Magnetic Field Indicators

Magnetic field indicators, e.g. flux indicators and field strength meters are used to assess the presence, intensity and/or direction of a magnetic flux either before, during, or after testing. In order to do this, the magnetic field indicators are made of materials which have very high permeability.

Note:

• Meter: a quantifiable value is given.
• Indicator: field is indicated but not an actual value.

Specific uses of these instruments are as follows:

• To ensure MT test machines are functioning correctly.
• To ensure that the specified field strength and field direction are sufficient while testing.
• To check for the presence of residual magnetism before testing.
• To check for the presence of residual magnetism after testing.

Magnetic field strength meters and portable flux indicators should not be used in conjunction with permanent magnets or with D.C. electromagnets for determining adequate flux density. A flux indicator may be used with these magnets but only to verify the suitability or correct application of an ink or powder.

Circular fields may be contained completely within a specimen, if this is the case, the field will not be detected.

Many terminologies are encountered when referring to types of magnetic field indicators, which can be confusing. There are also alternative methods other than those listed below to check for residual fields.

BS 6072 quoted two techniques for the presence of a residual field which were:

• using a compass
• paperclip on a thread