NTC Thermistor

Archiver

Hello
Has anyone done any projects which involved using a thermistor for
temperature measurements
Any pointers would do

Isaac

Archiver

Isaac,

I have built a fairly accurate (+/- 0.2 degrees C absolute) NTC-thermometer
using the command RCTIME.

The secret for my sensor was to linearize the curved relationship between
resistance and temperature. I did this linearization by cutting the curve
into pieces and then fit each each piece by a short straight line.

See code below to get an idea how this linearization works. For this
calculation you will need the resistance of the sensor at given temperatures
Tcal plus the slope of its curve.

Regards.

Klaus

'///////////////////////////////////////////////////////////////////////////
' NTC == miniature sensor, used for fever-thermometers
'
' Ts = Cx - RT/Cy Ts: Temperature of sensor * 10
' RT: output of RCTIME
' Cx = T_cal + R_cal/(dR/dT)_cal cal: 10,15,20,25 degree C
' Cy = K*(dR/dT)_cal (dR/dT): slope of curve at this point
' K = 29,57 K: system constant for BS2 and cap = 0.47 uF


'{$STAMP BS2}

OUTPUT 15

Ts VAR WORD
RT VAR WORD


Main:

OUT15 = 1 'just a flashing led to indicate the system is running

HIGH 12
PAUSE 10
RCTIME 12, 1, RT 'here the resistance of the sensor is measured

OUT15 = 0

' below are the four straight lines for linearization

Ts = 479 - (RT/107)
IF Ts>225 THEN Label1

Ts = 420 - (RT/139)
IF Ts>175 THEN Label1

Ts = 367 - (RT/177)
IF Ts>125 THEN Label1

Ts = 306 - (RT/231)

Label1:

' display output with decimal point, ie 19.7

DEBUG CLS, DEC Ts/10, ".", DEC1 Ts

PAUSE 440

GOTO Main

---

Oorspronkelijk bericht

---

Van: isaac_aiyanyo [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=0g93u_LQinISEjdhAFUABcYS1toFqPKtQXN9aAuKUFVUN1AdUjw_PbjLX762qmmwJxQ65iKPYv6bSM1dlaQR9tU8]isaac.aiyanyo@n...[/url
Verzonden: zaterdag 8 november 2003 23:12
Aan: basicstamps@yahoogroups.com
Onderwerp: [noparse][[/noparse]basicstamps] NTC Thermistor



Hello
Has anyone done any projects which involved using a thermistor for
temperature measurements
Any pointers would do

Isaac



To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.


Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/

Archiver

Klaus

Thanks for your reply you have given me hope as i was thinking it would be
almost impossible to do
the thermistor i am using have got 3k @ 25 degees C
i want to use the range from -20 to 80 degees C using the command RCTIME
and need at most +/- 0.5degrees C
accuracy.


> The secret for my sensor was to linearize the curved relationship between
> resistance and temperature. I did this linearization by cutting the curve
> into pieces and then fit each each piece by a short straight line.
For my range does this mean i have to do say _20,-15,-10,-5,0
then do say 5,10,15,20,25 etc
> See code below to get an idea how this linearization works. For this
> calculation you will need the resistance of the sensor at given
temperatures
> Tcal plus the slope of its curve.
On my data sheet i have got the resistance of the thermistor given over the
entire range example
at -20 degees C resistance is 19.41 K
at -15 degees C resistance is 19.29K
at -10 degees C resistance is 19.01K
at -5 degees C resistance is 18.79K
i got a bit lost with the maths trying to figure out how this would be
placed in the formula here could you kindly break it down for me RT is the
result of the command RCTIME thats clear

> ' Ts = Cx - RT/Cy Ts: Temperature of sensor * 10
Where would i get T_cal from and also what values would i use for (dR/dT)

Could you please expand on this using the values above

Thanks for Your Time
Very Much appreciated
Isaac