Table of Contents

Introduction

The physical quantity

Working principle

Pressure sensors types

Sensor characteristics

Applications 

References

The physical quantity

Static pressure is the ratio of the perpendicular element of force to the element of surface it is exerted on.

p_s = dF/dA

For the pressure inside a fluid we have:

p_s = p_atm+ ρgh , where p_atm is the atmospheric pressure, ρ is the density of the fluid, g is the acceleration at the place of measurement, h is the distance from the free surface

The Si unit for pressure is pascal (Pa) which is equal to one newton per square metre (N/m²).

For liquids the total pressure can be expressed as:

p = p_s + 0.5ρν² , where ν is the velocity of the fluid

For gases, using the ideal gas equation the pressure is:

p = nk_BT/V , where n, T and V are respectfully the number of molecules, the temperature and the volume of the gas, and k_B is the Boltzmann constant

Working principle

The majority of pressure sensors do not measure the pressure directly. The sensor consists of a sensing element that takes a primary reading (most commonly deformation followed by displacement, force or strain) and translates it to another non-electric physical quantity. Another sensor then translates this into an electrical signal that represents the output of the pressure sensor.

Types of pressure sensors

Pressure from resistance variation

In Figure 2A the applied pressure causes deformation of the sensing element (for example diaphragm), which is connected to a strain gauge that exhibits a change in resistance when mechanically strained. The value for pressure is then obtained by reading the variation of the output voltage which is proportional to the change in resistance of the strain gauge.

Similary, in Figure 2B a wiper (moveable electrical contact) is mechanically linked to the diaphragm and the applied pressure controls the wiper's position. This affects on the length of the wire (representing the resistance) between the wiper and the end of a potentiometer (variable resistance). The measurement for pressure is then obtained from the output voltage determined by the resistance value.

Pressure from capacitance variation

Here the sensing element, such as diaphragm, is connected to one of the electrodes of a capacitor. A change in the effective area of the plates , the distance between the plates, or the relative permittivity of the dielectric creates a change in capacitance which reflects on the output voltage, from where the pressure can be derived.

Pressure from inductance variation

The magnetic properties of the sensing element create a variation of the reluctance of the magnetic circuit which displaces the circuit's core. The direction and amplitude of the displacement are sensed by a diaphragm and translated into a signal, consecutively converted to electrical signal for the pressure.

Pressure from piezoelectric effect

These types of sensors are used for dynamic pressure measurements. The change in stress of the diaphragm creates a strain and when a piezoelectric is used as a sensing element, this strain is converted to electric charge. 

Pressure from oscillation

These sensors use a vibrating element which frequency of vibration depends on the force applied to it. This element is made from ferromagnetic material and induces voltage supplied by a detection coil, consecutively amplified and passed to an excitation coil. This voltage reflects the frequency of vibrations which follow a mathematical model to derive the pressure. The sensing element can be the vibrating element itself or connected to it. 

Pressure from light intensity variation

The displacement of the sensing element gives a variation in light intensity. Using a photodiode that senses the change, it can relate that to the change in the force exerted on the sensing element.

Pressure from heating variation

The temperature of the sensing element varies in accordance to the surrounding pressure. The pressure measurement corresponds to that change in temperature.

Pressure from ion variation

The electrical current through a gas is related to the number of ions that provide it. The number of gas molecules is the same as the number of ions created in the gas by the impact of electrons. Therefore, the amount of electrical current reflects the molecule count, which then is used to calculate the pressure in accordance to the gas law.

Data reliability

All pressure sensors are intrusive. Therefore, an incorrect installation can disturb the measurand or compromise the reliability of the system.

Range

Resolution

Accuracy

Signal Characteristics

Linearisation

Calibration

Noise

Other error conditions

Applications

Pressure sensors are widely applied in energy generation industry where a constant monitoring and control of pressures is crucial for the operation of the power plants. Another application is in robotics where a pressure measurement is required in controls or as a substitute for touch. The proper operation of machines can be related to data from pressures of compressed air, gas, vapor, oil or other fluids.

Some applications are

1. Touch screen devices

Smart phones and some computer devices come with pressure sensors. Sensors in those devices determine where the pressure has been applied and inform the processor by generating electric signal. There are two or more sensors fitted at the the corner of the screen to give precise location where actually the pressure has been applied.

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2. Automotive industry and security

Pressure sensors have great application in automotive industry and security system. They help to monitor oil and coolant pressure there by regulating appropriate power pressure in accelerators and brakes. Pressure sensors are usually incorporated in the radiators fill cap and the pressure inside the radiator ranges from 0.6 bar to 1.0 bar.

When the temperature of the coolant rises up then the pressured in the closed system also increases which is then notified by the pressure sensor which opens the valve and the excess fluid is dumped into the overflow container.

Besides, pressure sensors have very important role in security system such as anti-locking braking system (ABS) and air bag system. Pressure sensors fitted into the automobiles work under ECU. ECU detects uneven (fast/slow) rotation of wheels during driving which then inform the pressure sensor to detect the specific parts of the problem and apply more or less pressure to give even wheel rotation.

Similarly, when the pressure is applied to the airbag control unit, then the pressure sensor fitted into the system determines the direction of impact and the restraint device is activated within 15-30 millisecond of the crash. This  quick response of the system helps to prevent the passenger from possible dangers.

3.   Bio medical instrumentation

-       Digital blood pressure monitors and ventilators

4. Industrial Uses

-       monitor gases and their partial pressures

-       determine the depth in oil industry while exploring

5. Aviation Industry

-       Pressure sensors are used to balance the atmospheric  pressure with the airplanes’ control system.

-       gives appropriate external environmental situation to the system

-       helps to create a breathing condition in the cockpit

6. Marine Industry

-       Sensors in ships and submarines help to detect the actual depth creating a safe situation.

-       Sensors are of great use in underwater projects. They help to study the oxygen level and requirement.

What is a pressure sensor?

   Monitor the pressure of fluid, solid or gases.

  Most work based on piezoresistance.

  Level of electrical charges flow proportional to the level of pressure.

 Convert data into a form that can be displayed and understood by the user of another device.

Principle of Operation

The pressure sensing element is formed by diffusing semi-conductor strain gauges directly into the silicon diaphragm. Strain gauges are elements which change their resistance when they are deformed. The strain gauges are connected together to form a Wheatstone bridge circuit. When pressure is applied to the diaphragm, it deforms and stretches or compresses the strain gauges, which causes an imbalance in the bridge circuit, and a change in the output voltage.

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Selecting a Sensor

The easiest way to select a sensor is to choose the ones that will meet our technical requirements at the lowest cost.

• Pressure measurement range
• Media to be measured – the amplified pressure sensors are suitable for all media, whereas the modules are only suitable for use with non-corrosive gas
• Material – Titanium is lighter than stainless steel but is susceptible to attack by some cleaning solvents and by methanol.
• Operating Temperature – Make sure the operating temperature is appropriate for our installation.
• Connector – Certain models are available with integral connectors. Other types have a flying lead.
• Mounting – Flanged versions of certain models are available.

References

1. Ripka, P. and Tipek, A., 2007. Modern sensors handbook. 1st ed. Newport Beach, CA: ISTE USA.
2. Huddleston, C., 2007. Intelligent sensor design using the microchip dsPIC. 1st ed. Amsterdam: Elsevier/Newnes.
3. Engineers Garage. Available from: http://www.engineersgarage.com/articles/pressure-sensors-types-working

4. http://www.futek.com/pressure_sensor_selection.aspx