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Pepperl+Fuchs Blog

Which Proximity Sensor is Best to Detect Various Metals?

Posted by Zach Steck on Tue, Jan 22, 2013

When it comes to selecting the correct inductive proximity sensor to use in your metal application, there is a major factor to consider.  The composition of the metal being used in the process can greatly affect the functionality of the proximity sensor.  On a basic level, all metals can fit into one of two categories, magnetic (ferrous) or nonmagnetic (nonferrous).

Proximity sensors detect various metalsMagnetic metals are pretty straightforward when it comes to selecting a sensor, as any inductive proximity switch will detect this type of metal.  However, nonmagnetic metals can get a little more difficult, as you would need to take into account the decreased sensing range found with these targets.  The factor by which the sensing range is reduced, based on the target material, is called the reduction factor. 

Reduction facotr valuesThe table  shows the typical values for the reduction factor of common materials.  The smaller the reduction factor, the smaller the sensing range for that specific material.  So when using a standard inductive proximity sensor with a nonmagnetic metal, you will experience a decreased sensing range.

To counter this occurrence you may want to consider reduction factor 1 proximity sensors.  These sensors have no reduction in sensing distance when detecting nonferrous metal targets.  Standard inductive proximity switches can be subject to a sensing range reduction of up to 70% when detecting materials with nonmagnetic properties.  The reduction factor sensors allow targets of various metals to be detected at the same range as any standard ferrous metal.  Reduction factor 1 sensors will detect all types of metals with the provided sensing distance listed in its specifications.  This type of sensor is an excellent choice with nonmagnetic metals or if you are unsure exactly which metal is being used.

Along with the standard inductive proximity sensors and reduction factor 1 sensors, there are also inductive sensors designed to detect only ferrous or only nonferrous targets.  Sensors that detect only ferrous metals will completely ignore any nonmagnetic materials within the sensing range.  Similarly, nonferrous only sensors will not detect any magnetic metals that are near the transducer face.  This can be useful if you are sensing a ferrous metal that is embedded within a nonferrous metal or vice versa.    

Let’s take a look at some example materials and the type of sensor that would be best suited for that metal:

FE360 (ST37)

This is a plain carbon steel that is popular due to its strength, corrosion resistance, and good welding properties.  Common uses for this type of steel would include: automotive, machining, construction, and piping.

This ferrous grade of steel is the standard target for an inductive sensor and would have a reduction factor of one making it usable up to the maximum sensing range listed on the data sheet.  Any standard inductive proximity sensor would be a good choice with this type.

Type 304

This would be the most commonly used grade of stainless steel due to its corrosion resistance, toughness, and ease of cleaning.  It is used in a variety of home and commercial applications including: food and beverage (processing equipment), tools, furniture, architecture, and mining.

Type 304 steel can display magnetic properties due to its composition, but it would contain less ferrous materials than the standard target steel.  For this reason, type 304 steel is located on the reduction factor table shown above. This stainless steel has a reduction factor of 0.75, which translates to a 25% loss of sensing distance.  You would want to use a reduction factor 1 inductive sensor with this grade of steel to achieve the maximum distance from the target to the sensor.

Type 303

This material is another common stainless steel due to its ability to be easily machined.  It has decreased corrosion resistance and strength in order to retain this property but is still considered very tough.  This steel is used in making the following items: gears, shafts, screws, nuts and bolts.

Type 303 steel has slightly more ferrous materials in its make-up than type 304 but still less than the standard target.  It has a reduction factor of 0.85, which indicates a sensing distance reduction of about 15%. A reduction factor 1 inductive proximity switch would be the best choice to ensure the maximum sensing range.

Copper

Copper is a popular metal due to its high thermal and electrical conductivity.  The major uses of copper are: electrical wires, roofing, plumbing, and industrial machinery.

Copper is a nonmagnetic metal with a reduction factor of 0.3.  A standard inductive proximity sensor experiences a sensing range decreased by 70% for a copper target.  A reduction factor 1 sensor would allow the sensor to detect targets composed of this material at its specified sensing distance.

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Topics: Inductive Sensors

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