Selection and design considerations for pressure sensors

Pressure measurement definitions and related terminology

To understand pressure sensors, you will first need to understand some key definitions. Pressure is the magnitude of the force exerted by a gas or liquid on a unit area, expressed by the equation P=F/A. The traditional unit for pressure is the pascal (Pa), defined as one newton (N) per square meter. Pressure can also be defined as the force required to prevent the expansion of a fluid. A sensor is a device that detects or measures a physical quantity (force, temperature, length, pressure) and typically converts it into an electrical signal.

At a basic level, pressure sensors perform the same task as pressure transducers or pressure transmitters, so these terms are often interchangeable. However, the difference lies in the type of output signal. Pressure sensors detect a force and convert it into an output signal related to the strength of the applied pressure. Pressure transducers convert detected force into continuous voltage output (V), while pressure transmitters convert detected force into current output measured in milliamps (mA).

Pressure sensors find extensive use in monitoring and control applications. In everyday use, pressure sensors can be referred to by various terms, such as pressure transducers, pressure transmitters, pressure senders, pressure indicators, piezometers, and manometers. Regardless of the name used, these devices are employed to monitor and control pressure in various applications. Besides direct pressure measurement, pressure sensors can also be used for indirect measurement of other variables, such as fluid/gas flow, velocity, water level, or altitude.

When dealing with pressure measurement and pressure sensors, understanding several terms is necessary. Factors like the type of application used can affect measurement accuracy and system performance. Therefore, environmental considerations during pressure measurement, such as local atmospheric pressure at sea level, must also be considered.

1. The first common term is gauge pressure, which refers to pressure measurement relative to local atmospheric or ambient pressure. The indicated pressure will be higher or lower than local atmospheric pressure.
2. Another term is absolute pressure, which is pressure measurement relative to a reference of zero pressure or a vacuum. Regardless of where measured, pressure measurements using absolute pressure sensors should be consistent.
3. Differential pressure refers to the pressure difference between two points in a system, often used to measure the flow of liquid or gas in pipes.
4. Vacuum pressure measurement is a negative pressure range compared to ambient or local atmospheric pressure.
5. Lastly, compound pressure measurement combines positive pressure and negative pressure or vacuum, making it essentially a combination of gauge pressure and vacuum pressure.

Technologies used to measure pressure

The number of different technologies used to detect this basic force has continued to grow, particularly with the onset of semiconductors. Here is a quick breakdown of the major pressure measurement technologies and how they are used:

• Potentiometric is a kind of sliding arm that uses a resistance device connected to a Bourdon tube. As pressure fluctuates the arm moves and a relative signal is generated by the resistance device based on the amount of the force.
• Strain Gauge is a technology that converts pressure into a change in electrical resistance. This resistance can then be measured as it varies with applied force.
• Capacitive is a technology that detects shifts in electrical capacitance caused by pressure flexing a diaphragm between the capacitor plates.
• Inductive is a technology that detects minor deflections of a diaphragm connected to a magnetic core. This leads to linear movement in the core, which varies the induced current and converts it into an electric signal.
• Piezoelectric detects the amount of compression generated by the quartz or ceramic material and an outside pressure exerted on it. This technology uses the fluctuation in electrical resistance of a stretched material to measure pressure.
• Resonant detects the resonant frequency of a vibrating wire that is housed in a diaphragm and converts it into an electrical signal.
• Optical employs a light source that is increasingly blocked by a rise in pressure. This change in light is detected by a sensor that produces a corresponding signal. Fiber-optic sensors can also be used in a similar manner.

Understanding pressure sensors also requires examining the various types of applications that can be used in your designs. Here are the basic types that can match various environments and application requirements. 

