Moisture ingress is one of the most common causes of pressure sensor failure. To avoid this situation, select a pressure transducer or pressure transmitter with the right IP rating, measurement reference, and electrical connection.
Your pressure transmitter’s output is drifting, or the entire unit has failed. What happened? One possible culprit is moisture, which can quickly damage the circuit board or internal electrical connections. Protecting pressure sensors begin by choosing the right pressure instrument with the appropriate IP rating, measurement reference, and electrical connections.
Pressure Sensors and IP Ratings
Pressure sensors, also called pressure transducers, are used in a wide range of applications. Some sensors are located in climate-controlled laboratories with consistently low humidity. Others are subject to regular cleaning and sterilization with high-pressure steam, such as what is found in the food and beverage and pharmaceutical industries.
To minimize the chances of pressure transducer failure, always verify that the instrument’s ingress protection (IP) or NEMA rating meets the demands of its location. Note that the type of electrical connection makes a significant difference. For example, the A-10 pressure transmitter with an angular connector is rated IP65, which means protection against low-pressure water jets and sprays; some ingress is permitted. However, with a circular connector or cable outlet, the pressure transmitter’s rating rises to IP67, allowing the instrument to be fully immersed in water for a limited duration and to a certain depth.
Ambient vs. Absolute Pressure References
Whether a pressure transmitter is vented or sealed makes a big difference in the instrument’s ability to prevent water and other liquids from entering. Pressure transmitters come in three main types, each with different requirements to protect it against moisture:
- Gauge transmitters are typically used for lower measuring ranges, where changes in atmospheric pressure could be larger than the error band of the transmitter. The instrument is vented so that pressure readings can be compared with the ambient (atmospheric) pressure. Because gauge transmitters have a vent hole, they require a carefully engineered electrical connector and internal vent system to prevent moisture from entering and damaging the device. For example, some gauge transmitters have a small hole underneath the threads of an electrical connector’s retaining ring. This hole is protected by a Gore-Tex or Teflon-type material that allows air but not moisture to pass through.
- Sealed gauge transmitters measure higher pressure ranges where changes in barometric pressure are smaller than the error band of the instrument’s accuracy specifications; in other words, shifts in ambient pressure will not affect the output reading. This type of pressure transmitter usually does not need to be vented, but depending on the design, moisture can still find its way in. Choosing a transmitter with the required IP or NEMA rating will prevent this issue.
- Absolute transmitters measure pressure by reference to a sealed vacuum chamber located behind the sensor element. As an enclosed system, the conditioning circuitry is protected from the elements. However, the electrical connection remains a potential weak spot, as the soft sealing materials in the sensor assembly can break down over time, allowing moisture or process media to enter. One solution is the all-weld measuring cell, which eliminates the need for degradable sealing.
WIKA comprehensive portfolio of pressure sensors include:
- A-10 pressure transmitter for general industrial applications
- S-20 superior pressure transmitter for general industrial applications
- PSD-4 electronic pressure switch with display
- MH-4 and MH-4-CAN OEM pressure sensors for mobile working machines
The Electrical Connection: A point of vulnerability
The electrical connection is one of the most common spots for moisture to enter a pressure transmitter. Choosing the right one makes a big difference in protecting pressure sensors against moisture damage. Certain connections, such as the DIN 43 650 “solenoid” type, are designed for relatively dry environments. Others can withstand even significant amounts of moisture and water.
Cable with free ends
A properly connected cable outlet is an excellent solution for ingress protection. This cable usually terminates inside a dry junction box that contains other electrical components. Furthermore, cabling can also serve as a vent, allowing a gauge transmitter to compensate for atmospheric pressure changes while protecting it from water damage.
Submersible Transmitters Create Unique Challenges
Submersible pressure sensors are excellent solutions for measuring the level of sump pits, tanks, wells, and bore holes. But because they are immersed for long periods in media that can be viscous, corrosive, particle-rich, and/or very hot, these special types of pressure sensors come with several features that help it withstand extreme environments and receive an IP68 or NEMA 6P rating:
- Highly resistant casing made of 316L stainless steel, titanium, or Hastelloy
- Permanent hermetic seal
- Abrasion-proof cabling material
- Aramid fiber inside the cable to support it and the transmitter’s weight
Some manufacturers of submersible pressure transmitters install sealed bellows or rubber bladders over the end of the vent tube, but WIKA does not. Why? While these assemblies do protect the transmitter’s internal electronics from moisture, they can also introduce significant error into the level readings because the air in the bellows expands and contracts with temperature changes, creating pressure changes inside the sensor. A better solution is to allow the tube to vent directly to the atmosphere. Installing the cable end in a NEMA 4-rated junction box will protect the vent tube from water ingress. Rechargeable desiccant products can also be used to help prevent moisture buildup in the junction box and vent tube.
Expert Assistance in Selecting a Pressure Sensor
With so many factors to take into consideration, it can be a challenge to determine which pressure transducer or transmitter to purchase. That’s why WIKA USA encourages potential and current customers to contact our specialists with any questions about pressure sensor reliability, performance, service life, and suitability for their particular application. Because preventing pressure sensor failure begins by choosing the right products.
Products mentioned in this blog
• A-10 pressure transmitter for general industrial applications
• S-20 superior pressure transmitter for general industrial applications
• PSD-4 electronic pressure switch with display
• MH-4 OEM pressure sensor for mobile working machines
• MH-4-CAN OEM pressure sensor for mobile working machines with CANopen or SAE J1939 serial interface