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Dear all,

how do you determine the minimum distance between two high-field magnets?

Is it that their 5-gauss lines must touch?


asked Jan 25 '13 at 04:16

oxy's gravatar image


4 Answers:
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Hello OXY!

This question is not easily to be answered! It depends on the real location. Please contact the manufacturer of the spectrometer, so they will give you an answer for your purpose! You can overlap the magnetic stray field, but the console and computer has to be out of the 5 Gauss lines! Is your magnet shielded or even unshielded? Is this on ordinary rubber damping devices or on air-pressurized ones? It depends on too much possibilities, so ask the manufacturer for assistance.


Ulrich Haunz


answered Feb 19 '13 at 03:27

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Ulrich Haunz

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Not always possible for him to check with the manufacturer. Here's an answer that's generalized but should give you a few things to consider. In no way shape or form at all should the magnets 5 gauss lines cross, you'll bend the field of both and make them more difficult (even times not possible) and unpredictable to shim. Should one quench the other will instantly have very poor shims. The short answer is to put the magnets as far away from each other as possible but certainly beyond each others 5 Gauss line.

The next issue is cross talk. If they are both 400 Mhz systems or even a harmonic 600/300 you can easily have what's known as cross talk between them. In this case you're pulsing one and the other system feels it and produces a signal however slight and you see unknown random noise. This one is tougher to quantify but my advice would be to ensure the associated consoles are not located physically close to the other magnet. I used to see people stagger systems too, by putting a 400 next to a 600, next to a 500, next to another 400 and it would lesson the risk of cross talk if you located the 2 400's next to each other. The advice I'd give is use as big a space as possible and seperate as much as possible and most times you'll be ok. If you wonder if you're far enough away.. consider it's worth it to give a little more distance versus the cost of doing it again after. Good luck.



answered Feb 24 '13 at 07:01

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While often the subject of irrational fear and speculation, I can answer this from the confidence of actual experience. We have multiple successful installations here where the 5 Gauss lines cross between adjacent magnets by 1 or 2 feet. In no case do our 10g lines touch, but I suspect that would cause no problem more than resetting the cryoshims of the original system. In all but one case, our RT shims were strong enough to correct for the interaction. In one case, we reset the cryoshims of the 600 when we installed a 400 adjacent, but that was done out of the convenience of mapping equipment availability rather than empirical necessity. Later an 800 was added whose 5G line crossed those of both the 400 and 600 without any need to reset any cryoshims. In both cases, the mfr. approved of the arrangement. We have a combination of 400/600/800 interaction, and the other is 300/400/500. The systems are of mixed polarity, as well. From experience, I know that most consoles and computers will operate without problem in up to 50G stray field, and installations under those conditions are allowed by the mfrs. when necessary. I know of one installation where the console partly crossed the 100G line without any problems. Interference between harmonic fields is not a problem, since the mfr. practice is to offset the field from the nominal field value by more than the sweep width. In possible cases of identical field interference, the main field of the 2nd system can easily be offset slightly to avoid it.


answered Feb 25 '13 at 08:07

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Jerry Hirschinger

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Some good responses already. You can get the magnets relatively close but you must be aware that the fields will be additive (unless the magnets are powered up in opposite directions). Overlapping two 5 Gauss lines will give you 10 Gauss at that point. You will also be regulated by your local health and safety rules for exposure of people to the stray field.

Here is a little tip: Once you have installed the magnets and powered them up, you can use a modern smartphone to monitor the magnetic field (they are equipped with Hall sensors and there are apps which will read out the magnetic field values for you). I don't know what the accuracy of these are but my phone closely matches the values from our proper Gaussmeter.


answered Apr 22 '13 at 05:56

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John Hollerton

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Asked: Jan 25 '13 at 04:16

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Last updated: Apr 22 '13 at 05:56

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