Thermistor conversion formula is wrong?

[David Moerman]

There is a VI found in the DAQ>>Signal Conditioning subpalette called "Convert Thermistor Readings" (I have LabVIEW 5.1.1 and 6.0.2). There seems to be some blatant inconsistency with the thermistor formula inside.

The standard thermistor formula is 1/T = a + b(lnRt) + c(lnRt)^3. (This agrees with all the documentation I've seen, including within this VI!) However, the formula node contains a different equation:

T = 1/ (a + lnRt*(b + c*(lnRt**2)));

Note the bracket position around the 'b' and the exponent '2' vs the '3'.

What gives?

[Mikael_Garcia]

David,
Equations written in plain text are always hard to get a grip of. However, I guess the two equations you mentioned,

1/T = a + b(lnRt) + c(lnRt)^3

and

T = 1/ (a + lnRt*(b + c*(lnRt**2)))

are identical.

First invert the second eq. and multiply lnRt with c*(lnRt**2) in it. You will then get the first equation since
lnRt*c*(lnRt**2) = c(lnRt)^3.

I guess the confusion is that

lnRt**2 =(lnRt)^2 and not ln(Rt^2).

Hope this helps. /Mikael

[Mikael_Garcia]

Please also note that the ^ operator, which in pre-6.0 versions of LabVIEW Formula Node stood for exponentiation, now represents the bitwise exclusive or (XOR) operation. The new operator for exponentiation is **. That is also why it is very easy to get confused here. /Mikael