Pressure sensors for corrosive liquids and gases
Some pressure sensors are specifically designed to work with corrosive, aggressive or highly contaminated liquids and gases, collectively known as ‘corrosive media’.
These corrosive substances will destroy or damage other substances that they come into contact with, including metals and organic compounds. This makes them difficult and dangerous to work with and raises unique challenges for the design of pressure sensors with corrosive media compatibility.
Applications
Pressure sensors for corrosive media are used in applications such as industrial measurement and control, industrial boilers, monitoring the levels of chemical storage tanks, waste management, medical devices, instrumentation and analytical devices.
They also find applications in energy technologies such as those using natural gas, biogas, landfill gas and CHP (combined heat and power).
Measurement options
Pressure sensors for corrosive media 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.
Technology
Pressure sensors for corrosive media are transducers, generating an electrical signal in proportion to the pressure they measure. This allows pressure to be monitored by various devices with a suitable interface.
Most pressure sensors for use with corrosive media use the principle of piezoresistance. The sensor is based around a diaphragm made from a ceramic material that’s elastic but resistant to corrosion and abrasion.
The diaphragm acts as a semiconductor distortion gauge: when the corrosive liquid or gas presses on it, it is bent out of shape, which distorts the crystalline structure of the material. This, in turn, changes the electrical resistance of the diaphragm, allowing the sensor to reflect changes in pressure in the form of a change in current.
Options and specifications
A typical pressure sensor designed for corrosive media |
Pressure sensors for use with corrosive media often feature housings made from stainless steel or plastics such as PVDF, PVC or PPS. The sensor elements themselves are typically ceramic, although some sensors use a stainless steel diaphragm backed by silicon oil that transfers pressure to the sensing element.
The pressure sensors will usually specify a suitable pressure range (for example, 200 mbar–35 bar). Certain sensors may be able to measure both absolute and gauge pressure (see above), with a pressure range specified for each. Some sensors can be configured to measure negative pressure as well as positive.
Some sensors for corrosive media are temperature-compensated, so their readings are not affected by changes in media temperature.
Sensors will also specify the voltage output that they use to indicate pressure changes, and their output at zero pressure (for example, 100mV).
In some cases, the O-ring used to seal the ceramic diaphragm to the body of the sensor can be made from different materials, such as fluorocarbon plastomer (which resists mineral acids, petroleum oil, salt solutions and chlorinated hydrocarbons), nitrile rubber (which resists paraffin-based materials, fatty acids, glycerines or alcohols) or EPDM - ethylene propylene diene monomer (which resists many acids and alkalis).
Some sensors 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 flammable materials or dust.
Limitations
Many sensors are suitable for use with either corrosive liquids or corrosive gases. Their datasheet or other supporting material will specify which media they’re compatible with.
Other sensors are suitable for use with specific types of liquid or gas. However, manufacturers will sometimes be able to modify their sensors to suit specific requirements.
Engineers may need to consider what will happen in the event of the sensor failing. Corrosive or hazardous media could enter the sensor, leading to contamination. Conversely, internal parts or materials (such as oil used as a filling fluid) may leak into the process media.
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.
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