Re: Digital Camera as Sky Meter -- Ongoing Report

From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 09/13/04 

Date: Mon, 13 Sep 2004 05:42:20 GMT

On 13 Sep 2004 03:55:39 GMT, bllfs6@aol.com (BllFs6) wrote:

>This sounds like a bunch of baloney to me....but its late and a hurricane is
>coming and I am fighting computer problems and Ive had too many beers so who
>knows......
>
>BUTTTTT...iffff you have ice with ANY significant amount of liquid water in
>it.....its for ALLL practical purposes at 0 degrees Celius.....
>
>I worked on a project once where we used PURE distilled water/crushed ice as an
>calibration/reference point......and it so SOOO critical that the ACTUAL depth
>to a fraction of inch of the thermistor in that ice/water slush was a
>consideration.....

The trick is to make a good slurry, with the ice volume and the water volume
properly balanced. That can give you something pretty close to 0C, although I
wouldn't say it could necessarily be used as a good reference. I was taking
exception to using an ice chip as a reference. It is very easy to cool something
down to well below 0C with a piece of ice alone. If you want to be close to 0C,
you need to make sure it is the water that is your reference, not the ice, and
that requires a carefully designed cold finger for the ice.

In fact, with small Peltier coolers available for around US$15, that would be my
choice for cooling a sensor inexpensively. And since it isn't clear that dark
current noise is even an issue in this application, it might make sense to
actually heat the sensor resistively, and regulate its temperature that way.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

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