RE: The Joys of Cooling

[Tom Droege <tdroege2@earthlink.net>]

The two cameras on ROB make it to about 52 and 48 below water 
temperature.  I have not worried about which one is downstream.   But this 
was with 35 C cooling water.  The bad news is that TECs work better (are 
more efficient) at higher temperatures.  With 20 C water I might get to 
only -20 C with these cameras.  The good news is that this lets you pick 
and operating temperature with a good chance you can hold it through summer 
and winter.

Note, on purpose I designed the system so it would not blow up if you did 
not turn on the cooling water.

Tom Droege



At 03:52 PM 8/8/02 -0400, you wrote:
>I actually made this same cooling mistake the other day.  The symptom I saw
>however had the CCD cooling by about 20 degrees C and then rising about 10
>degrees (I'm doing this from memory).  After I turned on the coolant pump
>the "water" temperature rose swiftly by about 10 degrees C, the CCD temps
>started dropping to their target of about 35 degrees C below ambient and
>after a couple of minutes the water temp too started to fall by about 5
>degrees C.  This tells me that at least in my system the water temp is
>downstream of at least one of the TECs.
>
>I have no problem cooling to 35 degrees below the measured water temp.  I
>haven't tried to go lower so I don't know what the limit is but you should
>have been able to make it at least -15 degrees.
>
>
>Thanks,
>
>Mike G.
>
>
>-----Original Message-----
>From: Stupendous Man [mailto:richmond@a188-l009.rit.edu]
>Sent: Thursday, August 08, 2002 3:21 PM
>To: tass@listserv.wwa.com
>Subject: The Joys of Cooling
>
>
>   I've been experimenting with the Rochester Mark IV.  Last night,
>I ran a test that gave some strange results.  I have some ideas,
>but hope that others can give me advice.
>
>   The Mark IV has two CCD cameras.  Each one has a TEC inside to
>cool the CCD chip.  Tubing runs through the camera heads; one can
>circulate a liquid through the tubing to carry heat away from the
>TECs.
>
>   Last night, by mistake, I forgot to turn on the pump which forces
>water through the tubing.  I ran the cameras with the TEC units turned
>on to an intermediate voltage ("3", according to Tom's control
>program).  The ambient air temperature was around 15-20 C, and
>the cameras cooled to about -5 degrees.
>
>   Then I realized that the pump wasn't on.  So, I turned on the
>pump, and watched as:
>
>         - the V camera dipped by about 1 degree C for five minutes,
>                 then returned to its previous temperature
>
>         - the I camera dipped by about 3 degrees C for maybe 20 minutes,
>                 the gradually returned to its previous temperature
>
>   Huh?  Now, it's true that the fluid was probably warmer than the air,
>since it had been heated all day by the Sun (it sits in a garbage can
>outside).  But I expected some advantage with the fluid.
>
>   Later that night, I tried to drop the temperature of the fluid.
>The garbage can contained about 15 gallons of water and antifreeze.
>I placed 14 pounds of ice into the water, while the fluid was
>circulating, and watched the CCD camera temperatures.  Over
>the next 20 minutes or so, the CCD temperatures gradually dropped
>by about 1 degree C (both of them), and then they both rose
>back to their previous levels.  The ice hadn't fully melted
>(or even close to it) when the CCD temperatures were back to their
>pre-ice levels.
>
>   Hmmmm.
>
>   My guess is that the thermal contact between the block through
>which the water flows, and the finger running to the CCDs (or the
>finger-CCD connection), isn't as strong as it ought to be.
>If I'd been smart, I would have monitored the "Water Temperature"
>reported by the Mark IV, which I _think_ is the temperature of the
>block through which the coolant flows.  All I know is that
>the coolant
>
>       - started the night at 23 degrees C,
>       - was at 21 degrees C about 90 minutes after adding ice
>       - was at 19 degrees C about 130 minutes after adding ice,
>                  when I gave up and went home
>
>   It may be that the flow of coolant is too small to cool the
>block and finger effectively.  Because I'm using lots and lots
>of narrow tubing, only a trickle moves through the block --
>I measured it last January at something like 4-7 gallons per
>hour.
>
>   Tom, can you tell me exactly where the thermistors for the
>"Water Temperature" and "CCD Temperature" are connected?
>
>   Oh, one more thing: the tracking is fine when the telescope
>points to the East or South.  Once it moves about 45 minutes
>West of South, it starts to stick, so that 60-second exposures
>show short trails, often with "dots" (meaning that the telescope
>tracked well for a while, then stuck for a moment, then tracked
>well again afterwards).  Perhaps this is due to dirt on the
>portion of the threaded rod which touches the drive mechanism
>when the telescope points West.  I plan to clean the rod, and
>_maybe_ to lubricate it, before the next night's run.
>
>
>                                          Michael Richmond