Pumping Out, Cooling, Warming, and Venting

1. General Advice
2. Pumping Out
3. Cooling the Dewar
   1. Cooling to 77K
   2. Waiting for 77K Precool
   3. Cooling to 4K
   4. Cooldown Schedule Summary
4. Warming the Dewar
5. Venting the Dewar
6. Leak Checking
7. Revision History

General Advice

• The thin infrared-blocker filter is delicate.  Limit all pumping/venting to 1 Torr/sec.
  
  
• When the turbo is at full speed, it should never see more than a few hundred mT, and no more than 200 mT for  extended periods.
  
  
• Do not turn on the ion gauge when the pressure is above its saturation pressure of 7.5 mT, its filament lifetime will be reduced.  Actually, it's ok to turn it on briefly to check, but don't leave it on if the pressure is off scale.
  
  
• With any vacuum or cryogenic operation, think before doing.  Cookbooks like this are good as a reminder, but don't let them substitute for making sure you understand what you are doing.  If you've accidentally skipped an important step, you can catch it before damage occurs if you understand what you are doing.
  
  
• The pumpout speed is in general limited by water adsorbed onto surfaces.  Every effort should be taken to minimize exposure to air.  For example, even if you are going to open the dewar to air, it is still a good idea to vent the dewar with dry nitrogen gas because volumes you do not open up (the JFET box, RF filters, inside the array, between the lens and 4K snout lid) will see much less water vapor.
  
  
• Cleaning all KF surfaces and o-rings prior to use and using pumping lines you know to be leak-tight will reduce the time you spend hunting down non-dewar leaks!  This is especially important for the right-angle dewar bellows valve, as removing it requires completely venting the system.
  
  
• The baths must be capped whenever the outflow of gas is small enough that air could be sucked in and ice condense in the neck.  So the baths are capped pretty much all the time except right after a cryogen fill.  You usually have to defrost the fill ports whenever you want to open or close them.  This is done with a heat gun.  Be sure to aim the heat gun at the top of the fill port fitting; if you heat the bottom part, you may melt the RTV that helps seal up the ports.  Heat the top so that the ice melts and the surface is dry.  Then it should be easy to screw the bath cap on or off.  Also watch out for heating the rubber hoses when you are doing this or if you want to remove one.  Never hit them directly with the heat gun; rather, heat the metal of the port so that the hose warms up and unfreezes.  Finally, when you cap the bath, make sure you actually have an o-ring in the cap (people have been known to leave it out), make sure there is a rubber hose attached to the exhaust port hose barb, and check that, with the cap on, there is good outflow of gas through the exhaust port rubber hose.

Pumping Out

1. Attach the black right-angle bellows valve to the dewar, orienting it so the electronics box will not interfere with it.  Make sure the dewar valve is open.
   
   
2. Attach a KF cross to the bellows valve and attach the red Bolocam Pirani gauge (rectangular electronic gauge, part number starts with APG) to one of the side ports of the cross and the Bourdon gauge (round analog gauge, bottoms out at -30 inches Hg vacuum) on the other side port.  If the Bourdon gauge is not reading 0, note the offset so you can correct for it.  (If you want to, you can rezero it by popping off the face and turning the slotted screw that the needle is attached to.  Don't do this unless you know what you are doing!).
   
   
3. Connect the Pirani gauge readout line to a controller (there should be an Edwards Active Gauge Controller with the Bolocam equipment).  To get the gauge reading:
   
   
• Connect the telephone-type cable to one of the ports on the back of the controller.  Port 2 is known not to work as of 2004/01.
  
  
• Flip on the power switch.
  
  
• Hit the down arrow until the left-most display column hits the port number the gauge is plugged into.
  
  
• I don't remember the exact key sequence to get the gauge readout to appear, but it is some combination of hitting the Enter button and the button that looks like a circle made of two arrows.  Consult the day crew ahead of time if you don't know how to use the gauge controller (it's ok to run it while exposed to atmosphere).
  
  
4. Connect the Bolocam regulating valve fixture to the straight-through port of the cross.  This fixture consists of a valve sitting on an aluminum base plate and with copper 1/4" tube connecting to KF25 flanges on each end.  It serves to regulate the pumping speed during the early part of the pumpout.  Its orientation doesn't matter.  Make sure the valve is closed.
   
