Proximity Probe Target Material Compatibility

Many customers assume that proximity probes (also known as eddy current probes) will react exactly the same when pointed at any shaft made from any type of steel. The reality is that different steel materials create different signals when being monitored by proximity probes. This is due to the varying amounts of carbon, iron, and other trace materials used to modify the properties of the shaft so it will perform properly in specific situations or processes.

4140 Steel - Most Common Shaft Material
When proximity probes were originally invented, the most common steel used for machine shafts was 4140 steel, which made it an excellent choice for standardizing the probe outputs for industrial uses. Over time, the needs of the industrial sphere have changed due to new uses for old materials and also due to many new materials and alloys available. This makes it very important to be able to calibrate proximity probes to accommodate the newer alloys. 

Even though 316L stainless steel is similar to 420 stainless steel in material composition, it is readily visible from the table below that the output of a sensor attuned to 420 stainless steel would be similar to 4140 steel. 316L stainless steel is significantly different enough to cause possible errors in measurement, that could either cause an unwanted shutdown of a machine or worse yet, possibly cause a machine to fail destructively.

The good news is that with enough lead time and knowledge of the materials used in shafts, a sample target can be made, and in most cases probe systems can be tuned so as to offer adequate protection for monitoring shafts properly.