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From: "Bill Sloman"
Subject: Re: Temperature Measurement Stability
Date: Fri, 15 Nov 2002 00:59:57 +0100
Organization: Planet Internet
References: <3DD2EC20.2FB10158@maine.rr.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 14 Nov 2002 23:59:56 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Tony Williams" wrote in message
> In article ,
> Spehro Pefhany wrote:
> > On Wed, 13 Nov 2002 19:19:44 -0500, the renowned "Raymond E. Rogers"
> > wrote:
> > >Platinum RTD
> > Yes. Particularly if they won't be exposed to temperatures outside
> > the normal ambient range, otherwise they may show some small
> > hysteresis effects.
> I believe that Bill Sloman might know something
> about sourcing precision Platinum RTDs, about
> 10x better than DIN41617.
RS Components stock DIN B/3 parts, which is to say +/-0.1C
Hart Scientitic will sell you a calibrated Pt100 four wire sensor assembly
good to +/-0.02C for a hundred dollars or so, and the electronics to read it
to that accuracy for about $1500. We use a Thurlby-Thandar 1906 computing
multimeter as a four-wire ohmeter, and convert the resistances to
temperature by solving the quadratic in Excel - not quite as good, but much
> > What's wrong with an accurate mercury-in-glass thermometer? Not
> > expensive, and it sounds like you have a very controlled situation, so
> > the long response time and immersion depth requirement shouldn't
> > bother you.
We keep on breaking ours ....
> This is the problem.... you need some temperature
> reference that is traceable. We used to buy certified
> mercury-in-glass thermometers, narrow-range, only about
> +/- a few C around the specified temperature.
For a more comprehensive discussion of temperature stabilisation near room
temperature, see A W Sloman, Paul Buggs, James Molloy and Douglas Stewart "A
microcontroller-based driver to stabilize the temperature of an optical
stage to within 1mK inthe range 0-38C, using a Peltier heat pump and a
thermistor sensor" published in Measurement Science and Technology volume
7,. pages 1653-64 (1996), which is not only useful in its own right (if I
say so myself) but cites about a dozen earlier papers with useful content.
The +/-0.2C Betatherm thermistors we used are claimed to drift less than
11mK per year at 75C. Yellow Springs Instruments offers some +/-0.05C
tolerance parts that ought to be even more stable, and are available off the
shelf from Newark - they have 16 of the 10k at 25C part 46006 (stock no
46F3197) for $47.60 each.
A couple of the papers we cited achieved much better than millidegree
stability. Larsen's classic paper describes a 50 microdegree stability water
bath built around a 500R platinum resistance sensor, and Priel got down to
+/-3.5 microdegrees with a Fenwall thermistor sensor. Preil complained that
dissipating more than 16uW in his sensor produced appreciable long term
drift, but probably within the +/0.03C range asked for by the original
Interchangeable thermistors from reputable suppliers should be pretty
stable. Their great advantage is their high sensitivity - about -4% per
degree C, compared with +0.39% for Pt100 and Pt1000 sensors. This not only
makes the job of the measuring electronics much easier, but lets you get
away with cheaper and more easily accessible current-setting resistors and
so forth. A DIN B/3 Pt100 is a +/-0.04% resistor, and using anything less
than $10 Vishay 0.01% resistor throws away most of the accuracy you've paid
for. The YSI +/-0.05C thermistors are +/-0.2% resistors ...
The National Semiconductor LM35 semiconductor temperature sensor is a bit
noiser than a good thermistor or a platinum resistance sensor, but even
easier to use. My office mate at Nijmegen University used the metal-packaged
part for some millidegree stability temperature calibration baths and was
very happy with their long term stability. The Dallas parts may be suspect -
we had a posting here a couple of years ago about some nasty temperature
drifts that turned out to be humidity driven. Water vapour was getting
through the plastic packaging. I'm sure that the problem has been solved by
now, at least at the level of performance guaranteed on the data sheet.
Bill Sloman, Nijmegen
Bill Sloman, Nijmegen
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