   
5. Connect the free side of the regulating valve fixture to your pumpout line (stainless steel flex hose if at all possible).  For this stage, it doesn't matter what size line you use, but you might as well use the shortest, largest bore line you can find since it will matter for the next stage.  Adapt down to KF25 as necessary.  Find some way to support the weight of the line so it is not all hanging on the regulating valve fixture and black dewar valve.
   
   
6. Connect the open end of the pumping line to the turbo.  Make sure any bleeder valves on the turbo are closed.
   
   
7. Open the regulating valve 1/4 of a turn.
   
   
8. Start the turbo, and press the Low Speed button.  Running it at low speed is necessary because the pressure in the line is rather high in spite of the regulating valve.
   
   
9. Adjust the regulating valve setting to maintain a pumpdown rate of 1 Torr/sec (~1 inch/30 sec).  Use the Bourdon gauge near atmospheric pressure, trust the Pirani only below 100 Torr or so.  If the turbo shuts itself off during this process, then the pressure on the turbo side of the regulating valve is too high and you need to close the regulating valve by some amount (resulting in a slower pumpdown).  This shouldn't happen and is probably indicative of a very large leak somewhere (e.g., did you close the turbo bleeder valve?).  While the dewar is pumping out, you can attach the e-box (assuming you are confident you don't have a leak that will force you to open up again!).  Once you have the box on, you can start doing warm JFET checkouts too if you so desire.  Instructions for JFET checks are provided elsewhere.
   
   
10. Wait until you reach 50 T on the Pirani gauge (the Bourdon gauge will have bottomed out at about -20 inches Hg).  This should take 10-20 minutes from start of pumping.  At this point, you will execute the following procedure to remove the regulating valve and start pumping again without creating a big pressure change in the dewar.  Be careful!  One false move can result in the dewar jumping from 50 T to atmospheric pressure in seconds.
    
    
• Close the regulating valve completely.
  
  
• Note the pressure in the dewar as measured by the Pirani gauge.
  
  
• Close the black dewar valve.
  
  
• Stop the turbo.  Use the bleeder to vent the line to atmosphere.  This is safe, as both the regulating valve and the dewar valve are closed.
  
  
• Double check that the dewar valve is closed.  Open the regulating valve.
  
  
• Now the turbo and the entire line are at ambient pressure.  Remove the regulating valve from the line, connecting the pumping line directly to the cross at the dewar with the gauges.
  
  
• Replace the Bourdon gauge with the red ion gauge (circular shape, part number AIM-something).  Plug it into the gauge controller and make sure the controller recognizes the gauge (AIM), but don't turn it on yet. 
  
  
• Here's the tricky part: pump the line down to the same pressure the dewar is currently at (which you noted prior to closing the black valve).  Do this by quickly starting and stopping the turbo.  Usually you will overshoot, so you have to use the bleeder to bring the line back up to the desired pressure.  There's little risk of damage to the turbo (it didn't spin up all the way and now you've stopped driving it), but it requires only a small amount of air to bring the line to the correct pressure.
  
  
• Once you are sure the pressures in the line and dewar are the same to within a few Torr, it is safe to open the black dewar valve because there is no pressure differential with the line.
  
  
• Start the turbo up again in low-speed mode.  It is important to start from rest here, as the slow spin up of the turbo is what limits the pumpdown speed during this phase.
  
  
• Monitor the Pirani gauge as the turbo spins up.  If the pressure drops faster than 1 Torr/sec, you may need to use the large valve at the input of the turbo to throttle until the pressure decreases further.
  
  
• If you have any problems with the dewar pumping down too quickly in this phase, you are probably using a different turbo than we have used in the past and the 50 T switchover point needs to be reduced.
  
  
11. While waiting for the dewar to pump down, you can install the e-box if you haven't already done so and start or continue the warm JFET checkout if so desired.  Instructions are provided elsewhere: e-box installation, JFET checks.
    
    
12. When the Pirani gauge reaches 100-200 mT, take the turbo out of low-speed mode (hit the low-speed button).  It takes between a few minutes and an hour to reach this point, depending on whether the dewar was vented with nitrogen and how long it was open to air.  If it doesn't reach 100 mT in 90 minutes from start of pumping, you probably have a big leak somewhere.  Close the black valve and check that the line pumps down properly.  If the problem is not in the line but on the dewar side of the black valve, you will probably have to vent (see above) and look for the leak by overpressurizing and using a bubbling solution.
    
