Magnet ratings provide several pieces of information. The neodymium iron boron magnets (NdFeB) are designated with the letter "N" as the first character. Then the strength, as rated in MGOe is given as a number. For example - N40 tells us that it is a NdFeB magnet with a material property of 40 MGOe.
This does not tell anything about the actual surface gauss rating or strength of a magnet. The size and shape also play a big role in determinimng strength. This same material would have a Br of 12,600-12,900 gauss. This leads some people to say that this is a 12,600 gauss magnet. The effective usefulness of a magnet is determined by its geometry.
Now we look at the letters that follow some of the magnets. For example - N42SH - The N42 tells us that it has a material property range of 42 MGOe and it is an NdFeB magnet, the SH indicates the maximum operating temperature. Magnet ratings without any letters are good for 80C ( 176F). Using the magnet above this temperature will erase a percentage of its total strength depending on how high the temperature is and how long it is used at the elevated temperature. The SH means that it can be used at a temperature of 150C ( 302F). It is not possible to get N50SH. The highest working temperature for N50 (the maximum material strength available at this time) is 80C. As the maximum working temperature increases, the strength will decrease. The maximum strength that can work at 100C (212F) is the N48M. The max at 120C (248F) is N45H and the max at 150C (302F) is N42SH. Higher grades are available at temperatures up to 200C ( 392F) upon request.
Samarium Cobalt magnets have the highest working temperature of the rare earth magnets. SmCo magnets have a maximum operating temperature of 300C. SmCo will usually be 40-60% weaker than NdFeB but will cost up to 5 times as much due to the high cost of Cobalt.
When you see magnets advertised as haveing 12,000 to 15,000 gauss do not think that these magnets will actually measure that high. My N50 magnets which have a Br of approximately 14,500 gauss will measure from 1,000 gauss (smallest) to 5,600 for the center of my 2 inch square pyramid that is 1 inch thick and tapers to a 1 inch square on top. The poles of my large spheres like the 2 inch spheres (actually 2 pieces of 2 inch hemispheres) will measure about 8,500 gauss and the giant 4 inch pyramid that is 4 inches tall from N45 material and tapers to a 0.75 inch sqaure top will measure over 9,200 gauss on the corners.
Hopefully this will answer most questions about magnet ratings. if you have any other questions, just ask Mr. George.