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:
Quote:
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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 |
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. |
from 20C to 60C?
How do we accurately do that? I mean other than send it out to a calibrator :p your talking about the Callendar-Van Dusen equation? http://images.dr3vil.com/upload2/thertd_15.gif 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 |
[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. |
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 |
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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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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