is there an rtd response time VI?

[tempsensorguy]

I'm trying to measure the dc resistance of an RTD vs. time.  My requirement is to determine the time it takes for the sensor to reach 63.2% of final resistance value following a step change in temperature from room ambient to water at 150F flowing at 3 ft per second. 

It occurred to me that response time is a fairly common measurement so I was wondering if there is an express VI or something already written by someone that I could use for my application rather than write a new program.

[Dennis_Knutson]

In order to get a delta time, you just have to use the subtract function. It works with timestamps, numerics, etc. You would of course have to generate an initial time. I would store it in a shift register. Your program would be monitoring the value and as soon as the point is reached, make your calculation.

[tempsensorguy]

The resistance-time relationship is non linear.  I first need to wait until the resistance reaches final value.  Once I have final value, I need to calculate 63.2% of final value.  Then, determine the corresponding time variable.  Would some form of curve fitting express VI work?

[Henrik_Volkers]

Depending on your needed accuracy and how stable your bath temp is you need to define final value ....  

I think that is the more tricky part. The rest is collecting the data and find or interpolate the value you search.

If you can assume an exponetial (or whatever descriptive formular) curve you might get a better and faster result with curve fitting (parameter identification, levenberg -marquardt, ...)

[Dennis_Knutson]

I don't use express VIs so I'm unsure which might be used. Since you have the final value, you might be able to use Basic Level Trigger Detection on the captured waveform. This is on the Signal Processing>Waveform Measure>Waveform Monitoring palette. You pass it the level you are looking for (calculate) and can get the result as time.