Pressure sensors for different media types
Applications
Many normal pressure sensors are suitable for use with a wide range of liquids and gases, including water and air. However, more viscous liquids call for specially designed sensors. Examples of viscous media include melted plastics, paper pulp, bitumen, rubber, asphalt, crude oil, sewage, sludge, paint, sealants and adhesives, as well as certain foods (such as ice cream) and pharmaceutical products.
Measurement options
Pressure sensors for viscous liquids usually measure pressure in one of two ways: absolute or gauge.
Absolute pressure is measured relative to a particular value, such as zero or atmospheric pressure at sea level. With this method, the reading is always the same, regardless of where the unit is located.
Gauge pressure is measured relative to the surrounding atmosphere, meaning that readings can vary based on location and altitude. Sensors measuring gauge pressure within a liquid need a vent tube in order to measure the surrounding pressure, which is often combined with the electrical cable connection.
Technology
The flush diaphragm and accessible surfaces allow easy maintenance |
Viscous liquid pressure sensors are transducers, generating an electrical signal in proportion to the pressure they measure. This allows pressure to be monitored by electronic devices such as microprocessors, programmable controllers, or computers.
Pressure sensors for viscous liquids usually feature a physical diaphragm, often made of stainless steel or ceramic, which bends as pressure is applied. The diaphragm is a strain gauge, which increases in electrical resistance as more force is applied to it – in this case, from the pressure of the viscous liquid on the sensor. This resistance is used to modify the output voltage of the sensor.
Standard liquid pressure sensors often feature a relatively narrow vent that allows liquid to enter the unit and press on the diaphragm. However, this is impractical when working with more viscous fluids that have lower flow rates and tend to solidify or coagulate, particularly when a process is halted and the temperature falls and/or the media dries out. The sensor may get clogged up and take some time to begin working properly again when the process is restarted.
To address this, pressure sensors for viscous fluids usually have flatter, more open designs, perhaps with a flush diaphragm, that allow the fluid to move freely across the face of the sensor. They may also be designed so that all surfaces that come into contact with the fluid are accessible, to allow for easy cleaning and the removal of built-up residue (such as in the example to the right).
Many pressure sensors for viscous media have casings made from stainless steel, giving them strong resistance to harsh chemicals such as those found in sewage and sludge.
Options and specifications
Sensors for viscous fluids will typically be specified using features such as:
- Pressure range (for example, 0–0.4 bar)
- Measurement type (absolute or gauge; see above)
- Response time
- Output signal,
- Accuracy (expressed as a percentage)
- Installation type
- Housing and diaphragm material
- Process connection
- Cable length and type
Another important specification is the type of seal used on parts of the sensor exposed to the fluid, particularly if the media is volatile or corrosive.
Sensors will have an operational temperature range, which is vital to consider for media that are subject to intense heat, such as molten plastic or bitumen. Some sensors can be supplied with cooling elements that protect the electronics from the temperature of the media, extending their usable temperature range. For example, some sensors may feature an integral oil-filled capillary that transfers pressure from the diaphragm to the piezoresistor, putting extra distance between the media and the electronics within the sensor.
Some sensors for viscous fluids have nose cones that protect the sensor in use, but can be removed for cleaning. They may also include sealed cable exits to protect the sensor from cleaning processes used for surrounding areas, or from flooding in use.
Sensors for viscous fluids that are suitable for use in hazardous environments may be certified under a standard such as ATEX 95 (for Europe) or IECEx 02 (worldwide). Under EU law, ATEX 95 is required for all electrical and non-electrical equipment that’s used in hazardous environments, while IECEx 02 is intended only for electrical equipment in hazardous environments. Hazardous environments include those involving dust or flammable materials, including bio-gas that may be present along with viscous fluids such as sewage.
Sensors may be available with submersible cable connections, protecting them from spillage or allowing them to be continuously submerged in liquid when in use.
Limitations
Not all sensors are suitable for use with foods. Sensors that are suitable for sanitary food and biotech applications will usually be available with a food-grade oil behind the diaphragm, so the media will not be contaminated if the diaphragm is accidentally damaged and oil leaks out of the sensor.
If you want to learn more about the different types of media that pressure sensors can measure, the applications of each type, and the different sensor options for your design, click the links below to jump to the section you're interested in.
Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.