    
13. Soon after, you should be able to get the ion gauge to turn on and display a pressure below saturation (7.5 mT).  The ion and Pirani gauges do not match up well, so the ion gauge will start to function somewhere below 50 mT on the Pirani gauge.  How long it takes to reach this point depends again on whether the dewar was vented with nitrogen gas and how long it was open to air.  You don't have to wait for this; once the pressure is below 100 mT, it is safe to leave the turbo running at full speed unattended.
    
    
14. Overnight pumping should get you to somewhere between few x 10^-5 Torr and 2 x 10^-4 Torr.  The prudent thing to do is wait to get to few x 10^-5 Torr; if it never gets that low, you definitely should leak check the dewar.  If you are in a rush and are confident that the dewar is leak-tight, then you can start cooling once the dewar hits the mid 10^-4 Torr range.
    
    
15. You will leave the turbo connected to the dewar during the cooldown until the dewar has stabilized at 77K.
    

Cooling the Dewar

Cooling to 77K

1. Set the log filename for the fridge control program to LN_cooldown_YYYYMMDD.txt.  Set the big button to Fridge is off.  Set the sampling rate to once per minute.  Set all the diode thermometer vertical scales to read from 0 K to 300 K.  Start the program.
   
   
2. Note the starting time, dewar ion gauge pressure, and array, IC, and JFET temperatures.
   
   
3. Start the LN bath fill.  Insert the fill tube in the bath.  Open the storage dewar valve slightly; since gas, not liquid, is flowing right now, the volume flow rate is very high and the rubber hose can be damaged if you go too quickly.  Support the rubber hose so it freezes in a reasonable position.  As the hose freezes, you can open the valve more.  Once liquid is flowing through the hose, open the valve as far as you are comfortable with (the first thing to break will be your rubber hose, not the dewar).  Be a little aggressive; you will spend way too long filling if you're not.  It should take no more than half an hour to fill the LN bath.  It is full when liquid starts spurting out around the fill tube; at this point, close the storage dewar valve.  During this process, you should notice the vacuum pressure slowly drop as the bath cools, and then drop precipitously once liquid is collecting.  The pressure should get down to the low to mid 10^-6 Torr.  You will notice some reduction in the 77K and 120K diode temperatures, but they are separated from the bath by a long aluminum rod and so lag quite a bit, making them pretty useless as monitors of the LN bath itself.
   
   
4. Switch the fill line over to the LHe bath, defrosting as necessary.  Repeat the above startup process.  You will see the 4K Diode and all the fridge diodes start to cool immediately, with the 4K Diode leading.  Keep the pressure high enough to keep the 4K Diode cooling steadily, but not so much that the outrush of gas from the exhaust port seems dangerous.  It's good to attach a DMM to the BNC on the white thermometry breakout box (see elsewhere for details) labeled 4K Diode and monitor it, as it doesn't have the latency that the thermometry display has.  This is especially important as the LHe bath nears 77K because the pressure increases precipitously at this point and it may become necessary to reduce the flow from the storage dewar.  Pay very close attention to the exhaust pressure when the diode voltage gets to about 0.950 V.  77K corresponds to 1.020V, but you will notice an increase in the gas outrush a little before 1.000V.  The voltage takes a few minutes to reach 1.020V even after liquid has collected due to the impedance between the liquid and the thermometer.  Once you have started collecting liquid (as signified by the sudden bottoming out of the diode voltage or the 4K diode temperature reading), fill for 5-10 minutes -- you do not want to fill the LHe bath entirely, you only want 4-6 inches.  You can check how much you have filled using a LN dipstick (rod of G-10 covered with heat shrink); dip the stick in the bath, all the way to the bottom, pull it out, and look for the frost line.  Over the entire fill, the dewar vacuum pressure should continue to go down slowly, ending in the low 10^-6 to high 10^-7 Torr.
   
   
5. Once the LHe bath is sufficiently full, go back and top off the LN bath -- it evaporates a lot of LN immediately as the shield and JFET stage cool.
   
   
6. Once the boiloff from both baths has subsided a bit, put a rubber hose exhaust line on the LHe bath exhaust port and cap both baths (see above for defrosting and capping instructions).  You may need to tighten the dewar holder straps too -- the weight of the LN causes them to stretch a bit.
   
