Implement the Ideal Pressure Sensor

Without measurement, there is no control. As with any type of measurement, results should be clearly defined to allow for accurate interpretation and application of the results. Accurate measurements and good measurement practices are essential in industrial automation and process environments because they have a direct effect on the success of the desired outcome. Pressure, the act of a force on a specified area, is a common measurement used in many industries. Pressure sensors are used to measure pressure. A pressure sensor measures the amount of movement, or deflection, on an area with specialty devices. Many current pressure sensors use a strain gauge or diaphragm that creates a signal to be processed based on the amount of deflection to which the diaphragm is exposed. Other technologies may also be used to measure pressure—such as checking the changes in capacitance because of pressure fluctuation, the straightening of tubes or using optical fibers.

Referencing Pressure

Pressure can be referenced in multiple ways. To accurately identify and relay pressure measurements, the application must be considered. Pressure sensors may use gauge, absolute, differential or sealed pressure.

Gauge Pressure

Gauge pressure uses a reference to the atmosphere around the sensor. Because the sensing element has a deflection as a result of a pressure change, a reference point is needed to know exactly what pressure is being measured. Pressure sensors that use gauge pressure—typically indicated in psi(g), bar(g), kPa(g)—have some type vent. This vent can be built into the sensor or even through a tube in the electrical connection. The vent is in place to use atmospheric pressure as a reference point for the sensor to measure the media. A common reason for using gauge pressure is to ensure that at any location throughout the world, the sensor will always reference the location in which it is installed.

Absolute Pressure

Absolute pressure uses a reference to a perfect vacuum. This type pressure reference is the gauge pressure of the medium, in addition to the pressure of the atmosphere. As locations are changed, especially when dealing with elevation variations, the reference point can change because of atmospheric pressure differences. Using an absolute pressure sensor eliminates the reference to a varying atmospheric pressure and relies on a specific pressure range for reference.

Differential Pressure

Differential pressure can be a little more complex than gauge or absolute but simply measures the difference between two media. Although most gauge pressures are technically a differential pressure sensor—measuring the difference between the medium and atmospheric pressure—a true differential pressure sensor is used to identify the difference between the two separate physical areas. For example, differential pressure is used to check the pressure drop—or loss—from one side of an object to the other.

Sealed Pressure

Sealed pressure is less common than the previous three but still has a place in pressure measurement. Sealed pressure uses a predetermined reference point, not necessarily vacuum. This allows for pressure measurement in locations that will vary with atmospheric changes. Because of the predetermined reference point, no venting on the sensor is needed. In many applications, absolute pressure is specified unnecessarily. A common misconception is that all pressure measurement must be absolute. While a need for absolute pressure measurement exists, the majority of applications only need gauge pressure or another alternative. By understanding the application details, an appropriate pressure sensor can be easily selected. A correct pressure sensor allows for more precise processes and a proper outcome in the most efficient and economical way.

Types of Pressure Sensors

All sensor types measure pressure using a similar method but express the results in different ways.

Pressure Transducer

A pressure transducer measures the amount of force being applied and offers an electrical signal—typically a resistance or very small voltage—as a representation for the pressure. This type device is used for continuous pressure measurement and typically does not offer a visual display.

Pressure Transmitter

A pressure transmitter measures the same force being applied but offers a common process signal—such as 0 to 10 volts, 0.5 to 4.5 volts or 4 to 20 (4-20) milliamperes—as a representation for the pressure. Similar to a pressure transducer, a pressure transmitter is used for continuous pressure measurement but may offer a visual display.

Pressure Switch

A pressure switch measures the amount of force being applied and offers an electrical signal—such as 24 volts direct current (DC) or 110 volts alternating current (AC) along with other variations—when certain conditions are met. These conditions can be predetermined or user-defined. A pressure switch is used for applications in which specific pressures are a concern.

Pressure Gauge

A pressure gauge measures the amount of force being applied, but instead of an output, a visual representation of the pressure is given. Most pressure gauges use a needle on the face of a dial to indicate pressure. However, a digital representation may also be used.

