Deperm

Vessels wiped and depermed as outlined above were found to age towards their original state, so the processes as described were only effective for comparatively short periods and had to be repeated at regular intervals.

Our present system deperms more reliably and is arranged to deperm the sensor each time the power source for the magnetometer is turned on.

When it is not possible to deperm the assembly below 95% of the post exposure magnitude, ascertain if the component contains parts with permanent magnets such as relays which would not deperm in the 50 gauss field.

Deperming the boards individually lead to a significant decrease in the magnetic moment of the whole unit, this was then repeated on all units.

Fortunately a very pronounced extension of life was found to be possible for wipes and deperms by making the initial current very strong so that, for example, a positive component was made a larger negative one before being reduced to zero […]

One interesting feature of the S/N 008A and S/N 009 data is that the residual moment after deperms one through three increased from 735 to 870 pole-cm for S/N 008A, but decreased from 854 to 659 pole-cm for S/N 009.