurement, the bellow action is not sufficient enough for the accurate pressure measurement and that’s why spring is used. The spring would oppose against the applied force and as in the image a linkage would be connected to the bellow’s end section to indicate the direction.

• Construction is simple
• Less maintenance
• Economical

• The temperature could affect the measurement accuracy
• Hysteresis
• Over range protection is poor

Applications of bellows

• It is required for industrial control
• Aerospace
• Power transmission
• It is used in electrical interrupters

4. Diaphragm

A diaphragm is a disc that would be made up of a metal sheet. This device would provide the measurement of pressure with the electrical transducers. It is also used to do the isolation of the process fluids for high-pressure applications. In this sensor, the diaphragm will be placed in the middle and one of the chambers would be opened to the atmosphere and another will be connected to the pressure source which is to be measured. So if the pressure is applied to this sensor then the diaphragm would expand into the open chamber and the amount of pressure can be determined by the movement of the diaphragm.

Advantages of the diaphragm pressure sensor

• Economical
• Simple construction and low maintenance

Disadvantages of the diaphragm pressure sensor

• Sensitive to temperature change
• Poor over-range protection
• Hysteresis

Applications of the diaphragm pressure sensor

• Chemical
• Petrochemical
• Mining
• Power plants
• It is used for Offshore applications

5. Manometer

This is the simple method for the pressure measurement, the U-tube manometer would be U-shaped. The pressure measurement is done by using the reference pressure, so the pressure which is to be measured will be on one side of the tube. The other side of the tube would have the reference pressure, so by checking the liquid height on both sides we could determine the pressure.

• Very economical
• Operation and construction is, simple

• Low-pressure range measurement
• High-pressure range measurement would require mercury

Applications of manometer

• Measurement of fluid flow
• HVAC system
• Pressure measurement of furnace gas valve

Pressure measurement (Mechanical Output)

6. Strain gauge

A strain gauge is a device that would measure the pressure by applying the pressure to be measured to a material. When pressure is applied to the material then it would change its shape and this mechanical displacement would be converted into a variation in resistance. There are certain types of strain gauges like semiconductor strain gauges and metallic strain gauges. A metallic strain gauge would be composed of metallic wire and when we apply the pressure which is to be measured to this wire then the wire dimension would change and also the resistance. This device can be used to measure the force, acceleration, torque, etc.

Advantages of strain gauge

• Good range
• High accuracy
• Compact
• Quick response
• No moving parts

Disadvantages of strain gauge

• Sensitive to temperature
• Hysteresis in thermoelectric strain gauge

Applications of strain gauge

• Vibration measurement
• Torque measurement
• Strain measurement
• Tension and compression measurement
• Deflection and bending measurement

7. Vibrating wire

In this sensor, there is an electronic oscillator and this would vibrate the wire. So if the pressure in the diaphragm changes then the wire tension would vary according to it and this would affect the wire frequency. Thus, we could measure the pressure by the change in frequency.

Advantages of vibrating wire

• High accuracy
• Stable
• High resolution
• Low hysteresis
• Absolute gauge or differential measurement

Disadvantages of vibrating wire

• Large in size
• Sensitive to temperature
• Vibration and shock could affect the measurement

Applications of vibration wire

• Pressure measurement in tailings dams
• Foundation bearing pressure measurement
• Measuring the ground pressure in the construction sites
• Measurement of horizontal and vertical stresses in drilled boreholes

8. Piezoelectric pressure sensor

Piezoelectric crystals are used in this type of pressure sensor and the main advantage of these crystals is whenever pressure is applied to this sensor it would vary its potential difference. The sensor would be mounted on a dielectric base, the crystal would be connected to the horizontal shaft and a diaphragm would be connected to the horizontal shaft. When pressure is applied to the diaphragm then it would push the shaft and the crystal would deform. When the crystal deforms then it would produce electric signals.

Advantages of the piezoelectric pressure sensor

• High accuracy
• High-pressure measurement is possible
• Compact
• Quick response

Disadvantages of the piezoelectric pressure sensor

• Temperature-sensitive
• Only dynamic sensing

9. Capacitive pressure sensor

In this sensor, there would be two metal plates and there will be a dielectric between these plates. In this sensor, one of the metal plates can move and this movement would cause variations in capacitance. A diaphragm would be connected to the movable plate. The pressure which is to be measured would be applied to the diaphragm and it would move the plate and there will be a variation in the capacitance.

Advantages of a capacitive pressure sensor

• High accuracy
• Wide measurement range
• Quick response
• Linearity

Disadvantages of the capacitive pressure sensor

• Sensitive to temperature
• Costly
• Vibration
• Overpressure capability is limited

Applications of the capacitive pressure sensor

• Measurement of gas or liquid pressure in jet engines
• Car tires
• It is used as a tactile sensor

10. Optical pressure sensor

In this sensor, there would be an opaque vane that would be mounted on a diaphragm, and this would move in front of a light beam. So when the light is disturbed then the light which will be received in the measuring diode would show the diaphragm position. We can determine the pressure by the movement of the diaphragm.

Advantages of the optical pressure sensor

• Repeatability
• Temperature corrected
• Low hysteresis
• It can be used in a harsh environment

Disadvantages of the optical pressure sensor

• Expensive

Applications of the optical pressure sensor

• Oil and gas industry
• Medical applications

What are the applications of pressure sensors?

• Automotive industry
• Biomedical instrumentation
• Aviation
• Marine industry
• Electronic devices with touch screen

### Ashlin

post-graduate in Electronics & communication.

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