How to: Sensing against the elements

Nov. 23, 2008
How to select proximity sensors that resist everything from caustic wash downs to weld slag.

Proximity sensors have become more and more sophisticated as well as more reliable and rugged with IP-rated and sealed housings and other features. This lets them withstand some of the harshest industrial environments. For example, the current generation of sensors can handle exposure to cleansers, chemicals, extreme temperatures and weld fields while delivering accurate results. Here are just a few examples of how you can minimize downtime from sensor failures and lengthen the time between replacements by taking advantage of the latest proximity sensor designs.

Who, What, Where

Authored by Tony Udelhoven Director of Sensors Div. Turck Minneapolis, Minn.

Edited by Stephen J. Mraz [email protected]

Key Points

• Proximity sensors can survive in tough environments
• Find the sensor that matches your application
• IP ratings are a good place to start

Withstanding wash downs
Proximity sensors are often used in the food and beverage industries for inspection and to verify a package or product is on a conveyor or bottling and canning line. These sensors are placed in damp or humid environments where there’s a chance water might get in and ruin it. And much of the equipment in these applications, including sensors, undergo frequent wash downs consisting of water, foam, or cleaning/disinfecting agents. Sensors can fail if temperature shock causes wash-down residues to penetrate the front cap and connector insert, or the acidity of the cleaning agents deteriorates the housings.

To keep water and vapor out, manufacturers add features to the front cap and connector insert. Some manufacturers use plastic or liquid-crystal polymer caps, while others modify the inside of the cap by inserting an O-ring seal. To withstand high pressures and sudden temperature changes, sensors can be potted with material designed to handle those stresses, or the connector insert can be designed to seal out moisture. Sensors can also be housed in durable materials, such as 316 stainless steel, to resist cleaning agents. (Sensors that operate reliably in these environments are rated IP68 or IP69K.)

For particularly challenging applications, such as oil rigs, dams, dikes or locks, ships, and sewage tanks, there are now sensors that withstand complete submersion — up to a designated depth — in oil, water, and seawater. Many of these sensors use a polypropylene housing that keeps out the surrounding liquid and provides resistance to shock, vibration, and caustic chemicals.

Working in extreme temperatures
Sensors that operate in the extreme temperatures of ovens, freezers, semiconductor fabs, and glass and steel mills require the right combination of housing and sensor materials. Most sensors withstand some degree of temperature variation, but most are designed to operate best in certain environments such as exceptionally low temperatures, extreme heat, and sometimes both.

Factor 1 sensors level the playing field

Ferrous and nonferrous metals a ect proximity sensors differently. For example, targets made from these materials are sensed at different ranges depending upon composition. To detect di erent metals, the sensing range must be adjusted to the correction factor. With Factor 1 sensors, however, correction factors are not needed. That’s because these sensors use several coils, letting them detect all metals at the same range without adjustment. So instead of a single coil inducing and being a ected by eddy currents on a target as in standard proximity sensors, Factor 1 sensors use separate, independent sender and receiver coils. Because of this, ferrous and nonferrous metals have the same a ect on Factor 1 sensors and are rated for the same operating distances.

Plastic, stainless steel, Teflon, and chrome-plated brass are a few of the materials often specified for sensors destined for environments with temperature extremes. And some manufacturers use proprietary housing materials for the barrel, front cap, and connector insert to widen a sensor’s operating temperature range. Materials such as PTFE and silicon, for example are suited for temperatures down to –40°C, while others, including ceramics, PVDF, and polypropylene, can handle temperatures up to 160°C.

When welding is a concern
Welding applications are particularly challenging for sensors. Strong electromagnetic fields from resistance or spot welding can cause a standard (ferrite core) proximity sensor to falsely trigger or lock-on. And welding temperatures often exceed 1,200°F, with currents ranging from 15,000 to 35,000 A. This lets weld slag and splatter quickly build up on the sensor, melt the housing, and create small “pock” holes in the sensor face, which make the sensor even more vulnerable to weld slag and splatter. In general, sensor failure is a function of the amount of welding it is exposed to and where the sensor is in relation to the welding tips. A sensor 10 in. from the weld tips could easily experience 1,000 to 2,000 flashes/day.

An IP primer

IP (ingress protection) ratings are often misunderstood and misapplied. For example, many engineers assume that an IP67 or IP68 rating lets a device operate under water for the time speci ed by rating. Actually, the rating only ensures the device will work properly after being removed from water.

Some common IP ratings include:

IP67: The device is protected against the e ects of being immersed in water 15 cm to 1-m deep for 30 min and water will not get into it.

IP68: The device is protected against complete continuous submersion and, under conditions speci ed by the manufacturer, water will not get into it.

IP69K: The device is protected against hot-steam jet cleaning per EN 60529 and DIN 40050-9, as well as water pressurized to 100 bar (1,450 psi) at 80°C. The pressurized stream of water can be applied directly to the sensor in 30° increments (0, 30, 60, and 90°) for 30 sec at each point for a total of 120 sec without water getting in.

Another misconception is that a protection rating of IP69K automatically complies with IP67 and IP68. IP69Krated devices can withstand pressure and jet spray, letting them survive wash-down environments such as breweries, car washes, and food and beverage applications. But those devices may not be suitable in applications where they are immersed in water.

In welding environments, it is important to ensure the front cap (sensing face) can stand up to weld and splatter, so manufacturers add front caps made Teflon. And Teflon can be used with copper in housings. Manufacturers have also developed proprietary materials for welding applications.

For a sensor’s electronics, it’s more important to be protected from electromagnetic fields generated by welders. So-called Factor 1 sensors have separate, independent sender and receiver coils on their circuit boards. This design does away with the standard ferrite core and renders the sensors immune to magnetic field interference. It also lets Factor 1 sensors operate at higher switching frequencies.

If a sensor does malfunction in a welding environment, it may seem tempting to try and “repair” it by chipping off built-up slag with a screwdriver. A sensor that has been “fixed” this way will probably work for a while, but damage to the sensing face will eventually cause it to fail again and again, withstanding fewer welding flashes after each fix until it is rendered useless. Choose the right proximity sensor, however, and it will resist 20,000 to 30,000 weld flashes, saving significant replacement costs.

When choosing a sensor that will be near welding operations, keep in mind that it will still be susceptible to human and mechanical damage. In these cases, a housing more protective than Teflon or copper may be needed. For example, sensors can be fitted with protective sleeves to mitigate side and front impacts. Some of these sleeves are built into the sensor prior to sealing, making it almost impervious to physical damage from the side and weld damage from the front when used with special weld-resistant front caps or coatings.

These are just a few examples of how proximity sensor design has been refined to resist some of the most challenging plant environments. For each application, the key to reliable sensing and long operational life is taking time to investigate which sensors and corresponding components are best suited for your application. Selecting the right sensor right away will save time, replacement costs, and headaches in the long run.

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