In selecting the right pressure transmitter for individual applications, there are numerous criteria that must be considered besides the pressure range to be measured and the extant thermal conditions. Among these falls the subject of media compatibility: The housing and process connection must withstand the environmental conditions, so that the sensor can perform its service over the longer term.
Material selection therefore follows two important considerations: On the one hand, that there is a chemical tolerance to the contact media. The other factor is that preventative considerations also play an important role. It should not only be clarified whether the pressure transmitter will remain functional longer term. It must also be established whether the materials used in the pressure transmitter itself can lead to dangers when coming into contact with particular substances – the pharmaceuticals industry would be an obvious example here. In the following, we will be showing which media incompatibilities occur with which materials and what the solutions to this might be.
Chemical-physical media compatibility with sealant material & cable
It is not only the housing material itself that should be included in considerations of media compatibility. Other elements of the pressure sensor also come into contact with the surrounding or process media and these materials are to be particularly contemplated.
The majority of pressure transmitters come with a sealant made of elastomer. The problem here is that the elastomer can dissolve when it comes into contact with aggressive media such as biodiesel, for example. In this case, a front-flush, welded and elastomer-free sensor should be employed.
One further factor is the cable that serves in transmitting the measurement data. We will adopt the example here of using a submersible probe in a swimming pool. For reasons of hygiene, swimming pools use chlorinated water. As standard, submersible probes use PE or PUR cables. Although chlorinated water alone presents no issue to these cables, the chlorine vapor rising from this water does do, since this is much more aggressive than the water itself. These cables, over a period of time, will become porous above the water level (visible as a white discoloration) and water will then penetrate within. Subsequently, the sensor itself will also fail. For this reason, teflon cables would be used in such an instance.
Chemical-physical media compatibility with housings
With viscous media, using paints as an example, deposits within the sealant channel can be a consequence. To prevent contamination, smooth membranes free of any dead space and without an open pressure channel are needed for such applications, so that the sensor can be cleaned free of all residues.
When pressure transducers come into contact with abrasive media such as concrete, a simple membrane of stainless steel provides insufficient protection. In this case, a membrane coated with Vulkollan® foil will be required.
Galvanic & acidic liquids
A chromed pressure sensor may look better from an aesthetic viewpoint, but in practical terms it is anything else. When a pressure gauge with a metal housing is used in an electroplating bath, over time only a clump of non-functional chrome will remain. Even acidic fluids, such as sulfuric acid, will react with metals. For this reason, plastic housings are deployed for galvanic and acidic liquids. The most popular solution here being PVDF.
Image 1: Destroyed pressure transmitter due to incorrect material selection
Salt water (depending on its salinity) causes long-term pitting to stainless steel housings. This is why most submersible sensors and level sensors are also available in a titanium version.
Open waters / lightning protection
Lightning strikes cannot perhaps be described as a medium, but we will nevertheless look into this a little further. Should a strike hit a sensor directly, then no lightning protection at all will be of any use. Surge protection, however, can be recommended for submersible probes that are used in open waters. An excess voltage and damage to the measuring instrument by a lightning strike in the immediate vicinity can thus be prevented. This is particularly advisable when long-term measuring in remote places is being conducted. The replacement of a defective device here would then be much more expensive than surge protection itself might be.
Preventative media compatibility
The silicon chip of a piezoresistive pressure transducer is surrounded by a transmission fluid. A usual choice here is silicone oil. Although this fluid does not normally come into contact with the surrounding media, some things must nevertheless be observed here – since a defective housing, after all, cannot be totally ruled out. Depending upon application, this could lead to serious consequences.
Heavily oxidizing gases and fluids
When oxidizing gases and fluids come into contact with oils or greases, the threat of explosion then arises. All components exposed to the medium here must be free from oil and grease, and, in preventive terms, the transmission fluid also.
Foodstuffs and pharmaceuticals industries
In this case, the silicone oil must be replaced with a food-safe oil to rule out any contaminations either harmful to health or that act in other ways. Beer, for example, that has come into contact with silicone oil will no longer foam up, and nobody wants to have that.
Just one drop of oil can render a whole batch unusable. Here also, an alternative must be found.
The media compatibility of piezoresistive pressure sensors: Summary
The optimal pressure sensor for an individual application is dependent upon many factors. For this reason, a deep understanding on the supplier side of the respective customer application is required. STS always offers its customers a needs-oriented consultation that approaches all aspects in providing a reliable solution within the shortest of timeframes – even for lower device volumes.
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