Estimating exponential decay time constant - taking tangent lines

[ipatka]

Hello,

How would you go about estimating a first order decay time constant? I have a system that loses heat very slowly and I wish to estimate the time constant from the first part of the data to allow for more frequent tests (I know what it will end up decaying to). 

I know that I can make a good estimate based on the initial slope of the graph but I don't know how to implement this in LabVIEW. Does anyone know how I can make the VI calculate a tangent line to a graph and find where it intersects another line?

Thanks

[Henrik_Volkers · Accepted solution]

How about using the

Exponential Fit VI

Owning Palette: Fitting VIs

Requires: Full Development System

Returns the exponential fit of a data set (X, Y) using the Least Square, Least Absolute Residual, or Bisquare method.

This VI uses the iterative general Least Square method and the Levenberg-Marquardt method to fit data to an exponential curve of the general form described by the following equation:

f = ae^bx + c

where x is the input sequence X, a is amplitude, b is damping, and c is offset. This VI finds the values of a, b, and c that best fit the observations (X, Y).

[ipatka]

Thank you! I had tried this before but I had an exponential decay that didn't die out to zero. This was causing weird results with the damping indicator. I solved it by subtracting the vertical offset from all of my values so it died out to zero and now it correctly gets the time constant.

[Henrik_Volkers]

guess what the c is for 😉

[ipatka]

Ah....I feel foolish