   
7. At any time during the cooldown, you can check the 4K snout diode temperatures.  Remember the funny wiring pattern, where the two diodes are both connected to pins 7 and 8 of the fridge Cable G 9-pin D-sub connector.  They are antiparallel, so which diode you read out depends on the polarity with which you connect a diode supply to these pins.  There should be a connector lying around that adapts from DB9 to double male banana; this can be used to connect a diode supply and voltmeter to measure the voltages across the diodes.  These two diodes will lag quite a bit because of the large thermal impedance to the LHe bath, but they should reach 77K within a few hours.
   
   
8. Once the LN boiloff from both baths has subsided (a few hours), the turbo can be removed.  Make a last recording of the dewar vacuum pressure, close the black valve, shut off the ion gauge, and shut down the turbo.  Double check that the black valve is closed, then vent the pumping line, disconnect the line and the cross with the gauges, and put a KF blanking cap on the black valve port. 
   

Waiting for 77K Precool

Cooling to 4K

1. You first have to get rid of any remaining LN in the LHe bath.  This is very important!  If you try to do a LHe fill and there is any liquid in the bath, the LN will freeze and your LHe fill tube will get stuck.  Even if that weren't a problem, cooling the nitrogen to 4K is pretty much impossible, so you will just waste a lot of LHe and get nowhere.  To get rid of the LN, you can either blow out or dump:
   
   
• Blowing out: this is preferred because it is least disruptive.  Find a 3/8" metal tube that has a flat end and will reach the bottom of the bath.  Put a piece of rubber hose on the other end of the tube and set up an open-mouth dewar to accept the LN that will come out of the rubber hose.  Take the cap off the LHe bath, remove the bung and insert the 3/8" tube through the cap so that the o-ring will seal against the tube.  Insert the tube in to the bath slowly; LN will spit out the top and the tube will get cold, so use a glove.  When the tube is all the way in, back it off a very little bit (1-2 mm) and tighten the bath port o-ring seal.  Attach a nitrogen gas line to the exhaust port hose barb.  Gently pressurize with gas to blow the LN out through the tube you inserted.  Monitor and be careful not to overpressurize; you can build up a large ballast pressure in the bath without knowing it.  After all the LN has stopped coming out, continue to blow nitrogen gas through the bath until it warms measurably above 77K; this is necessary to ensure all the LN is gone.  Once this is done, defrost the blow out tube and pull it out.  Cap the bath while you get the LHe transfer tube set up.
  
  
• Dumping: If you have a lot of LN in the LHe bath, or can't find the equipment to do a blow out, you can just dump the LN by rotating the dewar.  Find a large open-mouthed dewar (the LHe bath can hold up to 16 L of liquid!).  Make sure any power or thermometry cables are either disconnected or being carefully guided to avoid damage when the dewar is rotated.  Remove both bath caps and the rubber exhaust hoses.  Check that the dewar holder straps are secure.  Pull the cotter pin in the dewar cart holder.  Raise the dewar up high enough so that it can be rotated and the dewar placed underneath.  Rotate the dewar partway (135 degrees from initial position); liquid will start coming out.  Catch it with the open-mouthed dewar.  Continue to rotate and move the LN reservoir until the dewar is vertical.  Let all the LN drain out of the LHe bath.  Remove the reservoir and rotate back to vertical.  Let the LHe bath warm for a bit to make sure all the LN is gone; you can refill the LN bath using a funnel and the LN you just collected in the meantime.  Cap the LN bath and reinstall its exhaust line.  Once the LHe bath has warmed measurably above 77K, cap the LHe bath (leave the exhaust line off) and get the LHe transfer tube set up.  You may have to blow some nitrogen gas into the bath to get it to warm up.
  
  
2. Stop the fridge monitor program, change the filename to LHe_cooldown_YYYYMMDD.txt, change the vertical scales to zoom in on the 0 K to 100 K region, and restart.
   
   
3. Bring in your LHe storage dewar, insert the transfer line and get it cooling.  I assume you know how to do a LHe transfer in general; if not, you shouldn't be doing this step!
   
   
4. While the transfer tube is cooling, uncap the LHe bath and put a short (2-4 inches) rubber exhaust line on the exhaust port barb.
   
