A. First Aid Measures
Safety -- Contact with liquid helium may cause frostbit or freeze burns in exposed tissues. Frostbite effects are a change in color of the skin to gray or white, possibly followed by blistering. The effect of oxygen deficiency resulting from simple asphyxiant may include: rapid breathing, diminished mental alertness, impaired muscular coordination, faulty judgment, depress of all sensations. As asphyxiation progresses, nausea, vomiting, prostration, loss of consciousness may result, eventually leading to convulsions, coma, and death.
Eyes -- In case of freezing or cryogenic "burns" caused by rapidly evaporating liquid: DO NOT WASH THE EYES WITH HOT OR EVEN TEPID WATER. Remove the victim from the source. Open eyelids wide to allow the liquid to evaporate. If pain is present refer victim to an ophthalmologist for treatment.
Skin -- For dermal contact or frostbite: Remove contaminated clothing and flush affected area with lukewarm water. DO NOT USE HOT WATER. A physician should see the patient promptly if contact with the fluid has resulted in blistering or deep tissue freezing. Skin can stick to metal that is refrigerated by liquid helium and can tear when pulled away. Immediately flood with large quantities of unheated water any area of the body that is "burned" by liquid or cold gas, and then apply cold compresses. If the skin is blistered immediately seek medical treatment. B. Preparation
1. Fill the NMR magnets with liquid nitrogen on Tuesday, or the day before the liquid helium is due to be refilled. The Chemistry Department has traditionally filled the NMR magnets with liquid helium on a 90-day cycle. Varian recommends a 80-day cycle. Current the fill cycle is set to 70 days, or five time a year.
2. Order 100 liters of liquid helium a supplier, Argyle Gas on a Thursday, the week before the filling date. Argyle place orders on each Friday for delivery from Phoenix, to Las Cruces on Thursday/Friday. The stainless steel (i. e. non-magnetic) Dewar is delivered to Las Cruces on Friday morning.
3. Physical Properties -- The freezing point of helium is -456.5° F (-271.3° C, or 2.17 K) it is a non flammable, colorless, odorless gas which is a simple asphyxiant, in a confined space. Liquid helium is delivered in stainless steel, portable (on wheels), vacuum-jacketed "liquid" cylinders. Liquid helium is extremely cold and un-insulated transfer lines may condense air. The density of liquid helium at saturation pressure is 0.146 kg/L.
4. Make certain the following requisite supplies are on hand for the transfer: - a drape cover for the Gemini magnet, - a cylinder of high purity gaseous helium, with a regulator capable of showing pressures of 2 to 10 psig, - a gas cart to transport the cylinder of gaseous helium, - a labeled transfer tube for each NMR unit with a deflector nozzle attached to the magnet end of the transfer tube; the transfer tube must be routinely pumped down (evacuated) once or twice a year, as preventive maintenance, - stinger tubes for each NMR magnet, - pressurizing collars, for each NMR unit, and - proper safety equipment for handling cryogenic fluids: face shield and several pairs of cryogenic gloves. C. Measuring the Liquid Helium Level
1. Slowly insert the flutter (thumper) tube into the liquid helium fill port until it touches the bottom of the can, then place an alligator clip on the tube, which is level with the top of the fill port.
2. While slowly raising the flutter tube, observe vibration by closing off the top end of the tube with your thumb (moist). When the end of the tube passes from the liquid zone to the gas zone above the liquid, the vibrations intensify and the frequency changes. At this point place another alligator clip on the flutter tube, level with the top of the fill port.
3. Measure the distance between the centers of the two alligator clips; this is the depth of the liquid helium in the can containing the magnet. Record the data in the Table on page 7.
You can also subtract the top of the liquid helium level from the depth of the can. The Gemini 200 can has a depth of 40 cm and the Unity 400 has a depth of 72 cm.
4. Determining Stinger Length for Storage Dewar.
The stinger may be necessary to extend the transfer tube into the liquid helium storage container. Take the following steps for each container.
Measure the top of the storage container port to the bottom of the liquid helium chamber (a flutter tube can be used). Measure the distance to the top of the internal storage container service port to the top of the helium chamber (a stiff wire with an inch long "L" shape which can be used to catch at the top of the chamber). Subtract the measurements to obtain the depth of the liquid helium chamber. This is also the measurement for the maximum length of the stinger for this container. CAUTION: The stinger on the storage container side of the transfer tube must not be longer than the height of the LHe chamber. Exceeding this maximum will cause filling difficulties and can cause a quench. Similarly determine if the correct transfer tube is being used for each magnet according to the protocol show.
