Magnetic Particles Wet Continuous Method
The continuous method implies that the magnetizing force is acting while the magnetic particles are applied. When the current is on, maximum flux density will be created in the test object for the magnetizing force being used. In some cases, usually when alternating current or half wave direct current is the magnetizing current being used, the current is actually left on, sometimes for
minutes at a time, while the magnetic particles are applied. This is more often needed in dry method applications than in the wet. To leave the current on for long durations of time is not practical in most instances, nor is it necessary. The heavy current required for proper magnetization can cause overheating of test objects andcontact burning or damage to the equipment if allowed to flow for
any appreciable length of time.
In practice, the magnetizing current is normally on for 0.5 s at a time. All that is required is that sufficient magnetic particles are in the zone and free to move while the magnetizing current flows. The bath ingredients are selected and formulated so that the particles can and do move through the film of liquid on the surf ace of the test object and form strong, readable indications. This is one reason why the viscosity of the bath and bath concentration are so important,
since anything that tends to reduce the number of available particles or to slow their movement tends to reduce the build up of indications.
Many methods are used to apply wet bath magnetic particles. The methods range from simple pouring of a bath onto a test object to large industrial systems in which the bath is applied automatically either by dumping or spraying. The most common method for application is through the use of a handheld nozzle and recirculating pump on stationary units. Occasionally, lever operated sprayers or aerosol containers are used.