Some comments
Quote:
Assuming a calorimeter type sensor placement, what is the necessity to use copper for the heat die ? (i.e. does the matl's conductivity matter so much ?)
while Ben's suggestion to use iron seems radical, why not use a more durable material ?
|
I think the only criterion for the material conductivity is that it is known. Steel has many grades and different k's which is why I didn't use it. If I could be sure of it I would have no hesitation using steel. Silicon carbide is another good option, machining aside. We use a variant at work with a very low CTE which has a k of 170, which I have been looking at. Not in my price range though, especially if machining is factored into the equation. If we could set up an accurate tester of thermal conductivity it would be rather useful. Secondary losses in this test are even more critical than the heat die.
There are other more trivial issues. Copper is so far ahead of all other suitable materials in terms of conductivity that I have been a little concerned about how much heat is generated over these long heat paths. Using heater cartridges mitigates this concern somewhat (versus the metal film power resistors I use). A positive aspect of this is that with the higher temperature gradients comes a lower dependence on measurement accuracy. Water temperature measurement resolution and accuracy remains of critical importance if heat to water needs to be measured.
Guard heaters are a direction I like, possibly an easier way around the secondary losses than proper insulation (vacuum).
Sensor holes and position. Depend of course on sensor size. Heat shadowing and other similar effects are problematic as is the fact that the waterblock itself will effect the temperature gradients in the die. One suggestion, one I will act on in future, with centred holes drill all the way though. The temperature cross section will be symmetrical, even though the heat shadowing is worse the compensations are much easier.
BTW Bill, the phenolic arrived, thanks. I will not be able to do anything with it for a while, work and move dominating life right now.