D. Filling Procedure (See section F for more details on filling the Unity 400 magnet.) Precautions: observe the lock signal during the helium fill. If there is a change in the lock signal stop filling as it indicates a problem. Use stingers that reach the bottom of the supply Dewar and fill both Gemini 200's before the Unity 400.
1. Drape the black- rubber-filling cover over the top of the magnet Dewar. Position the filling cover holes over the Gemini magnet ports.
2. Measure and record the amount of liquid helium in the storage container. The amount of liquid helium in the container is not the volume of the container but the amount that is accessible to transfer with a stinger. The bottom of the stinger should reach to within 2 cm of the bottom of the container when the stop tab on the magnet side of the transfer tube is resting on the lip of the magnet helium service port
3. Place the liquid helium storage container so the discharge port is about 26.5 inches (68 cm) horizontally from the helium storage port. This is the distance for the Gemini LHe transfer tubes available from Varian and allows the transfer tube to inserted into the storage container and the magnet helium service port, simultaneously.
CAUTION: Do not bend or twist the transfer tube when inside the magnet neck tubes. The filling-port tube is thin-walled stainless steel and is easily punctured with sharp objects. The same precaution applies to removing ice plugs. Never use an ice pick. 4. Depressurize the storage Dewar by opening the exhaust port, it could take 20-30 minutes.
5. Place the pressurizing collar on the Dewar side of the transfer tube.
6. Pre-cool the helium transfer tube by slowly inserting one end into the storage Dewar with the other end open to the room. Do not close the storage Dewar exhaust port during insertion. The room temperature heat will cause increased liquid helium boil off while being inserted. The helium boil off should not be allowed to cause increase in Dewar pressure.
Once the transfer tube has been fully inserted, pressurize the storage Dewar by closing the exhaust port. Adjust the flow meter valve so the flow rate is about 4 on the scale. The pressure will slowly rise. After 10 to 40 seconds, a plume of liquid helium (in the form of a very dense, milk-like white flamed-shaped fog) emerges from the deflector nozzle, indicating that the pre-cooling has been accomplished.
7. After observing the exhaust for 5 to 15 seconds, depressurize the storage Dewar by opening the exhaust port.
If the transfer tube frosts over it is likely that there is a poor or faulty vacuum within the transfer tube jacket. The transfer tube should be removed from use, warmed, and pumped down to at least 5 microns.
8. Remove the helium service-relief valve. Pull the transfer tube up enough to clear the magnet Dewar. Swing the transfer tube over the magnet and immediately insert it SLOWLY into the magnet access port for helium and the storage Dewar, simultaneously. As a guide, use 10 to 15 seconds for insertion, up to the Stop Tab, for the Gemini magnets.
If you observe excessive turbulence or a sudden increase in the exhaust rate, the transfer tube insertion is too rapid and hot gas is reaching the liquid helium in the magnet. This is dangerous as it could cause QUENCHING of the magnet, if the liquid helium level is below the top of the solenoid. Take immediate action and slow down the insertion rate.
9. The transfer tube is fully inserted so that the stop tab rests on the lip of the magnet access port. Start to pressurize the storage Dewar by closing the storage container Exhaust Port and then adjust the flow meter valve. Set the flow rate to about 4 on the scale.
CAUTION: Watch the pressure gauge on the head assembly. Any rapid pressure buildup may indicate a frozen air or water blockage in the transfer tube or the fill tube. If ice blockage occurs, STOP the liquid helium transfer, remove the transfer line, and again purge the line with gaseous helium. After the line is cleared, restart the transfer from the beginning. Be certain to plug the storage container and the helium service port as soon as the liquid helium transfer tube is removed from them in order to prevent air and moisture from entering the magnet Dewar.
9a. The pressure will rise slowly to about 0.5 psig. During the first two minutes a collapsing exhaust plume is usually observed. The collapsing exhaust indicates that the liquid helium is reaching the magnet end of the transfer tube and is filling the magnet.
9b. If the pressure is not increasing toward 0.5 psig when the flow rate is set to 4 on the scale, there may be a LEAK of the pressurizing gas that is preventing the storage Dewar from being pressurized.
9c. If the pressure exceeds 1 psig when the flow rate is set to 4, it is likely a blockage has occurred in the transfer tube. It will be necessary to remove the transfer tube, cap the magnet, warm the transfer tube, and start over again at the pre-cooling step.
