sensor calibration
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you read a temp, is it 'real' ?
what is the purpose of calibration ? - many certs state "in spec", but this info while welcome is not useful what is needed is a calibration curve indicating the sensor/instrument's deviation the first graph is an air cal of 2 RTDs and 2 TCs, the second a liquid cal of 3 RTDs; note that the yellow RTD is the same in both (this was the delay Joe) |
Re: sensor calibration
This begs the question; how does one run a calibration, which we've been over several times.
Then we have 3, 4, 5 and 6 point calibrations, and how it's applied. You have a Quartz reference, most DIY'ers will not. As a DIY tester, I'll have access to the following 'real' temps: -freezing (0.01 deg C) -boiling -a mercury thermometer, ranged from 19 deg C to 27 deg C, in 0.1 deg C increments (NIST traceable, but I do not hold the cert, I assume good to 0.1 deg C, until it is abused). (may be able to get various freezing/boiling of other substances) I believe I'll be able to run a limited 3 point cal, but over a range that is far wider than interesting/relevant. Or I could run a 3+ point cal using only the mercury thermometer, over its (narrow and interested) range, but I highly suspect it'll be useless (too low res). |
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(edit double post)
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Re: sensor calibration
when you have it caled ask for the points you want, say 5°C increments from 20 to 45
use The Meter Shop on Director's Row w/o a good cal you are lost of the many tools I've bought, no single one has had the impact on accuracy as the $900 spent on that thermometer (w/2 probes, all in cal) EDIT you after a sensor cal procedure ? |
Re: sensor calibration
Sounds good.
I'm still pondering wether or not I should calibrate it myself, but I do plan, at least, to check temperature readings on a regular basis (and log all the results). Since I've opted to build the 24 bit ADC, there is going to be a calibration procedure involved. I'm thinking more and more about having a starting point, i.e. one of those Fluke units, which I'd send out for calibration. Not sure what you mean by "The Meter Shop on Director's Row" ? With the wind tunnel, I've got more temp probes to keep up with than I care to count! |
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if you are building your own, you will need an in-house ref (not that mercury thing)
0.01 ref res = 0.1 cal capability The Meter Shop is on Director's Row (street) in Houston (610 & 290) |
Re: sensor calibration
Nice, thanks for the info.
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Another nice one!
"0.07 deg F" accuracy; do you believe that to be possible? Am asking because I don't know. Would guess it would be achievable because of the EM shielding?!? |
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multiple sensors, sure
that relates to the sensors only, see how they are read ? I would note that I can cal my setup a lot closer, but its ~6 hours of work + the necessary instruments |
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I wouldnt choose water boiling point as a reference, well i'm used to live not at sea level... Here water boils sensibly lower than the reference 100°C, and even at sea level it depends on atmospheric conditions. Or you make it boil in a pressure regulated chamber :P
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about only you, pH, and TM could make me respond, most burnt out with the clue level here
neither, a bath or environmental chamber depending on water or air cal the key is an HP 2804A w/dual sensors, all in cal of course (0.001°C resolution, but sloooow) I cal in 5°C increments from 15 to 45, 70 if a heat die RTD; then use that curve in a spreadsheet I'm in Houston so the Inst Shop cal (freezing, midpoint, boiling) is ok, just not sufficient for my desired accuracy note that the cal cert does specify the barometric pressure, so it can be corrected for (got to admit I've yet to see anyone actually do so, self included) |
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I have not investigated this well, however... I have seen (in scientific american) an article on producing an accurate temperature via triple point conditions. Doable by a hobbyist too. Let me plagarize below. BTW, I hope that with this, I can rouse your interest as well, Bill.
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actually i've seen it, in the Portuguese Institute of Quality (they "supply" the standard cal).
I got the tour. Anyway, it's done in several tubes (3 if i recall, per container) in a aluminium pressure chamber. If that's "hobbyist" friendly, then hell, i really have to move to the US. :) http://www.iop.org/EJ/S/0/M32185/pTF...2/9/me7209.pdf Hmm. Brians : this? Sounds simple. But these things never are. PS: had my laughts when they used a king size enviormental chamber to test a small pressure device for tyres. The thing is as big as a living room, yet the device is the size of a bread box. |
Re: sensor calibration
This copied from Scientific American. This is a great magazine, guys, well worth reading and supporting with real money.
