Deperming has critical importance and can’t be ignored. This process is different from the types of Degaussing as it nullifies permanent magnetic field. With time and lack of knowledge, the popularity of Deperming might have taken a toll, yet it is still useful and silently saves ships and submarines along with its crew. In this Post, we will discuss about Deperming, its importance, types, how to do it and how it is different from Degaussing.

What is Deperming

When a ship is incapable to neutralize its permanent magnetism during normal degaussing process due to the extreme value of permanent longitudinal/ athwart ship magnetism or any magnetization or an uneven permanent vertical-magnetism, then the ship is placed for Deperming method.

Deperming bay

The main aim of this process is to decrease/ nullify the permanent magnetism of the ship and bringing it to better standards. You can also say that this process is a broad scale adaptation of demagnetisation of a ship.

It is hardly carried out a single time (or twice in some special case) in the entire useful life of the ship. Sometimes, when a large scale changes are carried out in ship’s structure, then also it is required to be carried out. It can also be used to calibrate in built Degaussing Systems onboard.

This is one of best methods till to date to neutralize the effects of permanent magnetism on a ship’s body due to the influence of Earth’s magnetic field.

Why Deperming is important

The floating vessels (Ships and Submarines) used by Navy (s) are made of metals or metallic alloys. As metals or its alloys are highly susceptible to magnetism, the ships and submarines acquires a magnetism of its own. This magnetism of ships and submarines (also known as Magnetic Signature) can be detected by enemies and enable them to identify it and to track it down.

Magnetic Signature of a Submarine

The similar approach was extensively used in World War II by Navies to track and hunt enemies ships and submarines. It is therefore very necessary to nullify magnetic signatures of ships and submarines. And so, comes the importance of Deperming which helps in nullifying these magnetic signatures.

Types of Deperming

There are two types of Deperming.

  • Flash ‘D’ (Demagnetization) Deperming
  • Functional Deperming

Flash ‘D’ (Demagnetization) Deperming

In this type, a single time heavy flash is used to Demagnetize the vessel. Flash ‘D’ is most suitable for under water vessels where a fast demagnetization is required.

Deperming of Submarine

Functional Deperming

This method is carried out for vessels in shallow water. It may be Manual, Semi-Automatic or Fully Automatic depending upon the choice.

The Automatic approach for this uses 3 axis magnetometer, servo amplifiers (or its equivalent), selsyns and rheostats. The sensed value of ship’s magnetism is generated as an error signal and accordingly a similar value of magnetism is applied in opposite direction to nullify it. The process is repeated again and again as a close loop system till the time the error (ship’s magnetism) becomes negligible.

Deperming in a ship

How Deperming Process is carried out

It is carried out in specially designed bays which are also known as Deperming-bays.

During Deperming, The coil’s arrangement (under water and on ship’s structure) is such that whenever a DC current is passed through them, they generate a magnetic field which is same but in opposite direction to the ship’s permanent magnetism. This generated magnetic field counteracts the ship’s permanent magnetic field and nullifies it.

This process is repeated again and again, and the readings of permanent magnetism on board are taken out at different locations. The process is repeated till the time the magnetism in the ship’s structure becomes negligible.

Before starting the process of Deperming, it is necessary that every magnetic susceptible devices are taken out of ship.

Difference between Deperming and Degaussing

Permanent magnetic field components of ship

There are two types of magnetic field in a metallic floating vessel i.e. Induced Magnetic Field and Permanent Magnetic Field. A floating metallic vessel acquires Induced magnetic field mainly due to the Earth’s magnetic field. Exposure to external magnetic field in the vicinity may also lead to Induced magnetic field.

The strength of this Induced magnetic field depends on the strength of magnetic field and the time duration for which it was exposed to. However, Permanent magnetic field is the characteristics of metals used and is normally constant.

Deperming is removal of permanent magnetic field of the floating vessel whereas degaussing is removal of induced magnetic field.

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