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Message-ID: <3DD44A63.BBD5B13@maine.rr.com>
Date: Thu, 14 Nov 2002 20:14:11 -0500
From: "Raymond E. Rogers"
Reply-To: rrogers@plaidheron.com
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Newsgroups: sci.electronics.design,sci.engr.control
Subject: Re: Temperature Measurement Stability
References: <3DD2EC20.2FB10158@maine.rr.com> <3DD39EFC.8DE0895B@iinet.net.au>
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Thanks, comments follow inline.
dooley wrote:
>
> The stability requirements are certainly achievable with RTD's. I don't
> believe it is achievable with thermisters or semiconductors.
>
The NIST paper is at:
http://www.cstl.nist.gov/div836/836.05/papers/Wise92glass-disc_thermistors.pdf
> Ensuring performance is a different matter alltogether. Annual recalibration
> is typical but elements and instruments can change in that time. RTD's are
> more fragile than one might think so a serious bump can change the
> characteristics. To achieve the performance you are after and to do it
> consistently I would recommend an in-house calibration check facility as
> close as possible to the temperature of interest. A gallium melt cell is
> ideal. It has a temperature of 29.7646 deg C. You also need a bath in which
> to operate it although there are other less expensive ways. I think there
> are check systems specifically for body temperature that may be suitable for
> your use. Some of the contacts below may be able to advise you about that.
We did buy a $4000 NIST traceable thermistor thermometer to do
calibration. I gave up on interchangability; to expensive.
>
> I achieve an uncertainty of 0.01 deg C with an Isotech TTI5 meter and 1/10
> DIN RTD's. The TTI5 has two channels. I use an Isotech industrial standard
> PRTD with certified calibration with the TTI5 for calibration of the other
> RTD's. I have a stirred bath, a water triple point cell, a gallium melt cell
> and a precision low temperature coefficient resistor. I use the resistor
> before and after every series of measurements to check the instrument and
> periodically check each system combination using the WTP and gallium melt
> poimt cells. The largest deviation from calibration that I have seen so far
> using the cells is 0.006 deg C. The method using the cells has a certified
> uncertainty of 0.0005deg C.
>
> I would suggest that you talk to your nearest representative of either HART
I will have to rethink the RTD's. The common ones seem to drift about
.05-.1 degC/yr though. That means I am back to higher priced items.
Bummer.
> or Isotech. For thermister information I would recommend you contact YSI.
>
> Here are some links that might be helpful:
>
> http://www.isotech.co.uk/
> http://www.hartscientific.com/
> http://www.ysi.com/index.html
> http://www.its-90.com/
We obtained our present parts from YSI; pricey. I hope to bring the
price down a lot in production.
I would still like some authoritative reference to emphasize the
importance of doing this right up front.
Thanks again
Ray
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