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Subject: Re: Temperature Measurement Stability - +/-3.5 microdegrees
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Date: Fri, 15 Nov 2002 23:55:15 -0000
NNTP-Posting-Date: Fri, 15 Nov 2002 23:55:02 GMT
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I have to ask, what on earth requires that level of accuracy.
I am an engineer, not a scientist.
"Bill Sloman" wrote in message
> "Raymond E. Rogers" wrote in message
> > Thanks Bill, comments embedded
> > Bill Sloman wrote:
> > > For a more comprehensive discussion of temperature stabilisation near
> > > temperature, see A W Sloman, Paul Buggs, James Molloy and Douglas
> Stewart "A
> > > microcontroller-based driver to stabilize the temperature of an
> > > stage to within 1mK inthe range 0-38C, using a Peltier heat pump and a
> > > thermistor sensor" published in Measurement Science and Technology
> > > 7,. pages 1653-64 (1996), which is not only useful in its own right
> > > say so myself) but cites about a dozen earlier papers with useful
> > >
> > > The +/-0.2C Betatherm thermistors we used are claimed to drift less
> > > 11mK per year at 75C. Yellow Springs Instruments offers some +/-0.05C
> > > tolerance parts that ought to be even more stable, and are available
> > > shelf from Newark - they have 16 of the 10k at 25C part 46006 (stock
> > > 46F3197) for $47.60 each.
> > Yes we bought some from YSI that in fact have data to back up the
> > stability claims; and they are costing about $100. Okay for research
> > but probably to much for production.
> I checked out that NIST website, and it does look as if they are talking
> about the YSI 46006 - I don't know of anythihg else that is offered at
> +/-0.005C. The +/-0.05C parts from Newark are expensive. The Betatherm
> that we used are quite a bit cheaper, at about $5. Thermometrics does some
> +/-0.1C parts for similar sorts of prices, and in my current job we are
> actually using Fenwall thermistors - but a very small glass encapsulated
> part with good stability, but a +/-20% resistance tolerance, which we
> calibrate ourselves (and wish we didn't have to).
> > > A couple of the papers we cited achieved much better than millidegree
> > > stability. Larsen's classic paper describes a 50 microdegree stability
> > > bath built around a 500R platinum resistance sensor, and Priel got
> > > +/-3.5 microdegrees with a Fenwall thermistor sensor. Preil complained
> > > dissipating more than 16uW in his sensor produced appreciable long
> > > drift, but probably within the +/0.03C range asked for by the original
> > > poster.
> > >
> > > Interchangeable thermistors from reputable suppliers should be pretty
> > > stable. Their great advantage is their high sensitivity - about -4%
> > I will make a calibration fixture. We can spend money on capital
> > equipment but the shipped instrument must be cheap (relatively
> > speaking). I gave up on interchangability.
> Our calibration bath has been a pain in the fundament - it has been back
> the manufacturere a couple of times, and they don't turn it around fast.
> > > degree C, compared with +0.39% for Pt100 and Pt1000 sensors. This not
> > > makes the job of the measuring electronics much easier, but lets you
> > > away with cheaper and more easily accessible current-setting resistors
> > > so forth. A DIN B/3 Pt100 is a +/-0.04% resistor, and using anything
> > > than $10 Vishay 0.01% resistor throws away most of the accuracy you've
> > > for. The YSI +/-0.05C thermistors are +/-0.2% resistors ...
> > >
> > > The National Semiconductor LM35 semiconductor temperature sensor is a
> > > noiser than a good thermistor or a platinum resistance sensor, but
> > > easier to use. My office mate at Nijmegen University used the
> > > part for some millidegree stability temperature calibration baths and
> > I found the national data sheets .1 - .2 degC/khr , not good enough.
> That is what they are prepared to guarantee, not what they achieve. The
> is good because it was designed right, and it is made in a high quality
> analog fab. Lke all the National stuff it is produced in large volume at a
> low price, and that sort of production can't support the sort of testing
> that YSI does on their thermistors, and charges for.
> The risk always is that someone like National will find a cheaper way of
> meeting the published spec, and you stop getting the non-guaranteed
> performance that you have been relying on. You could have a second,
> design more or less tooled up against such eventualities, but nobody I've
> worked for has ever gone to that sort of trouble.
> The nearest analogy is the display tubes on the electron microscopes sold
> Cambridge Instruments in the 1980s - they were perfectly ordinary TV
> bought out of the huge batches made for the consumer market. The problem
> that the comsumer TV's were always being restyled for a slightly different
> look, and the tubes were never exactly the same shape from batch to batch.
> So our display housings were always big and sloppy, with plenty of room to
> accomodate an oddly shaped tube. The re-engineering costs were peanuts
> compared with what we saved on the display tubes.
> > > 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
> > > drifts that turned out to be humidity driven. Water vapour was getting
> > > through the plastic packaging. I'm sure that the problem has been
> > > now, at least at the level of performance guaranteed on the data
> > I have yet to find the Dallas drift specs. I will try to look up your
> > paper and follow the references.
> > Maybe I can use your paper as an authoritative reference instead of the
> >$100 books.
> No way. I'm quite proud of that paper but it reports a very specific piece
> of work, and is much more oriented to short term stability than long term.
> Some of the papers it refers to are more interested in long term
> you are going to have to do a bit of reading to assemble the ammunition
> seem to want and need.
> If you have any trouble getting a copy of my paper, e-mail me directly at
> firstname.lastname@example.org. At the moment the alias points to Melbourne,
> and it is something like 48 hours before I'll be responding to e-mails at
> the Australian address, but after that I should be able to be helpful.
> Bill Sloman, Nijmegen
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