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Date: Thu, 14 Nov 2002 21:02:52 +0800
X-Mailer: Mozilla 4.5 [en]C-CCK-MCD snapN45b1 (Win98; I)
Subject: Re: Temperature Measurement Stability
The stability requirements are certainly achievable with RTD's. I don't
believe it is achievable with thermisters or semiconductors.
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.
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
or Isotech. For thermister information I would recommend you contact YSI.
Here are some links that might be helpful:
"Raymond E. Rogers" wrote:
> I have a manager problem and would like a good Temperature Measurement
> reference to beat somebody over the head. We need to build an instrument
> that has .15 degC stability across instruments and over time; this is at a
> single fixed temperature 37 degC. I am trying for the temperature
> measuring part to be .03 degC (5:1) stable. I am trying to convince
> somebody that we need to recalibrate annually or biannually; or be
> convinced different. I would like a reference that has explicit
> authoritative statements and hopefully data, concerning the long term
> stability of:
> Platinum RTD
> Glass Encapsulated "Super" thermistors
> Semiconductor temperature sensors, like those used in Dallas
> Semiconductor units.
> The last is to be able to make statements about an old instrument that is
> supposed to be 1 degC stable.
> I looked up several books, boy are they expensive.
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