Originally Posted by Richard5
Thank you for your comments. But while you state that deperming and degausing are not the same and you state how deperming is accomplished you do not state why deperming is done. Please elucidate.
I thought I had made that clear; I guess not
When a warship is brought into service
the DG equipment
is set-up to counter the permanent magnetism, and it is "tuned" by the adjustable settings to counter the induced magnetism - these are explained in your original posts. The subpermanent magnetism happens over time - caused by thermal cycling, flexing, pounding, added/removed equipment, etc., etc. This causes a change in the ship's magnetic signature that can't be easily measured or accounted for, and is generally dismissed as insignificant for the purpose of degaussing. This is the reason ships run the DG ranges in various ports
- solely to measure the effectiveness of the DG equipment and give an indication that the subpermanent field is below limits. When there is so much subpermanent magnetism created that the DG equipment no longer satisfactorily negates the ship's magnetic field, then it has to be depermed. Deperming erases the subpermanent magnetism, effectively resetting the ship back to its original permanent signature, for which the DG equipment has been optimised. I suppose a ship could be depermed also to aid compass adjustment, but I don't imagine anyone would go to the expense of it.