Outside temperature readings with Honeywell C7089 exterior sensor

mc1069

Has anyone ever used a Honeywell C7089 Exterior Sensor to measure temperature with a Basic Stamp.
I have a Basic Stamp monitoring my HVAC system and I would like to add the ability to record the outside temperature.
I have used the AD592 temperature probe by using the rctime command and that all seems to work well.
I thought I would try using the Honeywell C7089 Sensor instead of the AD592. I can't seem to figure out the math.
I have the data sheet on the C7089 with the resistance values in Celsius and Fahrenheit, but I still can't make sense of it.
Here are some examples:
Ohms of Resistance F C
106926 -20 -28.9
61246 0 -17.8
27219 32 0
14356 60 15.6
4317 120 48.9
I have tried using the example in the WebApplied Sensors V2.0 manual, but I can't seem to get the temperature readings to workout from -20 to 120.
Any advice would be appriciated.

Thanks in advance.

Chris Savage

Welcome to the forums! It may help you to get help if you post a link to the datasheet for your sensor so that others can easily see its specifications and how it works.

mc1069

Here is the link for the datasheet.

http://controls.acmecontrols.com/Asset/Honeywell_C7089_Outdoor_Sensor.pdf

My problem is the difference in resistance is only 162 between 120 - 118 degrees,
but the difference between -20 and -18 is 6003.

I am basically using the code from the WebApplied Sensors Manual:

' Applied Sensors - AD592.bs2' Reading the AD592 temperature sensor using the RCTIME command.' {$STAMP BS2}' {$PBASIC 2.5}

Kal CON 1530 ' Constant to be determined. rct VAR Word ' A word variable.TK VAR Word ' Kelvin temperature.TC VAR Word ' Degrees Celsius.
TF VAR Word ' Degrees Fahrenheit.
DO ' Loop forever.LOW 5 ' Discharge the capacitor.RCTIME 5, 0, rct ' Time for the volts to rise to 1.3 V.TK = Kal/rct*10 + (Kal//rct*10/rct) ' Calculate KelvinTC = TK - 16 ' and Celsius.TF = TC * 9/5 + 32
DEBUG DEC rct, TAB, DEC TK, ' Show the results.TAB, SDEC TC, TAB, SDEC TF, CRPAUSE 50 ' Slows down the program.

LOOP ' Back to the beginning of the loop.

I changed KAL from 15300 to 1530 and TC = TK - 273 to TC = TK - 16.
That seems to get my lower temperature readings close with this sensor,
but once the temperature gets above 70 degrees,
my temperature readings start to rise alot quicker than the actual temperature.
I think my main problem is in figuring out the math.

Thanks again.