RTD calibration issue

So I just finished putting the finishing touches on my 15 gal. aluminum electric kettle and I am having trouble settling on a temperature offset for the RTD. I am using the Auber 2352, the 4" pt100 RTD and the 5500W camco ripple element. After 10 min. in crushed ice water (mostly ice) it reads about 33-34 while my cheap Taylor instant read probe thermometer reads 31.4ish. However at 100% output boil with constant stirring I can only get the RTD to hit 206, and at very brief moments hit 207 if I have a sheet tray over the top and a blanket around the sides.

So if it is 2 degrees high at freezing and 5 degrees low at boiling what do I do? I plan to use the kettle as both a HLT and boil kettle. I will collect the runnings from the mash tun (cooler) in 2 buckets and transfer back to the boil kettle after sparging is complete. I will be doing my first all-grain brew in a week or two and would like my mash temps to be dead on. I admit that I should invest in a better thermometer but I am still trying to stomach the price of my new system.

Here are pics of my setup.

https://picasaweb.google.com/116286224592627712791/BeerBrewing

And I would not have though of doing this if it wasn't for the great info on this forum.

I currently live in Ithaca NY where the boiling point would be 211.2 so that should only account for a small portion of the discrepancy.

Welcome to thermometry.

I would forget crushed ice water. I've done it in the lab many times and it's actually fairly hard to get a good standard with crushed ice; it really requires DI water, ice made with DI water, and an insulated container. If everything isn't just right it's easy to be a degree or two off.

Temperature stratification in boiling water is similarly significant. Barometric pressure makes a difference, and it should be DI water. Not that great of a standard.

What I would do is warm some water up to about 70C and calibrate your PT100 to match your Taylor thermometer, then call it a day. I do the same thing, only I use my mercury-in-glass Kodak Process thermometer, which is my personal temperature standard.

I had pretty much the same issue with the RTD in my boil kettle reading a little under at boil (211F on an average day here) and a smidgeon high at freezing temps. I approached it as I wanted the error to be nearly zero at mash temperatures, then I calibrated the sensor in my mash tun to the boil kettle (eBrutus2.0).

So, according to Wikipedia, the highest point in Albany is 324 ft above sea level. If your house happens to be sitting on that hill, your average boiling point is 211.4*F. Your ice water bath was busy melting, not freezing, so most likely was 33*F. This means you were maybe 1 degree high at freezing and 4-5*F low at boiling. Assuming linear error, this would put you at zero error at 62*F and (here come the technical terms) ABOUT *3F low around your critical mash temps.

This just happens to match mine pretty closely as I also am running a +3*F modifier. You didn't happen to get your RTD off the slow boat too, didya? Were the threads on it all sloppy loose? Hope this helped, cheers!

Yea, honestly you want to calibrate it as close to your mash temp as possible. