Stainless steel for tight seals

June 7, 2007
Total stainless-steel (SS) proximity sensors have long been widely used as rugged transducers that won't break when you hit them.

Rugged stainless-steel sensors not only survive physical abuse, but also prevent the ingress of water, dust, and other contaminants in the environment thanks to the one-piece construction of their barrel housing. The induction coil ignores the metal in the sensor housing and reacts instead to the metal beyond the face across the air gap.

But they also have advantages in damp, wet, and wash-down environments.

The reason, of course, is their entire housing is made from a single piece of stainless steel without seams or joints. Standard proximity sensors use other materials for their sensing head and front face — typically a type of plastic. Unless the joint is perfect between the two materials, water and other contaminates may get inside the sensor. This is especially true when the sensor sees high-pressure water spray as used for wash down. Because SS sensors are not made from separate pieces, there are no gaps for moisture, dust, or other elements to enter.

So how does an SS sensor detect the desired target and not its own metal face? First, it uses different oscillator frequencies to sense through the stainless-steel face. Second, and of fundamental importance, is the discontinuity or air gap between the front face of the sensor and the target of interest. The air gap lets the main eddy currents generated by the sensor work across the discontinuity into the target metal. The sensor ignores the small currents generated in the sensor face.

The stainless-steel housing does shorten the rated sensing distance when used with standard ferrite core/oscillator combinations. However, new designs now reach the same range as standard sensors. Stainless-steel sensors have rated sensing distances of 5, 8, 12, 18, and 30 mm (depending on the barrel diameter) and typically come only in barrel-style housings.

One final word of caution concerns the sensor target material. Most SS sensors are classified only for use with ferrous or nonferrous targets. Make sure the sensor target-type matches the target material.

Turck Inc. ( provided information for this column.

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