The Right Pressure Transmitter For The Right Environment

2023 Author : Hannah Pearcy | [email protected] . Last modified: 2023-06-01 01:49

At first only white deposits appear on the cable of the immersion probe, shortly afterwards the sensor is destroyed. What happened? The immersion probe was used in the chlorine water of a swimming pool - the user takes into account that the probe comes into contact with chlorine water, but not the harmful chlorine gas above the water surface, which ultimately destroyed the PE cable.

This shows how important chemical compatibility with the contact media is when choosing the material. The user also has to make a second consideration: The transmission fluid in the pressure transmitter can also come into contact with certain substances, which can be dangerous in the pharmaceutical industry, for example. We show which media incompatibilities exist with which materials and which solutions are available.

Picture gallery

Pressure transmitter in the swimming pool

Not only the housing material itself plays an important role in media compatibility. Other parts such as the cables and seals also come into contact with the surrounding media.

Seals Most pressure transmitters have an elastomer seal. The elastomer can dissolve if it comes into contact with aggressive media such as biodiesel. In this case, the user should use a sensor that is welded flush with the front and is elastomer-free.

Cables The cables that transmit the measurement data are also exposed to the media. The problem with the above-mentioned immersion probe in a swimming pool was the problem that PE or PUR cables are used as standard for immersion probes, but they cannot withstand the aggressive chlorine gases for long. The cables become porous above the water level, recognizable by the whitish discoloration, and water penetrates. That is why Teflon cables are used.

Which housing for which media?

Viscous media With viscous media such as paints, deposits can occur in the sealing channel. In order to prevent such contamination, smooth membranes without an open pressure channel and without dead space should be used so that the sensor can be cleaned without leaving any residue.

Abrasive media If pressure transmitters come into contact with abrasive media such as concrete, a simple stainless steel membrane does not offer sufficient protection. A membrane is better suited here, which is covered with a film made of the polyurethane plastic Vulkollan.

Galvanic & Acidic Liquids If a pressure gauge with a metal housing is used in a galvanic basin, over time you will only have a non-functional lump of chrome. Acidic liquids such as sulfuric acid also react with metal. That is why plastic casings, e.g. B. from PVDF used.

Additional information on how to find the right pressure transmitter

In addition to the right materials for the respective surrounding medium, the user also has to consider several other factors:

• Pressure range : A reference point is the pressure range to be measured and whether a measurement of the relative or absolute pressure is intended.
• Accuracy : With this factor, necessity and cost have to be weighed up. The pressure range to be measured is a good decision-making aid. If it is very far, it usually does not require great precision - because it has its price.
• Temperature : If the temperature range to be measured is known, each sensor can be optimized for a specific temperature range.
• Process connection : There are many possible electrical connections that can be selected based on the modular construction principle of the measuring instruments.
• Output signal : Should the pressure be transmitted as an analog signal or digitally?
• Space requirement : If there is little space available, piezoresistive pressure transmitters with a diameter of just a few millimeters are particularly suitable.
• Certifications : When used in hazardous applications with an explosion hazard, the transmitters must be certified.

Sea water Salt water causes pitting corrosion in stainless steel housings in the long term. Immersion and level probes should therefore be purchased in a titanium version.

Lightning protection in open water A lightning strike is not a medium, but should be dealt with shortly. Overvoltage protection can be advisable for submersible probes in open waters - especially if long-term measurements are carried out in remote locations and the replacement of a defective device is far more expensive than overvoltage protection.

What has to be considered preventively

In piezoresistive pressure transmitters, the silicon chip is surrounded by a transmission liquid, usually silicone oil. Although the oil does not actually come into contact with the surrounding medium, a defect can never be excluded, so the chemical compatibility must also be taken into account here.

Strongly oxidizing gases & liquids With strongly oxidizing gases or liquids there is a risk of explosion if they come into contact with oils or greases. All parts, also preventively the transmission fluid, must be free of oil and grease.

Food and pharmaceutical industry Here, the silicone oil must be replaced by a food-compatible oil in order to exclude contaminations that are harmful to health or have other effects. This way, beer that came into contact with silicone oil would no longer foam.

Modular pressure sensors

The optimal pressure transmitter for individual applications depends on many factors. Providers must understand that too. That is why it is necessary for the provider to understand the respective customer application. STS offers its customers advice tailored to their needs, covering all aspects, in order to provide reliable solutions within a short time - even in small quantities. (kj)

Ultrasonic sensor