Facility Maintenance Procedures

• Cryogen Refills
• Specifications Tests
• Physical Spectrometer Inspection & Cleaning
• System and Software Administration
• Super Shim Diagnostic Program
• User Billing

Cryogen Refills

• Liquid nitrogen fills
• Pictures of cryogen transfers
• Liquid helium fills

For information on safety considerations related to asphyxiation, please consult The OSHA recommendations and cautions regarding working in confined spaces. These are available at http://www.osha-slc.gov/Preamble/ConSpaces_data/CON_SPACE2.html (Occupational Safety and Health Administration) and are also cited at this facility's web page "Assessing risks of superconductive magnet quenches".

Liquid nitrogen fills

Liquid nitrogen is replenished weekly in the Oxford 500/51 the Oxford 400/54 and the Oxford 300/89 magnets. Somewhat over 80 L is required to complete all fills on a seven day interval. The stockroom personnel are not always available to fill the dewar promptly, so some planning is required. During holidays when the University is closed, NMR Facility personnel have a key to the outdoor liquid nitrogen tank and may fill the dewar themselves. A pink Stores Requisition form must be completed. The stock number for liquid nitrogen is 30-138-1036.

Liquid nitrogen is introduced into the magnet via a 160L medium pressure stainless LN2 dewar designed for the dispensation of either liquid or gaseous nitrogen. This has been modified with a 20 psi pressure release valve that can be closed off for higher pressure gas dispensation. However, liquid delivery requires pressures below 50 psi.

If the dewar has just been filled, it will have insufficient pressure to fill a magnet, so the pressure-building apparatus must be employed. Care should be taken to turn this off and re-engage the 20 psi blow-off when the fill is done.

1. Using the pressure release valve atop one of the black heat exchangers on the nitrogen ports, release pressure from the magnet's nitrogen can.
2. Remove all three heat exchangers.
3. Attach the stainless nitrogen transfer line to a conveniently located nitrogen port.
4. Attach short pieces of amber tubing to the remaining ports to prevent "snow" from falling into the magnet nitrogen can. Do not point these where people will get sprayed nor at the glass office window.
5. The magnets are full when liquid nitrogen comes out the top. Close the liquid transfer valve and any pressurizing valves.
6. Remove the amber tubing from the nitrogen ports and replace the heat exchangers.
7. Re-enable the 20 psi blow-off.

The NMR laboratory at SIUC is large enough that the atmosphere is not noticeably degraded during nitrogen transfers, even with the doors closed, so asphyxiation is not a serious concern. It has long been felt that magnet homogeneity of the 500/51 is somewhat perturbed during nitrogen fills but is restored quickly thereafter. Users are encouraged to suspend data collection during the procedure. The 300/89 seems unaffected by nitrogen fills, so it is our practice not to interrupt data acquisition for nitrogen fills. Users are permitted inside the NMR Facility during this time, but should be supervised by NMR staff.

Pictures of Cryogen Transfers

Click on image for full sized image
Apparatus for liquid nitrogen transfer	Low pressure for liquid transfer
20 psi blow-off closed for pressurization	Dewar pressure building controls
20 psi blow-off enabled for normal use	Precooling the liquid helium transfer tube outside the NMR lab
Precooling the liquid helium transfer tube outside the NMR lab	Beginning a helium fill on the Oxford 500/51 magnet

Liquid helium fills

Liquid helium is replenished every ninety days. It must be ordered a week in advance. Normally, a 100 L shipment will suffice to fill all three magnets. However, sometimes the dewars arrive partially depleted and an additional fill will be necessary.

Liquid helium fills are inherently dangerous. See "Assessing risks of superconductive magnet quenches". No one but personnel performing the fill should be allowed into the NMR lab during this time. The double doors should both be open to provide maximum ventilation and care should be taken that a clear path exists for personnel to rapidly exit the Facility in case of a quench.

Magnet homogeneity may be disturbed during the helium transfer and for two or three hours thereafter with all magnets. Because the anti-vibration devices on the 400/54 and 500/51 are lowered for this procedure, reshimming a magnet after a helium fill is strongly advised.

If at any time large quantities of helium (gas or gas plus liquid) begin issuing from a superconductive magnet, a very dangerous event called a magnet quench is occurring. It is imperative that everyone leave the area immediately. Pior to beginning a helium transfer, make sure that the roles of all personnel are clear and anticipate who might need assistance evacuating the NMR Facility. Loss of consciousness and death may result.

