AVR-Chat

Hi all,

I need to measure tempretures around 36C with the best acuracy I can 
get, I would like to measure with a precision of 0.1C so I guess I need 
acuracy or ~0.05C.

I am looking at IR based units, but I can not see why a Type K 
thermocouple can not be used and just calibrated Span is roughly 33-46C, 
so linearisation is not such a big issue for such a small span I THINK, 
correct me if I am wrong.

Any ideas ? will Type K get me there ? or do I need to do the IR probe ?

Cost will be a factor as we are looking at 1-100K units (I love vague 
customers)

Regards,

Kat.
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Hello Kathy,

The easy answer for me is : NTC in a half-bridge. I have used calibrated 
10k@25°C bead types from RS components to within 0.5°C, over a range 
10-40°C, and this without additional linearisation beyond the half-bridge.

RS databook characteristics support measurement to within 0.2°C  - i.e. 
this accuracy after calibrating with one then inserting another. I've 
also checked this - it's about right. I calibrated my circuits (with 
<50ppm/°C high stab resistors) with two NTCs in a water bath, then 
applied them to 32 NTCs fresh out of the packaging.

I am sure that your precision specification would be obtainable from 
individual sensor calibration (now, this may be a challenge...)

John Stiekema


Kathy Quinlan wrote:

> Hi all,
> 
> I need to measure tempretures around 36C with the best acuracy I can 
> get, I would like to measure with a precision of 0.1C so I guess I need 
> acuracy or ~0.05C.
> 
> I am looking at IR based units, but I can not see why a Type K 
> thermocouple can not be used and just calibrated Span is roughly 33-46C, 
> so linearisation is not such a big issue for such a small span I THINK, 
> correct me if I am wrong.
> 
> Any ideas ? will Type K get me there ? or do I need to do the IR probe ?
> 
> Cost will be a factor as we are looking at 1-100K units (I love vague 
> customers)
> 
> Regards,
> 
> Kat.

--- In AVR-Chat@yahoogroups.com, Kathy Quinlan <kat-yahoo@k...> wrote:
> Hi all,
> 
> I need to measure tempretures around 36C with the best acuracy I 
can 
> get, I would like to measure with a precision of 0.1C so I guess I 
need 
> acuracy or ~0.05C.
> 

How fast do you need to take the measurements?  In other words, is
the environment changing rapidly.  Many thermocouples have a very
long time constant.  Their thermal mass is still pretty large. For
quick measurements, I use bead thermistors and a small amplifier 
circuit.  You have to watch for self-heating and the resistance 
(gauge + length) of the wire you use to attach the thermistor. But
you can get them with time constants to 0.5 seconds.  With a good
circuit you can get to a hundredth of a degree C.

- ayman

----- Original Message -----

From: "Kathy Quinlan" <kat-yahoo@kaqelectronics.dyndns.org>
To: <AVR-Chat@yahoogroups.com>
Sent: Wednesday, November 10, 2004 12:22
Subject: [AVR-Chat] Tempreture Messurement


>
> Hi all,
>
> I need to measure tempretures around 36C with the best
acuracy I can
> get, I would like to measure with a precision of 0.1C so I
guess I need
> acuracy or ~0.05C.
>
> I am looking at IR based units, but I can not see why a
Type K
> thermocouple can not be used and just calibrated Span is
roughly 33-46C,
> so linearisation is not such a big issue for such a small
span I THINK,
> correct me if I am wrong.
>
> Any ideas ? will Type K get me there ? or do I need to do
the IR probe ?
>
> Cost will be a factor as we are looking at 1-100K units (I
love vague
> customers)
>
> Regards,
>
> Kat.

I found this which might be helpful.  It is basically the
design of a minature weather station (college project) where
the authors go into great detail about various temp sensors.

http://eelabinstruments.com/insmain.pdf

I'd try the forward biassed diode or an IC sensor..

erikc - firewevr@airmail.net
==
On 2004-11-02, the sheeple bleated and chose thier shepherd.
What sheep don't seem to realise is that the shepherd's only
interest in them is when they can be fleeced and when they
can
be slaughtered.