Thermistor calibration
Archiver
Posts: 46,084
Greetings all,
I am trying to measure a thermistor (10k @25C) with a BS2 or BS2sx.
A thermistor curve can be very closely approximated through use
of the Steinhart-Hart equation:
1/T=A+BlnR+C(lnR)^3
T= Degrees Kelvin
R= Thermistor Resistance
A,B,C = Curve-fitting constants
For the range I want to measure (-10C to +50C) the constants are:
A= 0.0010260068
B= 0.0002396354
C= 0.0000001548
I couldn't find a way to calculate this equation with those constants
and keep the accuracy at +-.05C.
If it helps:
R=47540 @ -10C
R=12260 @ 20C
R=10000 @ 25C
R=3893 @ 50C
Any idea will be appreciated.
Thanks
Aggelos Mallios
Design Engineer
National Centre for Marine Research
http://www.poseidon.ncmr.gr
mail: amallios@p...
I am trying to measure a thermistor (10k @25C) with a BS2 or BS2sx.
A thermistor curve can be very closely approximated through use
of the Steinhart-Hart equation:
1/T=A+BlnR+C(lnR)^3
T= Degrees Kelvin
R= Thermistor Resistance
A,B,C = Curve-fitting constants
For the range I want to measure (-10C to +50C) the constants are:
A= 0.0010260068
B= 0.0002396354
C= 0.0000001548
I couldn't find a way to calculate this equation with those constants
and keep the accuracy at +-.05C.
If it helps:
R=47540 @ -10C
R=12260 @ 20C
R=10000 @ 25C
R=3893 @ 50C
Any idea will be appreciated.
Thanks
Aggelos Mallios
Design Engineer
National Centre for Marine Research
http://www.poseidon.ncmr.gr
mail: amallios@p...
Comments
http://www.al-williams.com/awce/paktemp.htm
That particular code uses a polynomial fit, but with a PAK-II (which works
the same way as the PAK-I, but has more features and speed) you can even
compute ln.
We've done a lot of work with marine instrumentation with the PAKs (or
customized versions of the PAK) in the past.
Regards,
Al Williams
AWC
* Floating point math for the Stamp, PIC, SX, or any microcontroller:
http://www.al-williams.com/awce/pak1.htm
>
Original Message
> From: Aggelos Mallios [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=zyxj5X37Fu6Y6kQ6n35MHKZMFdA33FqU83PTXWczeRlOxeZ-lOc8xAeKQ_edJtzB6Fo6MFWDcssi6YfzshwSW-lD]amallios@p...[/url
> Sent: Thursday, October 12, 2000 8:39 AM
> To: basicstamps@egroups.com
> Subject: [noparse][[/noparse]basicstamps] Thermistor calibration
>
>
> Greetings all,
>
> I am trying to measure a thermistor (10k @25C) with a BS2 or BS2sx.
> A thermistor curve can be very closely approximated through use
> of the Steinhart-Hart equation:
>
> 1/T=A+BlnR+C(lnR)^3
>
> T= Degrees Kelvin
> R= Thermistor Resistance
> A,B,C = Curve-fitting constants
>
> For the range I want to measure (-10C to +50C) the constants are:
> A= 0.0010260068
> B= 0.0002396354
> C= 0.0000001548
>
> I couldn't find a way to calculate this equation with those constants
> and keep the accuracy at +-.05C.
>
> If it helps:
> R=47540 @ -10C
> R=12260 @ 20C
> R=10000 @ 25C
> R=3893 @ 50C
>
> Any idea will be appreciated.
> Thanks
>
> Aggelos Mallios
> Design Engineer
> National Centre for Marine Research
> http://www.poseidon.ncmr.gr
> mail: amallios@p...
>
>
>
I have a method to calculate the log function on the BS2 posted at
http://www.emesystems.com/BS2math3.htm
It is still a lot of work to get everything scaled right so that errors
don't accumulate above your 0.1% level in the integer math.
An alternative approach, especially if you have the additional memory of a
BS2sx or BS2e, is to use a big enough lookup table and interpolation.
Al Williams can tell you about the option using his Pak-I and Pak-II
external math processors.
-- Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
---original message--->
I am trying to measure a thermistor (10k @25C) with a BS2 or BS2sx.
A thermistor curve can be very closely approximated through use
of the Steinhart-Hart equation:
1/T=A+BlnR+C(lnR)^3
T= Degrees Kelvin
R= Thermistor Resistance
A,B,C = Curve-fitting constants
For the range I want to measure (-10C to +50C) the constants are:
A= 0.0010260068
B= 0.0002396354
C= 0.0000001548
I couldn't find a way to calculate this equation with those constants
and keep the accuracy at +-.05C.
If it helps:
R=47540 @ -10C
R=12260 @ 20C
R=10000 @ 25C
R=3893 @ 50C
Any idea will be appreciated.
Thanks
Aggelos Mallios
Design Engineer
National Centre for Marine Research
http://www.poseidon.ncmr.gr
mail: amallios@p...
a smaller temperature range where the approximation
equation will fit the curve better - in short your
equation will not be +/- .05 deg over 50 to -10 C deg.
However also consider that your thermistor may not be
this accurate or stable - see manufacturer's data.
I calculated
A = 1.029767 ^ -3
B = 2.390289 ^ -4
C = 1.570579 ^ -7
Using -10, 25, 50 deg. [noparse][[/noparse]and you can only use 3 points in
this curve fit...
This may be a case of too many decimal places for the accuracy
of the sensor - ! Each thermistor is a little different so
you may have to do an empirical approach with the actual
unit you are using and put those values in a table.
A glass encapsulated thermistor can be quite time stable...
as long as it isn't heated too high.
<><>
You wrote
Date: Thu, 12 Oct 2000 16:39:13 +0300
From: "Aggelos Mallios" <amallios@p...>
Subject: Thermistor calibration
I am trying to measure a thermistor (10k @25C) with a BS2 or BS2sx.
A thermistor curve can be very closely approximated through use
of the Steinhart-Hart equation:
1/T=A+BlnR+C(lnR)^3
T= Degrees Kelvin
R= Thermistor Resistance
A,B,C = Curve-fitting constants
For the range I want to measure (-10C to +50C) the constants are:
A= 0.0010260068
B= 0.0002396354
C= 0.0000001548
I couldn't find a way to calculate this equation with those constants
and keep the accuracy at +-.05C.
<><>
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I will work on that and I will come back when I have news.
Aggelos Mallios
Design Engineer
National Centre for Marine Research
http://www.poseidon.ncmr.gr
mail: amallios@p...