People may have notice that Paris, the walk up 500 MHz NMR in Malott B042, has been out of commission for a couple of days.  I have been working on the shims.  There were several problems.  First of all, the temperature of the sample in the probe was higher than room temperature even when the VT heater is off.  Setting all the shims to zero and letting the instrument sit overnight results in the temperature falling to 22 C.  Ah ha.  The shims, which are just electromagnets, are heating up the sample.  If you are wondering why the shims are producing so much heat, my only guess is that I (or more correctly, we) have slowly and steadily over many years configured our shims with too much current.  Since we can make lineshape (or at least get pretty close) I can only guess that we must be really lucky and able to compensate for one high order shim being too high with another high order shim.

At any rate, I set all the shims to zero and shimmed the magnet from scratch.  I also made an adjustment to the shim stack.  (There was a loose nut and we moved it a couple of years ago.  I was able to correct for this movement, but I wanted to make sure the nuts were tight and would not move again.  Mission accomplished!).  I have melted my brain shimming - fortunately I can get baseball gameday audio on my phone.  Shims look good.  Temperature looks good.  Temperature calibration looks good.  I just need to make new 2H shim maps and check pulse calibrations.  Instrument should be available to all tomorrow.

J