Pressure sensor principle

1. Introduction to Pressure Measurement

Absolute Pressure Sensor

Differential Pressure Sensor

Gauge Pressure Sensor

2. Pressure Displacement Conversion

Introduction to Displacement Conversion

Capacitive Airbag

Alumina Diaphragm

Semiconductor Diaphragm

Single Crystal Pressure Sensor

3. Comparison of Pressure Sensor Specifications and Performance

Specification

Characteristic

Pressure Measurement

The measurement of pressure can be divided into three categories:

(1) Measurement of absolute pressure

(2) Measurement of gauge pressure

(3) Measurement of differential pressure

Absolute pressure refers to the pressure measured in relation to absolute vacuum.

Gauge pressure refers to the pressure measured corresponding to the regional atmospheric pressure.

Differential pressure is the difference in pressure between two pressure sources.

If there is a pressure of absolute vacuum, the differential pressure is equal to the absolute pressure; if the pressure is regional atmospheric pressure, the differential pressure is equal to the "gauge pressure".

The unit of pressure measurement is pascal (abbreviated Pa), which is equivalent to Newton/m². In industrial applications, there are other different pressure units; such as Bar, psi (pounds/in²), atmospheric pressure, mmHg (the rise height of the mercury column is expressed in mm), and cmH₂O (the rise height of the water column is expressed in cm).

For the measurement of liquid and gas pressures, a variety of different pressure sensors can be used. By "sensor" is meant an energy conversion device that absorbs energy from one system and transfers it to another system in a different form (e.g., electrical). Therefore, some people call it "converter". As the measurement of pressure can be divided into three categories, pressure transducers can also be distinguished by these three categories:

(a) Absolute pressure sensor: This device contains a reference vacuum as a measure of the absolute pressure of the environment or a measured source of pressure.

(b) Differential pressure sensor: A measure of the difference in pressure between the two most connected pressure sources.

(c) Gauge pressure sensor: It is also a differential pressure transducer, but its pressure source is a regional atmospheric pressure and the other is a connected pressure source.

Pressure Displacement Conversion

In a pressure sensor, a diaphragm (or a corrugated film) is usually used to convert the pressure P(t) into a mechanical movement amount X(t). Diaphragm (pictured)

It consists of a metal (or rubber) disc that secures the edge of the metal disc to a solid support. The circular undulations are concentric with the outer edge of the diaphragm. The compressed liquid contacts one end of the diaphragm such that the membrane surface is deformed in bending in proportion to its internal pressure. Two corrugated metal diaphragms are combined to form an air bladder (as shown).

If the cavity inside the air bag contains a vacuum, it can be used to measure the absolute pressure; obviously, the pressure to be measured is added to both ends of the assembly. More diaphragm assemblies can be used in series if greater bending deformation is required. In order to measure the difference between the two variable pressures, one of the pressures must be added to the inside of the air bag. Other machines used to convert pressure into movement are bellows and Bourdon tubes. In recent years, with the development of single crystal pressure sensors, it has been possible to simultaneously perform conversion functions and pressure sensor function devices, which are:

Capacitive Airbag

2. Alumina Diaphragm (for piezoelectric transducer)

3. Semiconductor Diaphragm (piezoresistive)

Capacitive Airbag: Contains two ceramic components to support the capacitor pole pieces and are joined together to form a cavity to create a vacuum.

Alumina diaphragms (for piezoelectric transducers) support four bridge-connected resistors in a piezoelectric material.

Semiconductor film: consists of a silicon cell (semiconductor converter) on which a diffusion method is utilized to form a resistor.

Single Crystal Pressure Sensor

As previously stated, single crystal sensors are those that combine pressure sensing and conversion on a single component. The pressure-movement-to-voltage transition is done in one of the following ways:

(a) Capacitive pressure sensor: The pressure to be measured causes the ceramic diaphragm to bend, so that the capacitance of the component can be changed. By adding the necessary electronic circuits, the deformation and the change of pressure are interrelated as much as possible. Therefore, the change in capacitance is proportional to the change in pressure.

