How to Select the Ideal Pressure SensorJanuary 16, 2014
How to Select the Ideal Pressure Sensor
Product Specialist – Instrumentation Sensors
A pressure sensor measures the amount of movement, or deflection, on an area with the use of specialty devices. Many current pressure sensors use a type of strain gauge or diaphragm, which creates a signal to be processed based upon the amount of deflection the diaphragm is exposed to. In addition to a strain gauge or diaphragm, other technologies may also be used to measure pressure, such as by checking the changes in capacitance due to pressure fluctuation, the straightening of tubes or using optical fibers, as well as numerous others.
With a multitude of pressure sensors available, it is important to consider several factors, including the type of sensor, signal, pressure range, connection, media and external conditions.
Types of Pressure Sensors
There are various types of sensors that all measure pressure in a similar manner but express the results in different ways.
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 of device is used for continuous pressure measurement and typically does not offer any type of visual display.
A pressure transmitter measures the same force being applied but offers a common process signal, such as 0-10V, 0.5-4.5V, 4-20mA, etc., as a representation for the pressure. Similarly to a pressure transducer, a pressure transmitter is used for continuous pressure measurement but can possibly offer a visual display.
A pressure switch measures the amount of force being applied and offers an electrical signal, such as 24 VDC or 110 VAC along with other variations, when certain conditions are met. These conditions can be pre-determined or user-defined. A pressure switch is used in applications where specific pressures are a concern.
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 digital representation can also be used.
Factors to Consider When Specifying a Pressure Sensor
The right type of pressure sensor can vary significantly depending on the application. To ensure proper selection, it is important to consider the following factors:
What type of sensor are you looking for?
The first critical decision to make is the sensor type. To start, it should be determined whether or not the sensor needs to provide an output. The answer to this question can narrow down the possibilities quite substantially. If no output is needed, the most economical choice is to use a pressure gauge.
If an output is needed, what type of signal is needed and what is the signal being interpreted by?
This question generally has a straightforward answer that is decided by the individuals responsible for the application. Unfortunately, it is quite common for an output to be specified without knowing the details of the input device that the sensor is connected to. Available power and input/output (I/O) devices can all vary based upon applications and manufacturers of devices. Most manufacturers of I/O devices offer a wide variety of input types, such as 0-10V, 0.5-4.5V, 4-20mA, as well as AC and DC switch signals, but the requirements for the devices can differ due to multiple manufacturers of pressure sensors. In this case, it is important to consider the electrical signal needed, the operating voltage of the sensor and the input impedance of the I/O device.
What is the pressure range that needs to be measured?
The majority of applications are very simple and do not require much effort in selecting a pressure range. There is, however, a common misconception only a sensor is needed to measure the typical pressure range. Oftentimes, there are variations in the system that can cause a dramatic rise or fall of the pressure. A pressure spike—a sudden and sometimes very short burst of pressure—can be very damaging to the sensing device since the sensor is only rated for a specified range. In some instances, it is better to select a sensor with extra pressure range than to risk damaging the sensor. It is crucial for pressure sensor manufacturers to state what the range of the sensor is, along with any type of overpressure, or burst pressure, ratings.
How will the sensor be connected for the pressure measurement?
There are various ways to seal a pressure sensor to the area that needs to be measured. The type of application generally dictates which type of fitting is needed. In the United States, a tapered thread (NPT) is used for most common applications. A NPT connection seals by the tapered thread making a connection as it is threaded into the fitting. However, this type of connection still needs a type of sealant, such as Teflon tape, to keep the connection air/water tight. NPT connections are not rated for pressure over 15,000 PSI and typically not used on applications over 10,000 PSI.
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.
BSPP threads, also known as G threads, are a straight thread typically used in Europe but are seen in the Unites States. Similarly 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 as well as for higher pressure.
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 of connections allows for easy removal due to cleaning processes.
There are many other types of process connections available; all having the proper seal for various applications.
TURCK’s PS300 programmable pressure sensors target hydraulic and pneumatic applications.
What type of media are you trying to measure?
This can be very critical to know because of the type of material the sensor is made from. Chemical compatibility from the sensor materials that come in contact with the media, known as wetted materials, can play a key factor in the lifespan of the sensor, along with any possible contamination of the media. It is also crucial to make sure the sensor can withstand environment conditions, such as a wash-down application. Another key piece of information is the temperature of the media as well as the composition. Because all sensors are electronic devices, there are temperature restrictions that the sensor can be exposed to. Heating and cooling a sensor beyond the rated specifications can cause irreparable damage to the sensor. This not only applies to the media, but also the ambient conditions.
How do you plan on connecting the sensor to the input device?
Like any type of sensor, the way it is connected to a control system can impact the way it is oriented, as well as the time it will take to install/replace. Most pressure sensor manufacturers offer a few different ways to connect the sensor to a controller, such as an integral cable or quick disconnect. Knowing the application and understanding how the sensor is going to be used will help dictate which type of connection will be needed.
Are there any other external considerations needed?
As stated previously, knowing what conditions the sensor is going to be exposed to can have a significant effect on the usefulness of the sensor. It is also important to consider the environment’s presence of electrical noise. Noise is any type of electrical interference that can cause unexpected results from the sensor—this can be in both the performance of the sensor as well as the signal sent from the sensor. Noise can be generated by Variable Frequency Drives (VFD’S), wireless communications, as well as voltages running in cables. The chances of receiving successful signals from the pressure sensor are greatly increased by taking the proper precautions against noise. In addition to electrical noise, external considerations such as sunlight, moisture and physical damage should be taken into account.
A typical gas compressor offers many considerations in the choice of pressure sensors.
Before selecting a pressure sensor for any system, it is important to understand all of the details of the application and performance requirements. This will not only ensure accurate pressure measurement, but will also increase the lifespan of the pressure sensor—significantly impacting a company’s bottom line.
Kyle Horsman is a Product Specialist in TURCK’s sensor division. He has three and a half years of experience at TURCK and has a background in controls engineering with a focus on pressure sensing in the food and beverage industry. He can be reached via email at Kyle.Horsman@turck.com.
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