Reducing the magnetic susceptibility of parts in a magnetic gradiometer

TL;DR

This paper investigates methods to reduce magnetic susceptibility in materials used in magnetic gradiometers, focusing on material selection, machining effects, and acid washing to improve measurement accuracy.

Contribution

It demonstrates that machining increases susceptibility and that acid washing effectively reduces susceptibility variations in commercial plastics.

Findings

Machining with steel tools can increase susceptibility by up to ten times.

Acid washing reduces susceptibility variation in plastics.

Commercial materials still exhibit higher susceptibility than desired for precision applications.

Abstract

In this paper we report a detailed investigation of a number of different materials commonly used in precision instrumentation in the view of using them as critical components in the magnetic gradiometer. The materials requirement inside a magnetic gradiometer is stringent because the presence of magnetic susceptible material will introduce intrinsic errors into the device. Many commercial grade non-magnetic materials still have unacceptably high levels of volume magnetic susceptibility between 0.001 and 0.0001. It is shown here that machining with steel tools can further increase the susceptibility by up to an order of magnitude. The ability of an acid wash to remove this contamination is also reported. Washing in acid is shown to reduce the variation of volume susceptibility in several commercial grade plastics which already have low values of susceptibility.