Pressure sensors for harsh environments
You’ve heard the phrase ‘if you can’t stand the heat, get out the kitchen’. Well for pressures sensors, if they can’t stand the boiling heat, the freezing cold, corrosive substances, being submerged in salt water, constant exposure to the outside world, or even being sent into space, then they might not be fit for the job.
These hardy little sensors need to withstand some pretty challenging conditions.
In some cases, the ambient temperature may fluctuate widely and quickly, or the pressure media itself may be at a high temperature.
In other applications, corrosive pressure media can present a threat to sensor components such as the diaphragm, or to the integrity of the sensor as a whole. Commonly encountered media include industrial acids, alkalis, salt water in a marine environment, or even fresh water if the sensor is to be used outdoors or underwater.
Sometimes it’s not the environment that can damage the sensor, but the potential for the opposite to happen, such as in food-preparation equipment, where materials in the sensor may present a contamination threat to the pressure media.
Wherever the environment or pressure media are particularly challenging, the chemical compatibility and temperature capability of the sensor are important selection criteria.
Given that special sensors – designed to operate at extreme temperatures, or withstand exposure to salt spray or harsh chemicals – will likely carry a cost premium, engineers might consider isolating the sensor from the pressure media, if possible.
Physical separation from hazardous media
A fluid-isolation barrier (see diagram below) can be implemented to prevent corrosive media coming into direct contact with the sensor diaphragm.
Isolating a pressure sensor from corrosive media
The fluid-isolation system (as shown above) is closed to allow accurate measurement, taking advantage of the incompressibility of the liquid. A suitable liquid must be chosen that will not mix with the pressure media, or present risk of contamination to the process being monitored. Heavy industrial oil is often used.
Temperature isolation can be implemented in a similar way, by inserting a standoff such as an uninsulated tube between the main vessel containing the media and the sensor –industrial pipework or a flask might be used, for example.
Temperature isolation to protect a pressure sensor
Heat is dissipated from the media in the tube (see diagram above), thus exposing the sensor diaphragm to a safe temperature. The length of tubing needed is calculated according to the temperature of the media, thermal properties of the tubing, and the maximum temperature of the pressure sensor.
Isolation diaphragms for harsh environments
Similar principles to the above are applied to create upgraded pressure sensors, capable of withstanding exposure to extreme conditions. An isolation diaphragm can be designed to extend chemical compatibility by using a material such as titanium, tantalum, stainless steel, or another alloy, and filled with a dielectric oil to transfer pressure to the more sensitive diaphragm of the standard sensor. The isolation diaphragm can be an effective barrier to corrosive media or media at a higher temperature than the sensor is able to withstand.
Withstanding extreme temperatures
Pressure sensing at extreme temperatures is required in numerous industrial sectors, such as in the petrochemical industry where vast networks of pipelines must be monitored accurately. Pipeline pressures must be monitored at locations throughout an entire refining and distribution network that can span extreme climates from near-arctic conditions to desert heat.
Meanwhile, automotive, aerospace, and other industrial applications including mining, smelting and down-hole drilling provide further examples of operating conditions that demand rugged sensors capable of withstanding extreme ambient or media temperatures.
At very low temperatures, oil in the cavity behind the diaphragm in an oil-filled sensor can harden, leading to inaccurate readings. In addition, water mixed with gas passing through pipelines can freeze in cold climates and expand, resulting in excessive pressure on the sensor. The excess pressure can be enough to distort the sensor’s readings even after the water has thawed. Such damage may be temporary, or can be permanent.
In extremely low temperature conditions, other components of the sensor, such as rubber o-rings, can become embrittled, which compromises sealing and impairs accuracy. In some situations it may be practicable to heat the sensor continuously to prevent freezing. If heating is not an option, then a sensor designed for extremely low-temperature operation must be specified.
On the other hand, exposure to a high temperature environment can cause the materials used in standard pressure sensors – such as the bonding between strain gauges and substrate - to degrade, leading to inaccuracy or complete sensor failure.
High-temperature sensors feature upgraded materials or construction, using processes such as sputter thin-film deposition that creates a molecular bond between the strain gauges and the substrate, capable of withstanding higher temperatures.
