Temperature measurement

[bigben2k]

Following discussions on the heat die here, as well as other bench requirements, here is where we discuss temperature measurements.



A recent related question came up (here) about the type T versus type K (last page).

[bigben2k]

pHaestus: there's an outstanding question to you, about type T versus type K thermal probes. Do you want to answer it?

[pHaestus]

http://www.microlink.co.uk/microlink/tctable.html

I assume 0-200C is of relevance here. It's also easy to make type Ts with very thin wires if that's your thing (see JoeC). I like my dual linear thermistors more than thermocouples, but the Type Ts are handy for inserting into wb baseplates and die simulators.

[bigben2k]

[edit: rambling removed]


The temperature measurements is a really critical component of a test bench, so I'd like to tackle that question, and get to some kind of idea as to what level of accuracy we're all going to shoot for here. Everything else falls back on this.

[pHaestus]

Well the key thing is to have some way to periodically check your probes and calibrate them. This means that you need a single really accurate temperature monitoring device for cross calibration, a device specifically designed to standardize/recalibrate, or you need to budget for getting them professionally calibrated periodically. I honestly don't have a ton of experience with thermocouples but have been told before that linearity is a concern. It isn't like a thermistor where there's usually just an offset when they are calibrated, right? They work through Seeback effect and the equation is much more complicated.

On a budget, I suspect that something like a Fluke 2190 (get the multiplexer and you can have LOTS of probes) and type Ts is about as good as you can do.

For price/performance on temp monitoring, do not forget that being able to dump the readings to a PC is worth some money. I have analog temp readers now and it is a hassle.

[pHaestus]

Oh and as far as temperature measurement discussion goes:

You will need to measure (at a minimum):

water inlet
water outlet
waterblock baseplate
die

I also measure ambient air and radiator outlet temperature (just because I have the capacity to do so).

What accuracy is truly needed? I hear words like analytical vs comparative batted around in e-mail exchanges. Are you distinguishing the two based purely upon the accuracy of temperature measurement?

[Since87]

Originally posted by pHaestus
You will need to measure (at a minimum):

water inlet
water outlet
waterblock baseplate
die

Although I can see the desireability of knowing all these temperatures and more, I'm not sure I see the "need" to measure more than the die and the water inlet temperature.

Water outlet temperature can tell you the amount of heat the waterblock is dissipating, but assuming a 'perfect' die simulator, I'm not sure it is a necessity. (Obviously this test should be done to measure the quality of the die simulator.)

If it turns out that the die simulator 'leaks' too much heat, it may be desirable to make the delta T between inlet and outlet water the measure of heat transfer. This requires extremely good accuracy and resolution of the temp sensors though. (Not saying that extremely good accuracy is not needed anyway. Just that doing this would make the need greater.) It also requires extremely accurate flowrate measurement.

Alternatively, the water outlet temperature could be ignored (during actual waterblock testing) and 'leakage' of the die simulator characterized. This would require an ambient measurement though. (Since 'leakage' will vary with the deltaT between ambient and die temp.)

I don't see the value of, or a way to practically implement, measurement of the WB baseplate temperature. Few waterblocks of major interest these days are going to allow for placement of a temp sensor in a useful location.

[pHaestus]

Since87:

You are right; the baseplate and water outlet temps are mostly for the tester's internal checking and peace of mind and they aren't carried forward in any calculations. But here is how I am using them:

I find the baseplate temp useful as I compare to Bill's previous data (I am using a block that he also tested so the hole is in an identical spot). With my CPU I just don't know the power applied and so I am using the delta t die to water, delta t die to baseplate, and delta t water out to in all together to see if I can things to correlate. I guess this isn't explicitly needed when everyone is using die simulators, but then again if every unit is insulated differently...I also use the baseplate temp to track my reproducibility at waterblock mounting/paste application. YMMV I guess on the usefulness of it.

I look at the water inlet and outlet as important from a heat balance standpoint. If your die simulator is "leaking" then you can pick that up with these measurements pretty easily. If you don't do a check like that then you are going to have some pretty widely varying C/W values among different benches even with good thermal monitoring. This is probably why heatsink testing from commercial sources reports such widely varying C/W values.

[Since87]

Originally posted by pHaestus

I find the baseplate temp useful as I compare to Bill's previous data (I am using a block that he also tested so the hole is in an identical spot). With my CPU I just don't know the power applied and so I am using the delta t die to water, delta t die to baseplate, and delta t water out to in all together to see if I can things to correlate...