Specifying a Pressure Sensor

The right type of pressure sensor can vary significantly depending on the application. To ensure proper selection, end users should consider the following factors: What type sensor is required? The first critical decision is the sensor type. It should be determined whether the sensor needs to provide an output. The answer to this question can narrow the possibilities substantially. If no output is needed, the most economical choice is a pressure gauge. If an output is needed, what type signal is needed, and what will interpret the signal? This question generally has a straightforward answer that is decided by the individuals responsible for the application. Unfortunately, an output is commonly specified without knowing the details of the input device that is connected to the sensor. Available power and input/output (I/O) devices can all vary based on applications and the manufacturers of devices. Most manufacturers of I/O devices offer a wide variety of input types—such as 0 to 10 volts, 0.5 to 4.5 volts, 4-20 milliamperes and AC and DC switch signals—but the requirements for the devices can differ because of multiple manufacturers of pressure sensors. In this case, end users must consider the electrical signal required, the operating voltage of the sensor and the input impedance of the I/O device. What is the pressure range to be measured? The majority of applications are simple and do not require much effort when selecting a pressure range. However, a common misconception is that a sensor is only needed to measure the typical pressure range. Often, variations occur in the system that can cause a dramatic rise or fall in pressure. A pressure spike—a sudden and sometimes very short burst of pressure—can be extremely damaging to the sensing device because the sensor is only rated for a specified range. In some instances, selecting a sensor with extra pressure range is better than risking damage to the sensor. Pressure sensor manufacturers must state the sensor’s range along with any type of overpressure or burst pressure ratings. How will the sensor be connected for measurement? Pressure sensors can seal the area that needs to be measured in different ways. The type of application generally dictates which type fitting is needed. In the U.S., a tapered thread (NPT) is used for most common applications. An NPT connection seals as the tapered thread makes a connection when it is threaded into the fitting. However, this type connection still needs a sealant, such as polytetrafluoroethylene tape, to keep the connection air/water tight. NPT connections are not rated for pressure more than 15,000 psi and are typically not used in applications of more than 10,000 psi. Society of Automotive Engineers (SAE) fittings are a straight thread that uses a gasket to produce a seal. SAE fittings are generally used for higher pressure applications but are not limited to them. British Standard Pipe Parallel (BSPP) threads, also known as gas (G) threads, are a straight thread typically used in Europe but are also seen in the U.S. Similar to SAE fittings, BSPP/G threads are straight threads that use a gasket to seal the connection. BSPP/G threads can be used in general applications and higher pressure applications. For sanitary applications, a tri-clamp mount can be used. Tri-clamp fittings have a gasket between the mounting area and the sensor but do not require any threads. This fitting is sealed by using a special clamp around the circular connection. This type connection allows for easy removal during cleaning processes. Many other process connections are available, and all have the proper seal for different applications. What medium must be measured? This can be critical knowledge because of the sensor’s material of construction. Chemical compatibility of the sensor materials that contact with the medium, known as wetted materials, can play a key factor in the sensor’s lifespan, along with any possible contamination of the medium. Ensuring that the sensor can withstand the application’s environmental conditions, such as a washdown application, is also critical. Another key piece of information is the medium’s temperature and composition. Because all sensors are electronic devices, they have temperature restrictions. Heating and cooling a sensor beyond the rated specifications can cause irreparable damage. This applies to the medium and ambient conditions. How will the sensor be connected to the input device? Like any type sensor, the way it is connected to a control system can impact the way it is oriented and the time required to install/replace it. Most pressure sensor manufacturers offer different ways to connect the sensor to a controller—such as an integral cable or a quick disconnect. Knowing the application and understanding how the sensor will be used helps dictate which connection is needed. Are other external considerations needed? As previously stated, knowing the conditions to which the sensor will be exposed can affect the usefulness of the sensor. Considering the presence of electrical noise is also important. Noise is any electrical interference that can cause unexpected results from the sensor—in both the performance of the sensor and the signal sent from the sensor. Noise can be generated by variable frequency drives, wireless communications and voltages running in cables. The chances of receiving successful signals from the pressure sensor are greatly increased by taking the precautions against noise. In addition to electrical noise, external considerations such as sunlight, moisture and physical damage should be taken into account.

Conclusion

Before selecting a pressure sensor for any system, understanding all the details of the application and performance requirements is essential. This will ensure accurate pressure measurement and increase the lifespan of the pressure sensor—significantly impacting a company’s bottom line.