Volume 5, Number 1
January 2006
Q. Where did the number 45,000 in the MT coil formula for ampere-turns come from? J.T. Seattle, Washington
A. The constant, 45,000, was determined experimentally in the late 1940s, early 50s. It is base on the formula,
where n = the number of turns in the coil, I = the intensity of magnetization, k is a constant, B = flux density, L = part length and D = part diameter. The term nl is what we now call ampere-turns. By experimentations, researchers came up with the 45,000 as the number to replace kB that would detect most defects and had sufficient accuracy for most applications. The resulting formula was one that could be used by technicians base on the physical properties of the equipment and the part being tested. This evolution is documented in the paper. "Theory and Application of Coil Magnetization", presented by Mr. Glenn McClurg of Magnaflux Corporation at the Fall, 1954 SNT Conference. The paper was printed in the January/February issue of the SNT Journal.
Q. I sometimes perform inspections in poorly lit places where I need to hold a light as well as the testing equipment. What's a good way to do this without having to set something down or use a second person? A. The easiest solution is to use a hardhat-mounted light. Several types are available ranging from professional Mine Safety and Health Administration (MSHA) light with explosion-proof ratings to less expensive types such as those used in hunting. MSHA-rated headlights:
- are rechargeable, but have relatively heavy batteries
- are more expensive,
- require a serious recharger, and
- clip to a mounted bracket on a miner's hardhat designed for that type of light.
Sports-type lights found in gun shops or sporting good stores:
- are less expensive (20-30 dollars),
- use 2-4 standard D cell batteries (rechargeable or disposable),
- typically use an elastic mounting strap to fit most hardhats, and
- are reasonable durable.
Both types focus from spot to flood buy turning the rim of the outer light cover. Sport type light have a small, belt-mounted high-impact plastic battery pack. A set of batteries typically last 8-12 hours in continuous us with a standard bulb. Krypton bulbs give brighter light and are recommended but cut battery life by an hour or so. If your test require a minimum amount of light at the work surface, make sure your light meets those requirements.
TNT
E-mail, fax or phone questions for the Inbox to the Editor:
hhumphries@asnt.org
(800) 222-2768 X 206
(614) 274-6899 fax
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