MORE THOUGHTS ON THE SHORT CYCLES
written by Giles on August 2 2003
after the cycle of 7/28, we know that the problem is not incomplete condensation but rather it is
that the Helium-3 is boiling away too rapidly. The cycle of 7/23 gave 2 day hold time and
slightly elevated temperatures. The cycle of 7/28 gave 1.5 day hold time and quite significantly
elevated temperatures. (The cycle of 7/21 gave 1 day hold time but we did not measure the
temperature) I give a set of four plots that show how the signal has been getting lower as the
cycles have been getting shorter. These are from Huabai. They are consistent with a higher
detector stage temperature which results in a lower bolometer responsivity. The higher detector
stage temperature is produced by some additional heat load on the detectors which also produces a
faster boiloff rate and a shorter Helium-3 hold time.
The plots show the signal from RCW 108, in sparomap units, but corrected for the effects of tau:
THOUGHTS ON THE SHORT CYCLES OF JUNE/JULY 2003
written by Giles on July 25, 2003
I agree that the battery problem is troublesome, but Huabai and I have
been working on the cryostat problem. The short cycles started in June,
so this problem is independent of the battery problem that I assume is
Huabai and I have been working a lot on this. In particular, I have been
studying the cycles for Feb 8, June 6, and July 21, while Huabai is
working on studying signal vs. time, and also on plotting up the other
This is just a quick update on some developments. But before I go into
detail, please note that the information that I asked for in a previous
e-mail (attached at end) is still needed for a full understanding of the
problem. item # 1 (at end) is *especially* important.
so here is the update....
(A) a possible theory to explain short cycles:
I have a theory to explain why you got a 1 day cycle on July 21, and also
this theory can explain why the cycles have lasted only 2 days since
at least as far back as July 2.
I think you need to have have a longer time delay between when you stop
heating and when you close the heat switch. This time delay is necessary
to allow the He-3 to fully condense. Lets call this time delay the "final
condensing period". If the final condensing period is too short, you will
suck too much He-3 gas directly back onto the charcoal pump and then you
don't get enough liquid in the He-3 pot.
On feb 8 the final condensing period was 2.7 hours, and we got good hold
time (3 days + a little). On July 21 the final condensing period was 0.8
hours and we got 1 day hold time. 1 hour is too short. You were able to
succeed with such a short final condensing period earlier in the season
because the pumped pot cooling power was higher then, but as the ambient
pressure drops at South Pole, the cooling power of the pumped pot gets
less and less. Finally, on July 21, it was too low to even get 2 days
He-3 hold time, so we got 1 day He-3 hold time.
(This drop in the pumped pot's cooling power is expected, as the ambient
pressure drives the flow. but also I see direct evidence of this in the
cryofiles: the temperature of the pumped pot when pumped is dropping
significantly from Feb 8 to June 6 to July 21. A dropping temperature in
the pumped pot corresponds to a lower flow through the pumped pot system
and thus lower cooling power.)
once i get the rest of the information from you that i requested -
especially item 1, then i can further test this theory and some others
that I have.
(B) Huabai finds that the signal strength was only 15% lower on 7/21 than
in the earlier parts of the season (after he corrects for the effects of
tau). If there was a heat load that was causing our short hold time (e.g.
a gas leak) then the loss of signal should be much greater: more like a
factor of 1.5-2.0 loss of signal. But work on this issue is still
ongoing. we still have not calibrated the signal vs. temperature but we
will next do this using the pointing data of Feb-March.
Finally here is a repeat of that list of things we need from you next:
(1) please leave the cryocontroller running when the Helium-3 runs out in
SPARO. i want to watch the duration of the elevated temperature steps.
This will tell me if problem is in detector or guard stage. Once detector
stage stabilizes at around 1.5 K or higher, you can end the logger, and
e-mail the cryofile to me. Then start the logger again.
(2) tell me the formula that you are using to get from Volts to
milliKelvin. i want to convert all of the detector temperatures in your
excel log file into Volts, for uniformity.
(3) continue to work backwards with your excel logs...going back as far as
you can. I would like to get some normal 3-day cycles in your logs.
there are none yet.