Sensor Sense: Sensors and slag

Oct. 6, 2005
Sensors used around heavy-duty welding have a tendency to fail.

Sensors used near welding operations must shake off the effects of electromagnetic interference and coatings of slag and splatter from weld flash. Even weld-rated sensors may not last long in these types of environments.

It's not uncommon to find technicians replacing such sensors at the same location several times daily. Two factors bear heavily on how long a sensor lasts: the number of welds per hour and the sensor proximity to the welding.

No question sensors used around welding must function in a harsh environment. Weld temperatures often exceed 1,200°F. Weld flash spews molten metal, forming slag and splatter deposits on the sensor tip and body. Hot metal sparks melt sensor housings to form pockmarks that trap additional slag and splatter. Peak weld currents exceeding 35 kA generate severe electromagnetic interference for inductive proximity sensors.

Sensor manufacturers address weld slag and splatter problems in numerous ways. One way is by making the sensor front cap particularly resistant to slag and splatter. The reason is most often the sensor face is directly exposed to the weld flash. Thus slag and splatter are more likely to stick to the face than to the side of the sensor. Similarly, the housing gets greater shielding from the electromagnetic fields of the weld arc.

Some manufacturers form the entire sensor housing from special proprietary weld-resistant material. Others use different materials for the sensor housing and front caps. Front caps made from Teflon, stainless steel, and other materials all tend to reduce slag buildup on the sensor face.

Sensors with slag accumulated on their face may see reduced sensitivity or outputs that latch. Sensors that use stainless-steel front caps are particularly prone to false outputs. The sensor needs an oscillator tuned to the resonant frequency of the stainless cap. Slag buildup shifts the resonant frequency and thus trips the sensor. At times maintenance personnel may repair a defective sensor just by scraping off the slag built up on the sensor face. But the now-damaged face soon attracts more buildup and again breaks down.

Many sensors rated for welding applications aren't much more durable than their standard counterparts. Most weld-rated sensors won't exceed 5,000 weld-flash exposures before failing. But there are ways to select sensors used near welding that can boost their life expectancy more than sixfold. Identify why a manufacturer has weld rated a particular sensor. What sets its weld-rated sensor apart from its regular sensors? What type of electromagnetic shielding does the sensor have? Don't be afraid to try other sensors if one type isn't working. And even a sensor that withstands the hottest welding flash still needs physical protection from both human and mechanical damage.

Turck Inc. ( provided information for this article.

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