Pumping Out, Cooling, Warming, and Venting
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- General Advice
- Pumping Out
- Cooling the Dewar
- Cooling to 77K
- Waiting for 77K Precool
- Cooling to 4K
- Cooldown Schedule Summary
- Warming the Dewar
- Venting the Dewar
- Leak Checking
- Revision History
Back to ExpertManual
- 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.
You should have the dewar closed up and ready to pump out at this
point. You should note in the logbook the times and pressures
when you start pumping, switch out the regulating valve, take the turbo
off standby, and leave for overnight. These can be compared to
previous pumpouts if things appear to be going slowly.
- 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.
- 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!).
- 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 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.
- 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).
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.
Connect the open end of the pumping line to the turbo. Make
sure any bleeder valves on the turbo are closed.
Open the regulating valve 1/4 of a turn.
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
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.
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.
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.
- 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
- 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
- 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.
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
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.
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
You will leave the turbo connected to the dewar during the
cooldown until the dewar has stabilized at 77K.
Cooling the Dewar
Presumably you have already pumped out the dewar. You should have
also attached the electronics box and gotten the fridge monitoring
program running on andante,
including testing that you can see all
the thermometers. Instructions are provided elsewhere: e-box installation and cabling; fridge thermometry. Don't forget
to do your warm JFET checks before starting the cooldown (if you are
planning to do them at all); instructions are provided elsewhere.
The most important temperatures to log by hand are the Array Diode, IC
Diode, and 120K Diode. The cooldown of these stages limits the
speed of the whole process, so it's important to keep track of them and
note whether they are cooling anomalously slowly.
Cooling to 77K
You should have the day crew bring a LN storage dewar to the 3rd floor
and modify the plumbing so you can attach a flexible rubber hose
(3/8-1/2" bore) to the dewar. You will find a variety of
stainless steel 1/4" LN fill tubes that can be attached to rubber hose
in the 3rd floor electronics lab or on the cryogen deck by the AOS
lab. Find one that goes most of the way into the LHe bath (it's
ok if it bottoms out). Be sure not to drop your candidate tube
all the way in when trying it! If the tube is too short, the LN
won't reach the bottom of the LHe bath and it will never cool.
Also, make sure the tube doesn't completely seal the bore of the fill
port (it is 3/8" diameter) as this can be dangerous. Attach the
LN exhaust rubber hose to the barb on the LN bath port. Leave the
exhaust hose off the LHe bath. When you've got your plumbing all
set, go ahead:
- 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
- Note the starting time, dewar ion gauge pressure, and array, IC,
and JFET temperatures.
- 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.
- 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.
- 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
- 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.
- 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
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.
- 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
It takes about 24-48 hours for the array to cool from room temperature
to a temperature sufficiently low to start the LHe fill. This
depends strongly on the initial temperature due to the high heat
capacity at room temperature. A 10K difference because the summit
is cold can cut the cooldown time by 6-12 hours. Typically, we
can start LHe about 24 hours after the LN fill, but as late as 36 hours
is not atypical. You will probably need to top off the LN bath
before leaving at the end of the day or first thing the next
morning. If the LHe bath has equilibrated at 77K and there is
still liquid, don't add any more; there is very little heat load on the
LHe bath right now. But, of course, if you've run out, put enough
in to get it cold again.
It has been determined empirically that the optimal switchover point,
based on wanting the minimum cooldown time, is when the array diode
reaches 140K; this is where the rates for cooling with LN or LHe in the
LHe bath are equal, with the LHe winning when the array gets below
140K. If you want to minimize LHe usage and have time to kill,
you should wait for the array to reach 100K or so. In particular,
if the day crew is cooling Bolocam, there is no reason that one can't
start well ahead of time (LN is cheap!) and get the focal plane well
below 100K before starting LHe. This makes the LHe cooldown
faster and cheaper. See the cooldown schedule summary below for details.
Cooling to 4K
Once the array is cold enough, begin the 4K cooldown. You will
need about 150 L to get to base and do one fridge cycle. When to
start depends on what temperature the array is at. If you start
at 140K in the morning, the fridge will be ready to be cycled the
morning of the day after the next (~48 hrs). If you start at a
much lower temperature (110K-120K), you should delay the start of the
fill to mid-afternoon.
- 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:
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
- 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.
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!
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.
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!
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
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
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.
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.
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!
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
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.
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.
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
Optimized for speed, requires high maintenance and LHe usage.
Assumes cooldown is being done by a crew staying at HP who can return
to the summit for evening top-offs.
Minimal LHe usage, optimized to work with standard day crew hours:
- 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.
The fridge cycle program will be modified in the near future to cycle
the UC pump heater based on the 4K plate diode and array diode
temperatures so that remote shutoff of the UC pump will not be
- 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
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
You are presumably done with the run and ready to warm up. Make
sure you have ample LHe gas, a compressed air line, a hose-barb tee or
equivalent, and metal tubes and hoses for putting gas into both baths.
- Stop the fridge monitor program.
- 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.
both bath caps and the rubber exhaust hoses. Check that the dewar
holder straps are secure. Pull the cotter pin in the dewar cart
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
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.
- Reenable the array and IC diodes on the AUX board so these
temperatures are available during warmup. See the Fridge
page for detailed instructions.
- 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
- 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.
- 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
- 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.
- 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
Presumably the dewar is warm or almost warm. It needs to be above
the dew point if you are going to vent with air, only up to freezing or
so if you are going to vent with nitrogen.
- 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!
- Open the regulating valve all the way and pump out the vacuum
line and gauge cross.
- 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.
- 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
- 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.
- 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).
- 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.
- Close the regulating valve, close the black dewar valve, and
disconnect the regulating valve and gauge cross.
- 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.
A horrible experience, indeed. I am no expert on the CSO's
leak checker. I will only note that, when we leak check, we
usually tee the leak checker into the line running from the back of the
turbo to the turbo's roughing pump; this way, the leak checker doesn't
have to fight against the turbo. Other than that, the day crew is
really your best resource.
Leaks in the o-rings are usually the result of mistreatment, poor
cleaning and greasing, or an accidental speck of metal or aluminized
mylar on the seal.
The hermetic connectors develop leaks due to the mate/demate
stress. These can be sealed up with Kurt Lesker leak sealant,
there is usually some in the chemicals cabinet in the AOS lab.
But you have to hit it with a
heat gun to get it to cure. It never cures if one doesn't heat
Window leaks? Haven't ever had any as far as we can tell.
There are old/spare Bolocam windows at the CSO that can be used in a
- 2004/01/14 SG
- 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
Contact the Bolocam support person.