Find RTD inverse transfer function from Labview

[AA2]

RTD has transfer function, to calculate resistance from temperature as below:

R = R0( 1 + A*t + B*t*t + C(t-100)t*t*t )    for t <= 0C             .......equation 1

R = R0( 1 + A*t + B*t*t )                         for t >= 0C             .......equaion 2

Now I want to find inverse transfer function of it i.e calculate temperature from known resistance.

Now equation 2 is simple as it is quadratic & its inverse transfer function is easy to find.

But can I find inverse transfer function of equation 1 withh help of labview function.

[GerdW]

Hi AA,

as LabVIEW is a programming language you can certainly program a VI to do what you want.

You could calculate a lookup table for R=f(T). Then you use that lookup table to interpolate values for T=f(R). Or you create a polynomial interpolation to get T…

[Chinmay_NI]

Hi,

Assuming you know the values of constants R0, A, B, C, you essentially have a polynomial equation in t and to get t from a known value of R, you need to find the roots of that polynomial. This can easily be done in LabVIEW by using the Polynomial Roots.vi (found under Mathematics > Polynomial on the functions palette. Since your polynomial is of 4th degree, you will get 4 roots. Since this equation is for negative temperature, you should look at the negative, real root of the polynomial equation.

[Henrik_Volkers]

about 20 years ago I used the newton solver....

something like

The RTD conversion vi found under numerics-scaling doesn't cover 'correctly' temperatures below 0°C, however I never checked how big the differences are..