Gas flow failure.

7/2/2 SCH.

Gas flow failure is indicated in the edte window as gas MISSING.  This refers to a sample temperature control system in which refrigerated air flows through a heater to adjust its temperature and then across the sample tube.  Loss of gas flow causes the heater to automatically kick off, and loss of temperature control at the sample.  Gas flow failure can occur either due to loss of air pressure in the university supply lines or due to freezing up of the refrigerator unit.  In the former case, the lock lift will also not work.  One can tell if there is air pressure in the supply lines from the gauges where the supply lines attach to the wall.  There is a ball flow gauge behind the right hand door of the acquisition computer which will also indicate loss of gas flow specifically in the temperature control air stream.

Check the air dryer.

Blockage of the refrigerator unit may be a secondary failure due to failure of the air dryer.  The air dryer consists of a set of blue cylinders attached to the wall inside the utility room where the Avance500 and 600 spectrometers are located.  It also supplies dry air to the Avance700.  The dryer is not currently on emergency power, so if power is off for a significant time (~12 hours), enough humid air will circulate through the air refrigerator unit to freeze in the lines and block gas flow.  We've also seen the dryer off because someone kicked the plug out of the wall socket.  In any case, the first thing to do to correct a gas flow problem it to check that the green power light is on on the dryer unit.  If there is no power to the wall socket that it is plugged into, then move the plug to an active socket in the outer NMR room until that problem can be rectified.

Thaw and dry the feed line.

The gas flow goes from the dryer to the wall gauges, then to the acquisition computer console and through the ball gauge, then to the refrigerator unit (1.5 x 1.5 foot box on the floor), and then through an insulated hose to the bottom of the magnet where it connects in a cup and socket joint secured by a pinch clamp.  Turn off the refrigerator unit and disengage the pinch clamp.  After 15 to 20 minutes air flow should resume, indicated by 525 L/hr on the edte window and by palpable air flow at the exposed end of the delivery hose.  Let dry air blow through the system to dry it out for 3 to 4 hours before reconnecting the hose at the bottom of the magnet and turning the refrigerator back on.  You will have to turn the heater back on in the edte window, by clicking the on/off button next to the heater indicator.

Unresolved issues.

1) I see 525 L/hr. set now on the Avance600.  Is that the nominal rate?  The ball is 1/4 to 1/3 of the way up the gauge.  There appears to be a sensor in the middle.  Is this to indicate an over pressure situation?  Note that the flow rate show in the edte window doesn't fluctuate when the flow rate starts to fail.  It apparently either reads the set value or MISSING
2) I understand that people turn it up sometimes to get stronger temp. control.  What are the acceptable limits?
3) I once experienced that someone had left the flow rate up such that the lift failed to lower my sample.  It would be best to have a nominal rate recorded so that you could put the machine back into a known state, however I suppose any lower number that allowed the sample to go down and lock in but maintained constant temperature would be good enough.  I guess that people who need a higher rate turn it up after they get the sample locked in.