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Testing and Benchmarking Discuss, design, and debate ways to evaluate the performace of he goods out there.

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Unread 10-17-2005, 10:30 AM   #1
Ice Czar
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Default DIY Triple Point Calibration +

Tackling The Triple Point Shawn Carlson Scientfic American Jan 99
(build your own triple point cell, a sustainable calibration point of 0.01 C)

Calibrating With Cold Shawn Carlson Scientfic American Dec 2000
(based on the freezing point of mercury calibration point is –34.8 degrees C
also includes a proceedure to compensate for a boiling point calibration of 100C)

Homemade High Precision Thermometer Shawn Carlson Scientfic American Mar 99
(its a DIY 4 wire RTD)

Quote:
One of the horrible truths of scientific research is that simple and inexpensive techniques will get you just so far. Beyond some point, increasing accuracy can be obtained only with a disproportional rise in expense, sweat and frustration. That's partly because accurate measurements require an extremely well calibrated instrument, and providing such an exact scale can be a vexing challenge.
Quote:
Triple point of water

The single combination of pressure and temperature at which water, ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (ca. 6 millibars). At that point, it is possible to change all of the substance to ice, water, or steam by making infinitesimally small changes in pressure and temperature. (Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.)

Water has an unusual and complex phase diagram, although this does not affect general comments about the triple point. At high temperatures, increasing pressure results in first liquid, and then solid water (above around 109 Pa a crystalline form of ice which is denser than water forms). At lower temperatures the liquid state ceases to appear with compression causing the state to pass directly from gas to solid. It is, however, possible to melt ice by increasing pressure under specific conditions.

At a constant pressure higher than the triple point, heating ice necessarily passes from ice to liquid then to steam. In pressures below the triple point, such as in outer space where the pressure is low, liquid water cannot exist: Ice skips the liquid stage and becomes steam on heating, in a process known as sublimation.
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Last edited by Ice Czar; 10-17-2005 at 10:47 AM.
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Unread 10-17-2005, 12:45 PM   #2
BillA
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and . . . . . ?
RTDs are NOT linear in the range of interest to us
the inst will have one curve, the sensor(s) another(s)
you need to cal (or have caled) your RTD+inst in 5°C increments from 20 to 60° for each sensor, plot and derive a curve; then use that equation to correct your readings from that particular sensor
- an indicated temp in hundredths will seldom (aka never) be the actual temp

N.B. never use a differential thermometer as the temp lag depends on the flow rate
only if the temps are dead flat (seldom) can simultaneous readings be taken
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Unread 10-17-2005, 03:21 PM   #3
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Well, the article may be of marginal use for calibration over a wide temperature range, but I have to admit, it was an interesting method of getting to the triple-point of water.

I'll have to renew my subscription in SA.

Maybe.
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Unread 10-18-2005, 02:44 AM   #4
Ice Czar
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from 20C to 60C?
How do we accurately do that?
I mean other than send it out to a calibrator

your talking about the Callendar-Van Dusen equation?



for a second there I thought Id be able to affordably workup a curve with a -34.8 \ 0.01 \29.7 \100C 4 point

(29.7 being the melting point of Gallium)
http://www.isotech.co.uk/ga-guide.html
http://www.seabird.com/technical_ref...MPAccuracy.htm
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Last edited by Ice Czar; 10-18-2005 at 02:59 AM.
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Unread 10-18-2005, 03:32 AM   #5
Incoherent
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[quote=Ice Czar]for a second there I thought Id be able to affordably workup a curve with a -34.8 \ 0.01 \29.7 \100C 4 point

(29.7 being the melting point of Gallium)
[quote]


Good info Ice Czar. Why do you think you can't? Is gallium expensive? I hadn't heard of this, the third point between 0 and 100° is a problem for me. I have been using a medical thermometer but would prefer an absolute.
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Unread 10-18-2005, 10:07 AM   #6
BillA
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is a third point between 2 going to define the curve ? (sub-zero does not help RTDs, thermistors may be better ?)
sure, but with not so much confidence as if there were 6 points
the cal is done with a reference system having presumably a 10 fold greater resolution/accuracy (= somewhat expensive)

some typical RTD inst and probe cal data (to less than the optimum resolution !!)

true . . . .inst . . . . probe
temp . . .corr . . . . corr
20.00 . . 0.00 . . . . -0.10
25.00 . . 0.00 . . . . -0.12
30.00 . . 0.00 . . . . -0.14
35.00 . . -0.01 . . . . -0.16
40.00 . . -0.01 . . . . -0.17
45.00 . . -0.02 . . . . -0.18
50.00 . . -0.03 . . . . -0.20
55.00 . . -0.03 . . . . -0.22
60.00 . . -0.04 . . . . -0.24

as they were done separately, both corrections must be added to arrive at the 'actual' temp (ignoring uncertainty, etc.)
- a better procedure is to cal the inst and probe together to greater precision, now std practice w/me

I am suggesting that the assumption of linearity is such until demonstrated as fact, particularly in view of the resolution being described

Incoherent
I recognize, and admire, your doing all this yourself, but I question the ability of others to replicate your work - not stuff for a tyro
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Unread 10-18-2005, 10:13 AM   #7
Marci
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Thought ALL temp probes (loose generalisation there but u know what I mean) were non-linear in their response to temps?? Back when I was performing phasechange mods we had a nightmare of a time trying to find a suitable replacement temp probe for a MachII due to the fact that the original probes response was most definitely not linear... or is it only certain types of probes?
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Unread 10-18-2005, 10:30 AM   #8
BillA
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really depends on the probe AND the actual range of intrest, but under the scope few are so good
quartz ref RTDs are supposed to be special, but even so . . . . no correction ?; doubt it
I have limited experience with thermistors apart from the Digitech meters
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