Yes, that makes sense. I think one thing many people who are thinking about testing water blocks don't consider, is that you need more than the equipment which composes the test bench. You also need additional equipment to verify the operation of the testbench.

[bigben2k]

Ok, to recap:

-Need to measure:
1-the heat die temp
2-the block water inlet temp

-need a highly accurate probe, for periodic calibration.

-Alternate temp measurements:
1-block outlet (highly accurate, to measure the heat flux)
2-ambient air
3-water block baseplate (where available)
4-radiator outlet (water)


[edit: rambling removed]

[jaydee]

Originally posted by bigben2k



Off to Google for Fluke 2190... Thanks for the tip!

Do they still make these? Anyway I was going to buy one of these http://www.tequipment.net/FlukeThermo54.html The 54-2 series with data logging and dual probes. but it is only dual probes and it costs $300.

If you look here at the price page: http://www.tequipment.net/FlukeThermo54MO.html
and scroll down you will notice the probes are not exactly cheap either.

Probably not the lest expensive place to get this stuff though.

[bigben2k]

I was just looking at the same one.

The 50 series can return a value within +/- 0.3 deg C, regardless of the thermocouple used. The T type probe though, would have an absolute error of +/- 0.5 deg C, where a type K would be +/- 1.5 deg C.

[edit: rambling removed]

The different models have various extra features, like logging, dual input (and differential output), etc...

[jaydee]

Originally posted by bigben2k
I was just looking at the same one.

The 50 series can return a value within +/- 0.3 deg C, regardless of the thermocouple used. The T type probe though, would have an absolute error of +/- 0.5 deg C, where a type K would be +/- 1.5 deg C.

Couldn't we just calibrate a type K? Could we make/use our own thermocouples?

The different models have various extra features, like logging, dual input (and differential output), etc...

Yeah, my second plan was to buy one of the cheaper ($150ish) Fluke 50-2 series and just use it to calibrate the sound card mod thing OR I was going to get one of those maxim kits pH is using.

My head is killing me today. Damn cold front and rain! Bah, back to work....

[Since87]

Originally posted by bigben2k
Also, I'm going to bring up the coolant testing temp again: you suggested 30 deg C, because you're having difficulty maintaining 25 deg C. My concern with that, is that a higher delta T (block to ambient) may increase those secondary losses. (am I off my rocker here?)

Ben, if you can't answer this question yourself, what hope is there, that you are going to be able to cover all the bases in setting up an accurate test bench?

Originally posted by bigben2k
If we're stuck with Type T probes, then what's our margin of error? +/- 0.5 deg C?

Which is of course, absolutely useless when trying to measure the heat dissipation of the block by measuring the delta T between inlet and outlet water temperature.

Originally posted by bigben2k
Since I'll be using a heat die, if I want to measure those secondary losses, then I am stuck with measuring a block's coolant inlet and outlet temps, with high accuracy.

And if you don't measure the die simulator's secondary losses, then all of your measurements will be suspect.

[Since87]

Temp measurement link.

[bigben2k]

[edit: rambling removed]

[pHaestus]

since87: GREAT link. Informative +1. The comments about sources of error are most useful for tc users who are buying parts off ebay.

[myv65]

Originally posted by bigben2k
Exactly.

I already have a rough # for that secondary loss: roughly 2%. It's clear to me that it's significant enough to warrant a measurement of it. My question was: to what extent is a higher delta T, block to ambient, is going to skew the results?

On the other hand, who cares, if we're going to measure that secondary loss.

But +/- 0.1 deg C on each probe, isn't going to pick up that difference.

Ben,

Slow down and think a little before you post. I realize that rambling is one way to work things through, but I generally keep it to myself rather than putting it to paper, so to speak.

To your original question, yes, increasing the ambient will increase the amount of heat leaving via a secondary pathway. That should be self-evident as the delta-T from the die to air drives secondary losses. Assuming the air remains constant and you raise your fluid ambient, more heat will transfer to air. Unfortunately, it is not so simple as assuming a constant convection coefficient to the ambient air as you are off in the realm of "natural" or "free" convection where a significant portion of the air velocity is a function of the density change as it warms. You can pot-shot it and take copious measurements to back up your guestimate.

A rather fundamental thing is that the repeatability of pretty much any online temperature sensor will be worse than the delta-T of fluid through the block. This has been said by several here, but it isn't apparent that the meaning has truly sunk in yet.

