R

#### Rex

- Jan 1, 1970

- 0

before so I have been reading about thermistors. I have been trying to

reconcile a table of R vs T to the values computed by formula and B

value. They don't match. I am wondering if anyone here can explain why.

Background:

I want to build a PIC-based thing to record room temperature at

intervals over a period of days. I'll use a serial port on the PIC to

gather data to a PC to save it. Relative is more important than absolute

accuracy for this, but I wanted to try to get reasonably calibrated for

C or F.

I checked my junk drawer and found a 10K thermistor that I bought at

Radio Shack some years ago. After some web searching, I finally figured

out that it is a Semitec 103AT thermistor. I found a web page that has a

datasheet (unfortunately, only in a couple pages of their 800K catalog

pdf). At 25 C it has an R of 10K and its B value is 3435K between 25 and

85 C.

I did some general reading on the use and theory of thermistos via the

web.

Question:

I found this equation for R at a T value...

R = Ro exp( B (1/T - 1/To))

For my 103AT

Ro = 10K = 10000

To = 25C = 298K

B = 3435K

The data sheet says that B was determined by R values at 25C and 85C. If

I run the equation for T = 85C = 358K I get an R value that doesn't

match what is in the T vs R table for 85C.

If I calculate B from the table values for resistance at 25 and 85C I

get a B of 3477. At different T values in the table I get different B

values and none that I tried are 3435.

Is this normal? I thought the equation and table data would be closer.

I'm trying to figure out a way to generate a conversion table in the PIC

code but I'm not sure how to start. Do I have to do my own regression on

data from the table? This is looking more complicated than I expected.

If anyone wants to see the table data, here is a link to an Excel sheet

for the 103AT from the manufacturer.

http://www.semitec.co.jp/RT/AT/103AT1_11.xls

Other pages on that site have the datasheet but only as part of the

catalog, so I didn't post that link.