• Vacuum is used to measure pressure below atmospheric levels. This usually relies on piezo technology or by measuring the volume of gas in a defined space.
• Sealed use atmospheric pressure at sea level as the reference pressure.
• Vented is used to measure pressure relative to ambient barometric pressure.
• Diaphragm uses thin, flexible, and circular metal plates that deform under pressure.
• A Strain Gauge detects alterations in resistance resulting from external force-induced length variations and translates them into an electrical signal.
• Solid State employs no moving parts. It alternately incorporates a semiconductor switching element, such as a FET to sense pressure.
• Thin Film technology utilizes a slender film comprising resistive components that modify their resistance in response to alterations in length and thickness (deformation) brought about by pressure.

Considerations for selecting pressure sensors in design

As you decide on the use of a specific pressure sensor for your design, you will need to consider several key parameters of your system. This will help in identifying the ideal pressure sensor for your application. Some of these considerations include:

1. First, you should consider the type of sensor, including sealed, vacuum, strain, gauge, piezo, etc.
2. And then, you should consider the range of operating pressure. The device needs to detect pressure performing within the safe pressure range specified by the manufacturer.
3. Furthermore, you should consider the range of operating temperature. The device needs to operate within the safe temperature range specified by the manufacturer.
4. Moreover, you also need to consider the maximum pressure that device can handle. This refers to the upper limit of pressure that the sensor will tolerate before failure.
5. Next, you should consider the output type of the device. This refers to the type of electrical signal output from the pressure sensor (analog/digital) that will work with your design.
6. Another thing you should consider is the output level of the device. This refers to the range of the output signal, typically mV or Vdc.
7. The accuracy & drift characteristics of the device are essential. How accurate does the sensor need to be and how far will it vary from calibrated specifications over time? All of them are important parameters to be considered.
8. The device’s resolution refers to the smallest degree of pressure change that can be detected by the sensor. 
9. In addition, you should understand the supply voltage of the device. This refers to the voltage required to operate a pressure sensor.
10. Finally, you need to consider the environment in which the device is used. The external operating factors, including temperature, humidity, pressure, exposure to fluids, radiation, etc. to which a pressure sensor may be exposed. It may also be necessary to consider the physical distance between the sensor and any receiving device.

In terms of certification standards for pressure sensors, there is currently no single universally accepted standard that specifies the accuracy of pressure sensors. Some regional specifications include IEC 60770 and DIN 16086, both of which apply to sensor accuracy. ASTM F2070 covers requirements for general-purpose pressure sensors. Any technical standards involving pressure standards usually apply to their proper use in specific applications, such as medical devices or automobiles. Examples include SAE J1347 for engine applications or IEEE 1451.1 to 1451.7 for industrial pressure sensor applications.

High-quality and diverse selection of pressure sensors

Measuring pressure and using that measurement to monitor and control processes is a critical requirement for many manufacturing industries and service enterprises. From process control to healthcare applications, accurate and reliable sensing of fluid and gas pressure is essential to determining the quality and safety of the products or services provided. Modern pressure sensors come in various types, technologies, footprints, outputs, and accuracies. Ensuring you find the pressure sensor that best suits your application needs requires some effort, but it leads to high-quality results. Once these requirements are determined, CUI Devices offers a range of pressure sensors based on piezoelectric technology, with various pressure types and operating pressure ranges.

Featuring absolute, gauge, and sealed gauge pressure types, these models offer pressure ranges from 0 kPa to 100 MPa. These pressure sensors further feature temperature compensation, are constructed with stainless steel, and come with O-ring seals. Available with either analog or digital I2C output options, these models feature excitation ratings of 3.3 V, 5 V, 10 V, or 1.5 mA, and are equipped with 3-wire, 4-wire, 5-wire, or 6-pin terminations.

Conclusion

Pressure sensors have a wide range of applications, especially in industrial automation environments. Pressure sensors can detect pressure data needed in the manufacturing process to ensure process safety and improve product quality. The high-quality pressure sensors introduced by CUI Devices come with diverse specifications and product features, capable of meeting various requirements in industrial automation environments. You can visit the websites of CUI Devices or Arrow Electronics  to find the pressure sensor that best suits your needs.