I am very embarrassed to even ask this question, but after asking it many times, including recently in a ZWO camera Facebook group, it seems I continue to find conflicting answers. I have to think those posting here must know with certainty.
So my very simple question: What exactly does it mean, when we set our cameras to -10C or -20C? One would think the answer would be clear!!! Most people say it means just that, -10C actual temp. They in turn justify that answer by then saying that is why you can use darks taken at the temp when processing with lights taken at that same setting. OK fine!! But I also notice that when ZWO lists it's camera specs, they use the words "below ambient". That clearly to me is very different. And does not mean -10C is a specific absolute temperature reading that means the same thing every night . So which is it? Thanks for any help with this very simple question Al
What does -10C REALLY mean?????
Hello! I'm far from an expert, but I believe this is what it means:
The "below ambient" in the advertising indicates the maximum potential cooling capability of the cooling system. If the sensor starts out at +28C, the cooler could take it down as far as -7C.
The "cooling temperature" we set in our app (ASIAir etc.) is the actual temperature we want the sensor to be. So, in the app, -10C is -10C.
So, using the above example, if the sensor starts at +28C, the system could take it to -5C (33 below ambient) but not -10C (38 below ambient) .
Hope this helps!
Gary
Byrdsfan1948 So which is it?
Al, the camera cooling setting (e.g., -10ºC) refers to target (absolute) temperature of the sensor chip.
The "below ambient" temperature (e.g., 35ºC) is simply how much the Peltier cooler can maximally cool the chip below the current ambient temperature. I.e., it is how much you can cool the chip below the ambient temperature when you apply maximum (100%) current to the Peltier chip.
Chen
TXGaryW Thanks Gary. Yes this was what I have been supposing as I tried to reconcile this question myself, but was uncertain, and wanted to confirm. ZWO shows my camera's ( ASI183MC ) cooling ability delta as 45 below ambient. Of course I am assuming that |45|, means 45C. So if my ambient temp at the time I'm imaging is 68F/20C and I want to cool my sensor to -10C/14F I am well within the 45C capability of Peliter cooler to do that. 30C/45C. If the ambient temp were the 28C/82F you site and for some reason I wanted to cool my sensor to -20C/-4F. I have slightly exceeded the stated ability of the cooler to do that. 48C/45C.......... I think I have, it thanks Al
w7ay Thanks Chen, see my reply to TXGaryW above. BTW I was on another ZWO Facebook group site this week end and got involve with a discussion with the poster regarding a log graph of sensor temp and dark current. As I was commenting you came to my mind...... Long story as to why, but I enjoyed the discussion :-)) Al
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Byrdsfan1948 log graph of sensor temp and dark current
Logarithm based 2 is only useful (and only sometimes) for computer nerds. Science and engineering uses logarithm based 10 (at least for us who used slide rules -- have you ever seen a slide rule that is based on powers of 2? :-)
That graph smacks of redneck engineering.
Chen
Byrdsfan1948 ZWO shows my camera's ( ASI183MC ) cooling ability delta as 45 below ambient. Of course I am assuming that |45|, means 45C.
Yes, but 45℃ Delta T was tested at 30℃ ambient temperature. As the ambient temperature drop, delta T will shrink.
When you set the temperature to -10℃， it's sensor temperature.
Temperature (theta) in units of 'degree Celsius' is not a physical quantity, but absolute temperature (T) in 'K' (= Kelvin) is. The values corresponding to these units differ only in the zero point. Therefore temperature differences are identical, whether generated from two temperatures in units of 'degree Celsius' or from two temperatures in K.
Temperature difference values should not be specified in units of 'degree Celsius', they should be specified in 'K' (= 'Kelvin') in order to distinguish the result from a temperature value measured in degree Celsius.
Example:
t1 (ambient temperature) = 18 degree Celsius
t2 (sensor temperature) = -5 degree Celsius
t1 - t2 = 23 K
If this simple convention was met, no misunderstandings would arise at all regarding temperatures and temperature differences.
