Cooling Coil Bypass on a Varian A-60 NMR Spectrometer About a decade ago the Varian A-60 nmr was one of the quality instruments of its class. It is still a quality spectrometer and many are still in use today in academic and industrial laboratories around the world. One very common problem with the instrument. after more than a decade of use, is a narrowing or complete blockage of the water cooling coils imbedded within the magnets. These should be routinely cleaned with an aiidie detergent, hut even routine maintenan& cannot prevent some deposition of solid. In the particular ease a t hand, in a thirteen-year-old instrument, one of the three left magnet cooling coils became blocked and acidic detergent washes, high pressure, as well as attempts a t cleaning with Teflon-coated prohing wire were not sufficient ta allow continuous operation of the magnet. The factory cost of replacement was quoted as $1800 in addition to four man-days of labor for installation. We have overcome this problem with amaterials cost of $18 by performinga"coolingcoil hy-pass" operation in order to remove the blocked coil's share of heat from the left side of the magnet. A set of external cooling coils was fabricated t o fit the outer periphery of the left magnet, both on its flat sides as well as on the curved outer surface or rim. Thus, even though the heat transfer coefficient to the magnet outer surface is not very large, the extensive surface available to he cooled made this a very practical solution.
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As the photo shows, the cooling roils are made of soft copper tuhing PAs~in.nd.1 soft soldered t o a n appropriate copper base, either flat or curved in the ease of the rim coil. A wooden jig of the same radius as the magnet was helpful in constructing the latter. The flat coils were made in pairs (one pair for each side of the magnet) for ease of installation, hutthis would not he absolutely necessary. These pairs were interconnected with small tuhing sleeves covered over with epoxy. Epoxy was also used to cement the coil assemblies to the magnet. The rim coil does not extend to the lower half of the magnet because of interference from electrical connections and the thermal switch located there. Instead, two heavy mpper thermal shunts are used t o drain heat away to the flat side coils in this area. The coils are series connected as to water flow in the same arrangement as the existing coils, i.e., from the inside of the magnet outwards. No attempt has heen made (nor doe8 it seem necessary) t o balance this flow against the other two
condition a t minimal cost University of Vermont Burlington, 05401
470 / Journal of Chemical Education
Roy A. Paanenen Michael J. S t r a u s s
