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Originally posted by pHaestus
You need to start by making the following statement be set in stone:
"All temperature readings reported are under steady-state conditions"
This is a requirement!
Now, how can one arrive at steady-state? Over the length of the data collection then water, air, and die temperatures cannot change appreciably. Not so hard for one point on a C/W vs GPM curve; can get to be a big pita for 5-6 over several hours.
You have the cart before the horse though. The first step is to set a water temperature. Everyone will need to use the same one, you realize (right?). Assuming your die simuator is perfectly insulated (heh) then I don't think room temperature directly affects the results. The difference between room and water temperature though could have an effect on the observed delta T. Can the equipment being used pick this up? Not sure.
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Well put.
I have to agree that the die temp measurement really needs to be steady, at least within the error margin of the temp measurement. (i.e. it can't fluctuate)
I also agree that the water temp should be maintained at 25.0 deg C, as Bill has been doing. It's a fair representation of an average cooling solution (some people go as low as ~1 deg C (delta T between water and air), some people use a BIM with a small fan
), as 5 degrees above an ambient of 20 deg C.
I think that in general, anyone should be able to reach temp measurements within +/- 0.1 deg C, so let's use that assumption, for now (it's not a given, I just need a basis for discussion).
So now I'm down to (where I should be) how we measure the performance. At flow rate X (still needs to be defined), we're going to report a C/W. W is going to come from the measurement at the heat die and C is the temp delta (aka delta T) between the die and the water.
So obviously, we don't want fluctuations in the water temp either. So I'm back to my earlier statement:
Quote:
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Given that the heat die temp probe will return a result of say 35.7 deg C ... all with an error margin of +/- 0.1, then the coolant temp has to remain between 24.95 and 25.05, on average.
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Regardless, we want a steady water temp, at least within 0.1 deg C (the accuracy of the water temp probe).
And if we have +/- 0.1 deg C at the water and at the die, the resulting delta T is going to have an error of +/- 0.2 deg C.
An example: over a result of 0.20 C/W (not typical!), and given a 70 Watt source, the C/W is accurate to +/- 0.003 C/W. Then we add the error from the heat die power measurement, say 2%, and the accuracy is at +/- 0.007. Then there are other errors that will need to be factored in (all of this is theoretical!).
The thing is, I have no problem running a log of the temperature readings, but I don't know what sampling rate to use. The cooling solution is going to be a source for fluctuations, and that's what's really going to define that sampling rate, no?
I really like your idea of a PID merged with a PWM, for a rad fan combo. That's the cooling solution that I, personally, would like to focus on.
JayDee116: do you have any specs on that chiller? We're looking for "expected fluctuations", both in temp range, and over a period of time.