1. Open the helium supply dewar outside the NMR lab and attach appropriate fitting with O-ring to the liquid port. Direct venting helium away from the Facility and the NMR magnets. (Helium gas can penetrate magnet seals, softening the vacuums in the magnet dewars.)
2. Measure the liquid height with a flutter-tube and record the volume of helium received. Much less than 85 L requires that the transfers be performed as efficiently as possible.
3. Equip the helium transfer tube with a tygon feed line from a helium gas tank and gradually insert the transfer apparatus into the supply dewar. Allow some boiloff to be vented, but use as much as possible to precool the transfer tube.
4. Loosen the input and output port hardware on the magnet to be filled.
5. Using gas pressure if needed, precool the transfer tube until a liquid plume emanates from the tube's output. Discontinue pressure and vent excess pressure.
6. With supply dewar exhaust port pointed out of the lab, walk supply dewar to magnet. Fill-person #1 ascends the ladder with the transfer tube in hand, removes the magnet hardware, and prepares to insert tube into magnet while fill-person #2 moves the dewar into position. (See pictures above.) Fill person #2 must lift the transfer tube out of the supply dewar to facilitate insertion into magnet. The ceiling must be avoided while putting the tube back down into liquid-transfer position.
7. Person #2 must close supply dewar vents, and regulate gas pressure at helium gas tank so that the transfer apparatus on the supply dewar shows 3-5 PSI.
8. Person #1 should come down from the ladder when transfer is underway. In the case of the 500/51 magnet, the supply dewar touches the ladder, making this step somewhat difficult.
9. The fill is complete when the characteristic plume appears at the magnet exhaust port. #2 shuts off the gas pressure and depressurizes the supply dewar while #1 ascends the ladder to remove the transfer tube (#2 must lift it out of the supply dewar simultaneously) and reinstall the magnet hardware.
10. If another magnet is to be filled, it is best to proceed to it promptly in order to minimize the helium waste from re-cooling the transfer tube.
11. Outside the NMR lab, measure the remaining liquid helium volume. Record this and compare quantity used to other fills at similar intervals. Greater helium consumption may indicate that the vacuums of either the magnet dewars or the transfer tubes are degrading.

Specifications Tests

Some laboratories run "specs" on a monthly or weekly basis. That is fine if one has an abundant supply of spectrometer time and technician-level labor, but a better rule is to run spec tests occasionally - and whenever you are wondering if there's a problem. Many "shortcut" tests are more helpful, less trouble, and more efficient.

For instance, most users of the Inova 300 and Mercury 400 run routine ¹H and ¹³C in CDCl₃. Occasionally, we put the proton sensitivity standard, ethylbenzene in chloroform, into the magnet and look at the spectrum. If the peak shape is nice and the signal-to-noise looks about normal, then everything is very likely okay. 90 degree pulse widths for all nuclei have held steady over the years.

Since the Inova 500 is used for much more demanding work, it should be scrutinized more closely when there is free instrument time. This is made easier by an automated diagnostic routine called Autotest that examines all aspects of the radio frequency section and generates a report. This test should be performed as often as is convenient.

The chloroform lineshape specification test should be performed periodically also. This is made considerably easier by the presence of Z-axis pulsed field gradients.

Download the Microsoft Word Document Deuterium Gradient Shimming.doc for the procedures for making shim maps and gradient shimming. Perform these procedures with a sample that relaxes quickly - like D₂O doped with GdCl₃ so that the process is fast. Recable the instrument and manually adjust the non-spinning shims. Perform gradient shimming again. Insert the chloroform lineshape standard sample, regulate temperature to 30 degrees, and gradient shim with a 10 second relaxation delay necessary for d6-acetone. Adjust the non-spinning gradients and then gradient shim the Z gradients again.

The Varian specifications for our 5-mm ¹H{¹³C/¹⁵N} triple resonance PFG probe with 28-gradient shim stack are

Other specification tests are largely academic exercises. The resolution test, for instance, uses o-dichlorobenzene in d6-acetone and takes forever to get a good result. It doesn't tell you anything that lineshape doesn't tell you.

Similarly, the signal-to-noise test is important when signing off during acceptance of a new instrument, but it is a time-consuming, highly shimming-dependent test that can almost always meet spec with enough effort - unless there's a problem - and that's when you should do it.

Physical Inspections and Cleaning

The days when the magnets are filled are opportunities to open the RF cabinetry and do visual surveys looking for problems. Note any dirt accumulation on air intake filters, etc., and clean. Inspect all cooling fans to make sure they are working. Heat is electronics' worst enemy and dirt buildup and restricted air flow cause heating.

System and Software Administration

Shim Supply Self Test

Here is a text file of the output from the Super Shim Diagnostic Program as described in the Inova Technical Reference, pub. no. 01-999047-00, Rev D0801. The instructions indicate that you need a special communications program like Procomm Plus, but there is a utility built into Windows called Hyperterminal that works just fine.

Serial and parallel ports are not present on some new computers. Luckily, our Dell laptop had the serial port (DB9 male). The serial port on the shim supply, however, is a female DB9, so we had to use a gender changer to use our female to female cable.

User Billings

Using the VNMR Accounting program requires root-level access and must be performed at the spectrometer console. There is a script file called nmrbills that will call the accounting program:

vnmrsystem=/vnmr
export vnmrsystem
/vnmr/bin/vnmr_accounting

The accounting package is programmed to apply the standard rates from 8 AM to 8 PM Monday through Friday and the discount rates at all other times.

Print the main window, the console log and the group billings pages and retain these for filing and distribution with the invoices. Doing this every three months is usually sufficient. Send billing information to Suzanne Garoian, mailcode 4716 or email sgaroian@siu.edu.

To change rates, root must manually edit /vnmr/adm/accounting/multirate. The dollar figures are contained at the end of the paragraph for each group. For instance:

kohli 3 {3,Fri 0 3,Thu 0 2,Mon 1 3,Mon 0 1,Tue 1 2,Tue 1 3,Tue 0 2,ho 0 1,Wed 1 2,Wed 1 3,Wed 0 1,Sun 0 1,ho 0 2,Sun 0 3,Sun 1 1,Sat 0 3,ho 0 2,Sat 0 1,Thu 1 1, Fri 1 3,Sat 1 2,Fri 1 2,Thu 1 1,Mon 1} {1 08:00 2 20:00 3 0:00} {1 10.00 2 5.00 3 5.00}

The time periods 1, 2, and 3 for the kohli group have charges of 10.00, 5.00, and 5.00 respectively. When normal and discount charges change, edit those three numbers.

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