Bernd
@Tech@ZWO#58875 Thanks for your response, however to be sure I understand you let me try to put your response into my words. I believe you are saying that the listed 45C degree, below ambient temperature, cooling ability ( using my camera, the ASI183MC as the example ) is the result of testing done at a 30C/86F ambient temperature. Testing results done at that ambient temperature, show that my camera's cooler should be able to cool the sensor down to as far as -15C/5F. Therefore if I set the sensor cooling to be -10C/14F, I am well with the ability of the cooler to reach and hold that temperature. Therefore in this case, -10C/14F Is in fact the actual temperature of my sensor!!
HOWEVER, as the ambient temperature drops, that 45C degree delta T which was measured at 30C drops as well.
Of course that makes sense, in that I am sure there must be some actual absolute limit to the cooler's ability to cool further. If I understand this correctly my original question is answered. If I do not please you, or anyone else here let me know!!
As an old engineer who loves details, and hates ambiguity, I'm just wondering do you have any data that shows how the cooling Delta diminishes as a function of ambient temperature? Thanks Al
bulrichl Sorry but I'm not 100% sure I'm getting your point. I assume you are referring to my comment that ZWO shows the cooling delta for my camera as "45 below ambient." Since the 45 they show is only an magnitude, and does not describe what units of measure it represents I used the term |45| to show that as simply an absolute number. That being the case I assumed those units of measure to be Celsius to completely define that value.
I of course agree that a delta of 45 degrees is the same delta in both degrees C as well as K...... Perhaps you are suggesting that if there were some sort of agreement in particular within the scientific community to standardize on the Kelvin scale we would then all understand that values like "45 below ambient" is in degrees Kelvin and therefore no question as to C or F or any other unit?? Al
Byrdsfan1948 degrees Kelvin and therefore no question as to C or F or any other unit??
Al, for what its worth, I have never seen air (ambient) temperature referred to in Kelvins instead of Centigrade :-). Can you imagine the weatherman saying "today's high will be at 297º Kelvin!"
But you are the Mechanical Engineer who knows thermodynamics, and I am just an EE. I had skipped the required thermodynamics class by substituting with a nuclear engineering class instead.
By the way Al, your alma mater beat out Stanford again in this year's ranking of U.S. graduate engineering schools. 1 and 2, as usual, yeehaw.
To my surprise, my undergrad school Purdue has popped up all the way to number 4 now, tied with CMU. Bezerkeley came in as No. 3. Purdue had in the past struggled to get into the top ten, even though they had the country's first official Computer Science department at the time when everyone else had lumped that field in their Math department.
Chen
Byrdsfan1948 Yes, you are correct.
My point is: a value specified in 'degree Celsius' always is a certain temperature, with freezing point of water as the reference. Therefore temperature differences must not be specified in 'degree Celsius'. However, temperature differences are not numbers without unit, the unit needs to be specified. For temperature differences, the correct unit is 'K'. 'Degree Celsius' is wrong, numbers without unit are wrong as well.
Cited from ZWO's specification of the ASI183MC Pro (color):
Delta T: 40°C-45°C below ambient
This should be changed to:
Delta T: 40 - 45 K below ambient
Bernd
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bulrichl Therefore temperature differences must not be specified in 'degree Celsius'.
You might also want to also request a change in the Wikipedia page on the Seebeck coefficient where they referred to ∆T in ºC:
https://en.wikipedia.org/wiki/Seebeck_coefficient
While the Seebeck coefficient is indeed specified in Kelvins, it is quite common in the engineering world to refer to temperature difference in ºC (as the above Wikipedia page did).
The common nomenclature of "∆T = n ºC" simply means that you change the temperature on one surface of the Peltier cooler at x ºC to (x-n) ºC on the second surface.
You factor out the º (both x and n become unitless numbers) and then apply º to the result to get units of temperature again. Being more pedantic would not help anyone to better understand the problem at hand.
Saying "xºC - nºC" would be wrong. But no one is saying that.
While you are at it, you might get the manufacturers of Peltier coolers to change their specs too, starting with this one (that specifies ∆T in ºC):
https://lairdthermal.com/products/thermoelectric-cooler-modules
(Hint: all of them do.)
Chen
The fact remains that a value specified in 'degree Celsius' always is a certain temperature, with freezing point of water as the reference.
Citing papers using the wrong unit '°C' for temperature differences does not prove correctness. I could cite other papers that use the correct unit, but I leave well alone.
Have a nice day,
Bernd