   
5. When the transfer tube plume appears, start inserting the tube into the bath.  Go slowly, as the pressure from the escaping He gas will be high given the high temperature of the bath.  Keep the storage dewar pressure high, 2-3 psi; it has to fight the backpressure from the escaping gas.  You will sometimes see the exhaust suddenly stop; pressurize the LHe dewar a bit more to get it going again.  If this fails, pull out the transfer tube and wait until you see the plume again, then go back in.  But be careful -- if this continues to occur, you may still have LN in the bath!
   
   
6. Monitor the 4K diode using the voltmeter as above; you should see it start to cool.  All the LHe bath layer diodes should also start to cool.
   
   
7. You should already have the fridge heater supplies connected and have tested them by this point (during the installation of the e-box).  Turn on the UC pump heater to 35 mA current.  It is very important to have the UC pump heater running; the desorbed gas sits in the pump tube and conducts heat out of the array into the 4K bath.  For instructions on how to program the fridge heater supplies, see the Fridge page.
   
   
8. Maintain the storage dewar pressure high enough to ensure continued cooling of the 4K diode.  This will typically require 2-3 psi because of the high boiloff during the cooldown.  It should take less than 10 minutes for the 4K diode to have cooled to below 10K.  The diode voltage will continue to slowly decrease.  You must continue to maintain a high enough pressure to overcome the boiloff pressure.  Continue to fill until the bath is full, which is evidenced by a sudden change in the thickness and length of the plume from the exhaust port -- it looks like a jet during the fill, but changes to a thicker, whiter, more cloudlike form when the bath is full.  Stop the fill.
   
   
9. You will need to leave the bath open for a few minutes until the boiloff calms a bit.  Unless you are about to leave for the night, you can leave the short exhaust hose on.  Set the bath cap on top of the fill port so that the gas is redirected through the exhaust port.  You can then use a heat gun to defrost the cap and get is screwed on.  The seal will freeze almost immediately.  That's fine, as long as there is good outflow through the exhaust hose.
   
   
10. Turn on the bias board to power the JFETs.  If you are in a rush to get the JFETs up to 130-140K, you can also turn on the JFET heater, anything below 40-50 mA is ok.  Watch out, they'll heat up quick with 50 mA!
    
    
11. You will need to do a top-off 2-4 hours after the first fill because a good deal of LHe is used to cool the radiation shield down.  Once the 4K snout diodes have stabilized, then the remaining LHe boiloff is dominated by the UC pump heater and you will need to refill every 8-10 hours or so.  In general, you can keep the short exhaust hose on between fills if you are around to make sure it doesn't plug; otherwise, defrost the short hose and put on a long one. 
    
    You will probably not be able to do the last fill of the night and the first fill of the morning close enough together to avoid running out of LHe.  This is ok if you start the cooldown at the time advised above.  If you do the last fill of the night around 9-10 pm, then the LHe will run out before you come back in the morning, but the array will still be so warm (>70K) that it won't notice that the LHe has run out.  You can then recool.  Since it will then be daytime, you can refill as soon as the LHe has run out and avoid letting the bath warm.  The array will cool at about 3K per hour until it reaches 40K, at which point the cooling will speed up (about 6-8 hours from 40K to done).  Keep filling when the LHe runs out until the array has dropped below 10K for greater than 1 hour.  At that point, you can shut off the UC pump heater.  You must allow this extra hour; the array will not have fully cooled off if you do not, causing your first fridge cycle to fail.
    
    If you are able to make use of remote access to the fridge control program, you can monitor the cooldown and turn off the UC pump remotely when the LHe has run out; this will minimize reheating of the array.
    
    JFETs: You can minimize LHe usage by leaving the JFETs off until late in the cooldown -- just leave the bias board power off.  You can heat them up to their operating temperature in only a few hours using the JFET heater, see the schedule summary below for instructions on when to do this.  Once the JFETs have reached their nominal 135K operating point, you may still need to apply DC power to get the JFETs to keep them there.  In the 2003/10 run, we had to apply 10 mA to the JFET heater.  This may change if the internal configuration of the JFET box is changed; it's pretty easy to figure out, just do a binary search.
    
    
12. Disable the array and IC diodes on the AUX fridge board; they dissipate too much power to leave them on for a fridge cycle.  See the Fridge page for detailed instructions.
    
