The High Sensitivity Low-Mu Permeability Indicator is a simple-to-operate device to be used for indicating the normal permeability of low permeability materials.
The operation of the Indicator is based on the mutual attraction of a permanent bar magnet for a known standard and an unknown material. In use, an insert is screwed into the top of the case. The magnet is then attracted to the insert by a force dependent upon the insert’s permeability. The end of the magnet projecting from the opening in the bottom of the case is then brought into contact with the material being tested. It is essential that the contact surface be clean and free from oxide scale or foreign material. The Indicator is then moved away in a direction normal to the contact surface. If the material being tested has a permeability higher than that of the insert value, the magnet will first break contact with the insert as the Indicator is moved away. Only full, complete breaks should be considered as indicative of a higher permeability than the test material. A complete break occurs when the metal collar holding the magnet strikes the bottom of the plastic case. On the other hand, if the permeability of the material being tested is lower than that of the insert value, the magnet will first break contact with the test material as the Indicator is moved away. Thus, by interchanging the inserts, it is possible to bracket the permeability of the materials under test.
Two features of the Indicator deserve special mention. First, the balanced beam to which the magnet is attached permits the use of the Indicator in all positions without correction due to gravity. Secondly, the hemispherical magnet ends provide point contact with the inserts and the test materials.
Test Specimens: The test specimen is recommended to have a minimum area of 1 cm2 and a minimum thickness of 0.3 cm. The specimen may be laminated. Test specimens having a volume in excess of the minimum value quoted above may be in any form, shape or condition (for example, castings, forgings, bars, weld beads, etc.) The indicator may be placed on any location on the specimen to be tested provided the surface is suitably flat and in full contact with the magnet. The indicator is capable of detecting surface permeability differences, if present, of large objects.
The High Sensitivity Low-Mu Permeability Indicator must be handled with care. Do not adjust the nuts on the indicator. It will affect the accuracy of the measurements. Also, the following precautions should be observed:
- Remove metal filings, chips and dirt from the surface of the material under test. Filings and dirt on the end of the magnet can be removed with masking tape.Contaminants on the magnet will affect measurement accuracy.
- Under no circumstances bring another magnet in contact with the indicator magnet. This will disturb the calibration of the Indicator to such an extent that it will necessitate its return and subsequent recalibration.
- Be sure inserts are screwed firmly in place so as to establish contact with the magnet.
- Do not jerk the Indicator away from the test material, especially with the 1.01 insert in place. This will tend to give a false indication.
- Avoid as much as possible contacting the Indicator with strongly magnetic materials such as steel, cast iron, or straight chromium steels. This can be accomplished by first screening the materials under test with a hand magnet.
- Do not drop the Indicator. Do not expose to temperatures below -18° or above 48° Centigrade.
- When not in use keep the Indicator in its box with the highest value insert in place in the Indicator.
- Inserts are not interchangeable between indicators.
- The Indicator needs recalibration on a yearly basis to maintain measurement accuracy.