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Now that solves an issue of a single point, if that is accurate enough. For multiple point calibration, you need to either use something that is known to be linear temperature through the interesting range (is a mercury thermometer linear enough?), or use other temperature set points like the melting point of mercury and watch the calibrated probe as the mercury does the phase change. I think that mercury is less sensitive to atmospheric pressure than water, although, you could put it into a sealed cell and try to measure the temp through the glass walls. Anyways, hope this is interesting. |
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As for the size of the env chamber... real accuracy is expensive and space consuming. Edit: btw, ordered a CD from amazon of those Amateur Scientists projects. Will see how good it is. |
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Also, found the prior thread that talked about this. It is by Ice Czar here, and I see Bill's objection to the issue. RTDs are NOT linear through the range we are interested in, so this calibration may be insufficient without other physical references.
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Re: sensor calibration
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Honestly i dont really like SA and PM (Scientific American and Popular Mechanic) because of the holes on the info they provide. It's like that Dilbert comment "Build your own helicopter from common household items, on PM!."
The first problem is the cell itself. I admit that small variations in wall thickness might not be a big issue, but when you increase the required accuracy, they could be. Just saying. So having some guy from your local glass shop make one ... well. I'd go to a medical supply, or even scientific supply store and get one. I'd venture that it's not that expensive. Another point is that glue thing. regular glue doesnt attach to glass very well, and neither for a long time. And using (as it stated) sand, which creates a very diffuse contact point, with glue, doesn't seem very stable, specially through temperature differences. Third point is very simple. Any welding or soldering on sensor is something to avoid. Any weld made will change the ohmic value of the sensor, per wire. Add that to the ohmic value of the wire, which you should know, and size of each, which sould remain equal, and you get a mess to calibrate. but hey, that's why labs exist :) . PS1: for that particular range, you really have to use a platinum resistance thermometer (PRT). PS2: for the ones that haven't figured it out what a "cell" is supposed to be: (10m, paint! -> best app ever) |
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there is a guy selling these things pretty regularly over the last several years on eBay (search Schmermund)
for different temps different matls are used the thermometer is only $350, but the cells . . . whew it still gets back to the PRTD's non-linearity over our range of interest no way to avoid creating a cal curve that I know of, at 1/3 DIN they are pretty much the same - but still have a curve of course, on the censored forum the resident wizard has concluded that 0.1°C resolution is sufficient for WCing component testing, we do love to pick the pepper |
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lazy again?
http://cgi.ebay.com/ICE-POINT-CELL-S...QQcmdZViewItem http://cgi.ebay.com/SCHMERMUND-DUAL-...QQcmdZViewItem 750 for a bong? :) Eh. Anyway, they (cells) look decent. |
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there is an ad running now during the Olympics showing a patient receiving what the Dr. had removed in an operation
his 'lazybone' take mine away and I'd be jello no TM, the pride is in doing - not being shown I'm not sure that guy has ever sold one, rather thin market I suspect |
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thin indeed. but i suspect he had it done for "pennies".
i checked his feedback, a few sold (that i could see): http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7578300064 example. |
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There's a limit to the number of reference points you can create until you start using another measurement device like boiling point of water at X mm Hg and then try to accurately measure air pressure. Quote:
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industry (where I live) generally use ASTM or similar type equipment and procedures; good enough to produce repeatable results over time and between facilities - the 'acceptability' of data is quite predicated on others being able to replicate it individual companies will often develop in-house procedures for greater precision and accuracy; in Intel these are called BKMs (Best Known Method) and the writers of such are held in very high esteem, and the docs are Secret - one such that made it into the 'public domain' is the TC to IHS soldering procedure in the stepped groove (but I would note that a TC is still used) in the P4 docs being who I am I attempted to use an RTD instead bit could not source a 4-wire to fit into the groove, lol I have led many here down a merry path with my personal pursuit of ever greater 'accuracy' on a budget of pennies; but this is the consequence of what I was doing - product design, where it was essential to be able to distinguish the very small differences resulting from very small changes now all this is wonderful for comparative product testing also, but the minutiae so exposed are generally invisible to a typical user yes, I started the WCing 'spec wars'; but I did also elevate the level of performance in available products returning to the original question "What do the labs use, then?"; ideally the SAME equipment used by the company for whom the data is being generated; why Intel provides TTVs, eh ? OR equipment and procedures SO GOOD that a room full of double PhDs will record the data points and murmur asset (what I like, lol) - this was done recently (by some friends) with a vacuum bell on the die backside to eliminate the heat flux question, but the temp was still taken with a (calibrated) TC refer back to the cal curves in the initial post, these can only be done with a certain type of setup - and are only necessary for those after greater accuracy than a thermometer with a cal sticker can provide - the greater the resolution, the greater the possible differentiation (do note the difference in both the cal and instrument 'quality', but as I caled them they are all good - understanding that the indicated temp is NOT the 'real' temp) this is procooling, zeros are our friends |
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