    
13. You can now set up a fridge cycle for whenever you like.  (Remember to change the log file name).  If you want to do it immediately, put in a 15 minute delay to allow the UC pump heater to cool back down.  It's also a good idea to check all the heater supply settings before starting the cycle.  For details on checking the supplies and starting the cycle, see the Fridge page.  Don't forget to tilt the dewar!

Cooldown Schedule Summary

• Start LN in the morning on day 1, top off before leaving summit on day 1.  Should allow 2-4 hours between end of first LN fill and top off to ensure boiloff has reached quiescent level.
  
  
• Start LHe mid-afternoon on day 2.  Go down for dinner, return to top off around 8 to 9 pm. 
  
  
• If possible, monitor temperatures remotely and shut off UC pump heater remotely if LHe runs out and bath goes above 30K.
  
  
• Recool bath on morning of day 3, let it cool all day on day 3.  Wait until array diode is < 10K for more than 1 hour before shutting off UC pump heater.  Should be cold and ready to cycle by end of day, though may require a trip back to summit in the evening to top off LHe.  Leave JFET power off, turn JFET heater on to 29 mA so they heat up overnight to 135K.
  
  
• Cycle fridge overnight between day 3 and day 4, focal plane at base on morning of day 4.  Switch the bias board on and turn off or reduce the JFET heater current to whatever is needed to maintain the JFETs in the 135K to 140K range.
  

• Start LN on day 1, same as above.
  
  
• Top off LN on day 2 and day 3, allow system to continue to cool.  Top offs can be done by observers, it's just LN.
  
  
• Start LHe fill on morning of day 4.  Top off before leaving at end of day,
  
  
• Monitor temperatures remotely during evening of day 4, shut off UC pump heater remotely before going to sleep.
  
  
• Turn UC pump heater back on on morning of day 5 remotely.  Turn JFET heater on to 50 mA at the same time.  Leave UC pump heater on until LHe runs out (don't wait for it to go to 30 K as the array diode should be pretty cold by know and letting the bath warm that much may warm the focal plane) or when array diode has been below 10K for 1 hr.
  
  
• When you arrive at the summit, you will need to either recool the LHe bath or top it off.  Also, turn the JFET heater down to 29 mA once the JFETs have reached 135K.  Switch the bias board on and turn off or reduce the JFET heater current to whatever is needed to maintain the JFETs in the 135K to 140K range.  Regardless of whether you ran out of LHe, you should be able to start the fridge cycle by mid to late morning.
  

Warming the Dewar

1. Stop the fridge monitor program.
   
   
2. Dump the cryogens: Find a large open-mouthed dewar (you may get a lot of LN, of order 10L).  Make sure any power or thermometry cables are either disconnected or being carefully guided to avoid damage when the dewar is rotated.  Remove both bath caps and the rubber exhaust hoses.  Check that the dewar holder straps are secure.  Pull the cotter pin in the dewar cart holder.  Raise the dewar up high enough so that it can be rotated and the dewar placed underneath.  Rotate the dewar partway (135 degrees from initial position); liquid will start coming out of the LN bath and a plume out of the LHe bath.  Catch it with the open-mouthed dewar.  Continue to rotate and move the LN reservoir until the dewar is vertical.  Let all the cryogens drain out.  The LHe bath will take some time since the liquid turns to gas before escaping.  Remove the reservoir and rotate back to vertical.  Reattach the rubber exhaust hoses.
   
   
3. Reenable the array and IC diodes on the AUX board so these temperatures are available during warmup.  See the Fridge page for detailed instructions.
   
   
4. In the fridge monitor program, change the filename to warmup_YYYYMMDD.txt, change the vertical scales so they go from 0K to 100K, and restart (with the Fridge is off button set).
   
   
5. Attach a metal tube to the He gas supply, run a little gas to clean the air out of the tube, and insert into the LHe bath slowly.  You will get a lot of blowoff due to the hot tube.  Once the tube is in, start the He gas flowing, gently.  Monitor the 4K diode voltage.  Initially, you will see no change; the cold plate has a lot of thermal inertia.  Increase the pressure and eventually (a couple minutes) the 4K diode will start to warm.  The bath will warm quite quickly to 20-30K, then take 10 minutes or so to get to 77K.  Just keep blowing He gas -- no other way to do it.
   