As for measuring secondary losses, one way is to install a well-insulated block with no flow. Then you modulate power to the die simulator and measure temperature of the die. You get a feel for how much power is dissipated vs die temperature when (almost no) power is going into the block. The obvious goal is to insulate the simulator such that you can scarcely turn the power down low enough to avoid a high die temperature.

[bigben2k]

[edit: rambling removed]

[RoboTech]

Hey Ben,

IMHO thermocouples (whatever type) are good for applications that don't require high accuracy. Even with a quality TC and calibrated instrument you won't be able to do much better than about +/-0.5 deg C. TCs big advantages are they are inexpensive and widely used throughout industry (available in all shapes and sizes). If you need better precision and accuracy then 4-wire platinum RTDs are the next step up. They are more expensive but in a best case situation can provide +/- 0.01 deg C accuracy.

So I recommend looking at RTDs for the die temp and water inlet. Don't even bother with TCs. Even with good RTDs installed on the inlet and outlet of the block, I doubt we will be able to measure the Delta T across the block accurately enough enough to define the actual heatflow. To define Q based on mass flow rate and Delta T would require even more accurate (0.001?) temp readings and very accurate (0.1?) flow rates.

In regards to calculating the thermal die secondary losses - my plan is to...

1) insulate the block quite well (thermal stand-offs, air gaps and ceramic insulation) to help minimize secondary losses as much as possible.

2) Measure the secondary losses by firing up the die and applying just enough power to bring it up to a typical steady-state operating temperature (i.e. 40 C) at a known and controlled amb air temp (with the die surface insulated). Once the die equillabrates at temp, virtually all of the power going in (E x I) is going out via secondary heat paths (since there is no WB sucking heat out the top). Since I'm not far enough along to have actually done this I don't know how well it will work.

Well, so much for lunch...

[bigben2k]

Going over RTDs now. That Omega site has some info.

I should also point out that Bill used an RTD in his original heat die.

[pHaestus]

You realize you are doing this in exactly the wrong manner, right? I would suggest:

1) Come up with goals for the testing
2) Run some numbers with different levels of accuracy in the temperature and flow rate and power measurement.
3) Price out gear that will get you where you want to be error-wise. This will let you meet your goals with the minimum outlay of cash.

I get the impression that your goal in #1 is "to get the exact correct answer", and that's going to result in a $ value for 3 that's near infinity.

I also get the impression that there is a bit of snake oil being sold in this whole WBTA thing. Quality testing just can't be done without a big investment in both time and money.

An allegory for the interested: I recently made some arsenic samples of known chemical composition in my lab. I started with a 1000mg/L standard that I diluted to 0.1 mg/L using an analytical balance accurate to 0.1mg and with density of water adjusted using room temp. I sent these to 4 professional laboratories in my region and asked for As analysis. The point? (1) none of the labs reported 0.1 mg/L (but one was "close enough" with 0.12ppm) (2)%RSDs were 1-2% max (3) there was a range from 0.02 to 0.4mg/l for the concentrations.

So what happened there? Each lab used probably a $100,000 instrument with a professionally trained technician. Each lab had good reproducibility of measurement, and none of the labs were correct.

Rather than spend thousands of posts on talking about the minor details, I would suggest starting with error and uncertainty and how to propagate error forward. That would be my first standard if I were a WBTA; all points must have error bars. Equipment needed would all follow naturally from that standard.

[bigben2k]

[edit: rambling removed]

[bigben2k]

[edit: rambling removed]

[jaydee]

I don't know....

What I am not sure about is why we need to make such an analytical approch to this. Do we really have to know what the block does through out various flow rates? Do we really need to know the Base plate temp? Do we really need to try and measure inlet/outlet temps?

I just want to tell of block A is better than block B on my test bench with in a resonable amount of error. I was planing to sort blocks by groups. Group A would fit into high performance, Group B in middle performace, Group C low performance, and Group D not worth the metal used to make it.

Do I really need .01 accuracy to do this? Seems to me this is all the readers want. I don't think they want to hear all the flow rates, pressure drops, blah blah blah... They just want to know what level the block is in. If it is worth their money or not and how wrong they might go if they buy something else. Remember most consumers don't know jack about what they are buying, they just want to be told how good it is without a lot of technical mumbo jumbo they not only can't understand but do not want to understand, and quite frankly shouldn't have to understand. They got better things to do as they should.

I don't know.....