(b) Piezoresistive or "thick film" pressure sensors: The principle of action of this converter is to apply a piezoresistive effect, which changes its resistance as the material is deformed. The four resistors are connected to the Wheatstone bridge using thick film technology and placed on an alumina (Al₂O₃) diaphragm. When the pressure to be tested causes the diaphragm to deform, the differential voltage output of the bridge changes.

(c) Semiconductor pressure sensor: This device is also a measurement result obtained by applying a piezoelectric effect and a bridge resistance form. A diffusion method is used on a silicon support to generate a diaphragm, and a unit including a bridge resistance is electrostatically treated. Fixed to the support glass. Therefore, it forms a mechanical isolation from the outside world. When the silicon diaphragm is deflected, the output of the bridge changes.

(d) Piezoelectric pressure sensor: The principle of operation of this converter is the application of the piezoelectric effect, which refers to the nature of the voltage generated when the material is pressed (force or pressure). These properties are used for pressure measurements at high frequencies and for measuring sound levels (in this application, the most famous is the "crystal microphone").

Comparison of Pressure Sensor Specifications and Performance

★ Since there are various types of pressure (absolute pressure, differential pressure, gauge pressure), it is necessary to use the sensor for measurement. So it's important to understand the other peripheral features that affect the sensor's use. Therefore, it is important to understand that the object to be tested that is in contact with the sensor is a liquid or a gas. As a sensor used for fluid measurement, when the fluid to be tested is likely to damage the sensor, its application is different from those of ordinary sensors. Other important factors are the measurement range (expressed in units of bar, psi, Atm, etc.). That is, the pressure range that the sensor can measure while maintaining the accuracy of the measurement specification. The measurement range can be unipolar (pressure or vacuum) or bipolar.

★ "Over pressure" or "preventive pressure" (the maximum pressure that the sensor can accept without causing damage) is quite important in the choice of sensor. Knowledge of the operating temperature range is also important. The temperature of the liquid or gaseous substance to be tested must never exceed the operating temperature range of the sensor.

Other temperature-related peripheral factors are temperature errors. That is, the range over which the temperature can vary within a given accuracy of the measurement specification. Another factor is the reserve store temperature. Other factors that affect converter selection are vibration, thermal fatigue, and temperature. The most important characteristics are: linearity, sensitivity, stability, repeatability and hysteresis.

Linearity: The offset between the converter's indicated value and the best straight line. This parameter is usually expressed as a percentage of the full value.

Sensitivity: (or resolution), which produces a minimum input variation of the output signal value. It is expressed in terms of the output signal size per unit input (mv/PSI).

Stability: The ability of the sensor to maintain an output signal when the amount to be converted (at a fixed temperature) maintains a fixed value at the input. Stability over a given range is usually expressed as a percentage of full value.

Repeatability: The ability of the sensor to regenerate the output signal when the same amount of conversion is present at the input at different times. Repeatability is usually expressed as a percentage of full value.

Hysteresis: The same pressure is applied to the sensor in the opposite direction, the maximum difference between the two readings indicated by the sensor.

For interface systems, the more important factors are the excitation voltage, FSO, and sensitivity of the unit pressure.

Excitation voltage: The supply voltage used to deliver power to the sensor.

★ FSO: (full scale output) The difference between the sensor output voltages under the limits of the relevant pressure range.

Unit pressure sensitivity: The relative change in output voltage when the pressure changes by one unit value.

★ For some sensors, the output voltage associated with full scale pressure is a function of the supply voltage, which is expressed in mv/v. The output value (mv) indicated by the sensor when full scale pressure is applied to the sensor and the excitation voltage is one unit value (volts).

Sensor Characteristics

1. Type of Measurement:

(1) Absolute pressure

(2) Differential pressure

(3) Table pressure

2. Used in:

(1) Gas

(2) Liquid

3. Scope of Measurement

4. Test Pressure

5. Operating Temperature

6. Vibration

8. Linearity

9. Sensitivity

10. Stability

11. Repeatability

12. Hysteresis

13. Excitation Voltage

14. FSO (Full Scale Output)

15. Sensitivity of Unit Pressure