How to Assess a Used Superconducting Magnet

The superconducting magnet is the single most expensive component of an NMR spectrometer. There is no reason why a properly maintained superconducting magnet should not have a lifetime spanning decades. The console of an NMR spectrometer, on the other hand, begins to fail or becomes obsolete long before the end of the useful life of a superconducting magnet. As a result many laboratories find themselves buying new consoles for existing magnets. Also, when spectrometers are decommissioned, their perfectly good magnets may be available at a very much reduced price as "used". Acquiring a used magnet can represent a huge financial saving when obtaining an NMR instrument. It is useful therefore to address the points to consider when assessing a used magnet. These are outlined here.

1. Cryogen consumption - All NMR laboratories maintain detailed logbooks for the maintenance of their magnets. If you are considering a used magnet it is essential that you have access to these logs. They will detail the date of liquid helium and liquid nitrogen fills as well as quantities of cryogens used and regularly measured cryogen levels. From this information, you will know the boil off rates for both helium and nitrogen. A comparison of these boil off rates to the published boil off rates for new magnets will allow you to do a financial assessment of cryogen costs for the used magnet vs. the capital cost of purchasing a new magnet. It is very common for cryogen boil off rates to slowly increase over time. This is often the result of helium leaking into the vacuum through the magnet seals. The seals of magnets should be replaced periodically (every 10 to 20 years). This should also be considered when evaluating the cryogen boil off for a used magnet. The seals should always be replaced when recommissioning a used magnet.

2. Drift rate - Superconducting magnets are remarkably stable, however they do have measurable drift rates. It is important to know whether or not the drift rate is acceptable for you applications. Many laboratories will periodically measure the drift rate and keep a record of it.

3. Homogeneity - Superconducing magnets have cryoshims which are used to make the magnet homogeneous. This is done by the service engineer during installation. He/she will have noted the linewidth and lineshape (typically of water) the last time the magnet was charged up. The currents in the cryoshim coils will have also been noted. These currents should not be near their maximum values. Magnets are further shimmed by the user with a set of room temperature shim coils which are inserted into the magnet. NMR laboratories will routinely evaluate the lineshape and resolution using the globally accepted lineshape sample. If the same room temperature shim set will be used, this lineshape data should be compared to that for a new magnet. If the magnet is to be used for solids NMR, the homogeneity requirements are not as critical as for liquids NMR.

4. How was the magnet decommissioned? - The magnet should be decommissioned by a qualified service engineer. Magnets that are decommissioned by quenching may have suffered damage as a result of the quench.

5. Shipping - Superconducting magnets are very sensitive pieces of equipment and must be shipped only with the proper shipping restraints. Any magnet moved without the proper restraints should be avoided.

6. Stray magnetic fields - Most new NMR magnets are actively shielded and have very small stray fields. Many older magnets are unshielded and have much larger stray fields. It is essential that you know whether these stray fields are acceptable for your workspace. Unshielded magnets require significantly more space than shielded magnets.