9d. If a frost spot occurs in any section of the transfer tube that is not directly exposed to cold exhaust, there is probably a metal to metal touch across the vacuum jacket. The transfer tube should be replaced. A defective transfer tube can cause a magnet or shim QUENCH. The transfer tube can be routinely pumped down once or twice a year, as preventive maintenance. A special connections are used to evacuate the Unity 400 transfer tube.
10. After the initial 2-minute period, adjust the flow meter value to about 8 on the scale. The pressure indicator should slowly rise to about 1-2 psig.
11. The filling continues at this pressure and rate until the magnet is full, which is indicated by a marked increase in the rate of exhaust from the magnet access port. The increased exhaust is accompanied by an increase in the density of the exhaust cloud. If there is any doubt about the level, use a flutter tube measurement to verify the content level.
For the Gemini 200, length of the helium neck tube is 50 cm, the depth of the helium can is 40 cm and nominal refill level is 35 cm (8 L required), see Figure 2. For the Unity 400, the top of the helium can is 72 cm, with top of the magnet at 46 cm and the nominal refill level is 69 cm (45 L required).
12. Depressurize the storage Dewar. Remove the transfer tube from both containers. (Removal is much less risky than insertion.) Immediately cap the magnet access port. Close the appropriate storage Dewar valves.
E. Consumption of Cryogenic Liquids
1. The following table lists approximate values concerning consumption and measurements for both liquid helium and liquid nitrogen used to cool the two types of magnets in the Department of Chemistry at New Mexico State University. The three magnets are filled with a 100 L (Cost has increased from $418, to $425, to $743 in October 2004) of liquid helium every 90 days. Air Liquide demurrage charges are $7.00 per day or $100 per month. Every 60 days increases yearly cost by $862.
2. The weekly filling of liquid nitrogen for three magnets uses a single Dewar containing 180 L (Cost $80) for two fillings for each of the three magnets. Care must be taken to return the empty Dewars two days before the end of each month, to avoid a $40 demurrage charge. 3. The minimum yearly charge for cryogenic fluids for the three NMR magnets is $1860, plus any demurrage charges. Demurrage charges for liquid nitrogen could add an additional $500 a year to the yearly charge. An additional $862 will be required to fill every 60 days. Examine relationship between depth and volume of cryogens for the NMR instruments to prevent errors in filling the magnets. The general relationship between depth and amount of liquid helium in the storage Dewar may be a critical
4. Always record both the levels of liquid helium before and, as-filled, or after filling. See the recent helium fill data or history for each magnet.
F. Additional Details for Helium Filling of the Unity 400 Magnet
1. There is a washer and o-ring on the short leg of the transfer tube which will be inserted into the magnet. Remove the black cap and o-ring attached to the vertical tube on the right side of the magnet.
2. Insert the long leg of the transfer tube a little way into the storage Dewar and tighten it in place. Do not push the long leg of the transfer tube into the liquid helium storage Dewar at this point. If the transfer tube is in the liquid helium some liquid will be wasted.
3. Use dry helium gas and pressurize the storage Dewar to about 1 psig. After about a minute helium will start to cool the transfer tube. When the cloud of helium gas changes to a dense white fan or flame shape the tube is cool enough to insert into the magnet cryostat.
4. Release almost all pressure in the storage Dewar. Use a gloved hand to dislodge any liquid air that is at the end of the transfer tube. Insert the leg until it rest on the core then lift about about an inch (25 mm). Remove the helium check valve between the two towers.
5. The filling process will take about 30 minutes, depending on existing level of liquid helium. The boil-off rate will increase when the vessel is almost full. When this happens, release the pressure in the storage Dewar. Check the actual level of liquid helium using the level probe, it should be nearly 100% full (refer to the Table on page 7).
6. Use a hot-air blower to unfreeze the o-ring at the entry fitting. Unscrew the nut and remove the transfer tube from the cryostat, complete with nut, washer, and o-ring. Immediately fit the stopper, nut, washer, and o-ring which were previously removed and tighten the nut to form a gas-tight seal.
7. Remove the transfer tube from the storage Dewar store it in a dry place.
8. When the manifold has returned to room temperature make certain that all clamps are tight. They may have been loosened by the freezing process.
9. Located the transfer-hose fitting to re-evacuate the Unity 400 transfer line. A new vacuum was put into the three helium transfer tubes on October 6, 1999 and on March 17, 2008.
10. There maybe frozen air in the upper part of the Unity 400 helium filling tube.
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