Sensors can be built to operate in various ambient-temperature ranges, with the peak temperature rating of ruggedly designed sensors extending to more than 200°C.
Operating in corrosive environments
Pressure media in industrial processes, such as acids or alkalis, can have particularly aggressive corrosive properties. Liquids, such as fruit juices in contact with food-processing equipment, can also present a significant corrosion risk.
The pressure-diaphragm metallurgy is critical, to ensure suitable corrosion resistance. Titanium has excellent resistance to corrosion by acids, alkalis, or salts, and can be used to fabricate the diaphragm and other parts of the sensor that may come into contact with the media.
Other applications may require resistance to the corrosive effects of seawater, or sea fog. Some examples include platform stabilisation equipment, desalination equipment, pipeline control valves, or oil-tanker piping systems.
Seawater has several corrosion mechanisms, including chemical corrosion due to salts, oxygen, and carbon dioxide contained in the water.
In addition, bacteria in seawater cause microbial induced corrosion, by feeding on iron and manganese content in steels and ultimately promoting further microbial action resulting in chemical waste that attacks the surface of steel membranes. The severity of this type of corrosion can vary with geographical location, depending on factors such as the microbe species present and typical water temperature.
Low-grade austenitic stainless steels, such as common 304 or 316 grades, have poor corrosion resistance in seawater. Although higher-grade duplex steels can offer greater corrosion resistance than the standard grades, nickel-based superalloys such as 625 (nickel-chromium) or C276 (nickel-molybdenum-chromium) are superior - although more expensive – for applications that are exposed to seawater or sea fog. Titanium also offers good resistance to seawater corrosion.
The chemical compatibility of other important parts of the sensor, such as o-rings, should also be considered when selecting sensors for use in corrosive environments. Sensors specially designed for such applications may feature parts made from a material such as Viton, which has broader compatibility than plain rubber.
Coping with dynamic environments
In some applications such as industrial air-blasting equipment, fluid-flow measurement or combustion performance analysis, sensors are needed to measure fluctuations in pressure superimposed on a static background pressure. More extreme applications include monitoring combustion in gas turbines or jet engines, for purposes such as engine control, fault detection, or acoustic analysis.
Piezoresistive elements have fast response times, suitable for dynamic pressure measurement, and can allow a wide bandwidth ranging from a few Hz to over 10kHz. Some sensitivity may need to be traded for faster sensor response, and vibration compensation using accelerometers may be required to increase signal-to-noise ratio.
Creating robust high-pressure sensors
Extremely high pressure environments exist within hydraulic actuators such as aircraft flight controls or test rigs for equipment like landing gear.
Industrial processes such as injection moulding, hydroforming, or powder metallurgy such as hot or cold isostatic processing are also dependent on generating extremely high pressures that must be monitored for safety and process-control purposes.
Other applications include mineral-extraction equipment, high-pressure cleaning equipment, and industrial test equipment such as burst-pressure or fatigue-test benches.
A high-pressure sensor featuring robust construction and metal-to-metal screw sealing |
Heavy-duty high-pressure sensors can measure static and dynamic pressure of up to 100,000 psi (7000 bar) and more. Special features for high-pressure operation include stainless steel construction, a robust threaded pressure port, and metal-to-metal screw sealing (see image to the right).
In spite of the enormous challenges faced by pressure sensors in extreme environments, it’s possible to specially engineer them to operate under a wide range of harsh conditions.
Compatibility between the sensor materials and the pressure media is vital, whether the media or environment presents a hazard to the sensor materials, or whether the sensor could present a hazard to the media, such as in medical or food-processing applications. Additional considerations in the latter application might include the use of special food-grade materials, such as non-toxic cavity oils.
Stainless steel construction is widely featured among sensors for use in harsh environments. Whereas 316 or 304 alloys are economical and robust, only higher-grade steels like nickel-based superalloys – or titanium – can withstand the most extreme corrosive media, including seawater, which has multiple biological as well as chemical corrosion mechanism.
If you’re not sure where to start, our sensor experts can guide you through your options and help find the right balance between sensor performance and protection from hazardous environments.
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.