   
6. Once the bath is about 85K or so, it is safe to start blowing compressed air instead.  The fridge diodes will probably still be cold, that's ok, they'll catch up quickly.  Get your compressed air line set up, tee it into two hoses with metal blowout tubes, one for each bath, insert the tubes in the baths, and start the air going.  You want to slowly increase the pressure.  You want as high a pressure as you can stand in order to get the system to warm up quickly, but you have to be careful about pressurizing so much that you blow up your rubber hoses or blow the blowout tubes out of the baths!
   
   
7. In about 24 hours, the system should be at room temperature.  You can put a couple manifolds of nitrogen gas into the vacuum space to help it.  To do this, take the blanking cap off the black dewar valve (but of course leave the valve closed), blow lots of nitrogen gas into the open end of the valve to get rid of any air, hold the blanking cap on by hand, open and close the valve to let the nitrogen in, pull the blanking cap off again (it will now be held on by the vacuum) and repeat a couple times.  It's not clear whether this really helps or not, so don't feel obligated.
   
   
8. If the next morning the system is not quite warm enough, it's ok to vent the system with nitrogen as long as it's reasonably close to room temperature (say, about freezing).  The nitrogen will in fact help to warm the system, and it's perfectly safe as long as you don't let air in.  See below for venting instructions.

Venting the Dewar

1. Set up the gauges and turbo pump in the same way as was done for the first stage of pumping out (see above), with the regulating valve, Pirani gauge, and Bourdon gauge.  Of course, leave the black dewar valve closed!
   
   
2. Open the regulating valve all the way and pump out the vacuum line and gauge cross.
   
   
3. Once the line is down to below 100 mT, close the regulating valve and open the black dewar valve.  You can actually do this part of the process whenever you like; it is a good way to monitor the dewar pressure on warmup.  Though you should pump the line down as low as you can get it if you are going to open it to a cold dewar, just so you can actually be sensitive to the dewar pressure.  Note the pressure and dewar temperature.
   
   
4. Double check that the regulating valve is closed.  Shut down the turbo and vent the pumping line.  This should leave vacuum on the side of the regulating valve that is connected to the dewar and gauge cross.
   
   
5. Disconnect the pumping line from the regulating valve.  Replace it with a KF25-to-hose barb if you want to vent with nitrogen, otherwise just leave it open.  If you want to vent with nitrogen, get your nitrogen line, blow some at the hose barb to get rid of the little bit of air in the line behind the hose barb, and connect your nitrogen line with a slight overpressure.  Remember, if you are venting with air, all parts of the dewar (including the focal plane) must be above the local dew point or you will condense water everywhere.
   
   
6. Open the regulating valve 1/8 turn and monitor the pressure using the Pirani gauge.  You are aiming for 1 Torr/sec rise; adjust the valve to get this.  Once you hit 100 T, use the Bourdon gauge (1 Torr/sec = 1 inch/30 sec). 
   
   
7. If venting with nitrogen, overpressurize the dewar just a little bit (1 psi).  If you are planning to store the dewar as-is, you can just leave it with this overpressure.  Or leave it like that if it is still a bit cold and needs to warm up (though watch out for Boyle's law!  Don't do this when the dewar is too cold or the dewar will be severely overpressurized when it reaches room temperature).  If you are planning to open up, close the regulating valve, remove the hose barb, then open the regulating valve 1/8-1/4 turn and let the overpressure bleed off.
   
   
8. Close the regulating valve, close the black dewar valve, and disconnect the regulating valve and gauge cross.
   
   
9. If you are planning to open up the dewar, slowly open the black dewar valve; if somehow you have made a mistake and there is significant under- or overpressure, you want to catch it before there is much gas flow.  Once the dewar is equilibrated with ambient, it is safe to open.  Otherwise, just cap the black valve and leave it until you want to open it (in which case you have to do the same equilibration) or pump it out again.
   

Leak Checking

• 2004/01/14 SG
  First version
  
  
• 2004/01/29 SG
  Add links to get back to main pages.
  
  
• 2004/04/24 SG
  Update filenames for fridge monitoring.
  
  
• 2004/04/28 SG
  Update instructions for cooldown to allow for long LN precool, quick heating of JFETs.
  
  
• 2005/06/03 SG
  Make various links to ExpertManual to more specific locations, minor error correction.