LVDT Free Cores - The Proper Handling & Marking
The core of an LVDT is made of a high permeability ferromagnetic material that has been carefully annealed to maximize its magnetic properties. For this reason, the core must not be dropped on a hard surface, machined or clamped in a vise or pliers. All these actions could damage the surface of the core and change its operation in the LVDT. It also should not be welded or brazed, or otherwise subjected to high temperature heating. Anything that affects the annealing can result in the calibration data for that particular LVDT no longer being correct.
Mechanical connection to the core should be done with a non-magnetic material, preferably 300 series stainless steel or a fiberglass or plastic rod that is attached by a suitable adhesive or thread locker. Highly conductive aluminum or brass connecting rods should be avoided as they may introduce eddy currents within the LVDT which will produce unacceptable results.
Occasionally, a user may want to mark an LVDT’s core with its serial number to be able to match them up if the parts become separated. There are several ways to mark a core without affecting its performance. Electrochemical etching or low power laser marking are commonly used methods. Using an ink jet marker or a paper or polyester label are also used, but usually require a thin-wall transparent shrink tube to be applied over the marking or label as protection.
Engraving with a tool is definitely not a satisfactory method as this would change the properties of the annealed surface of the core and effect its operation in the LVDT. For OEM users of LVDTs, the sensors should be ordered with serialized cores to begin with.
Each LVDT free core is commonly marked with a blue dot on one end. This blue mark should be aligned with a blue mark on the LVDT body for proper orientation. This is the orientation of the core with which the LVDT was tested and calibrated and that offers optimum performance.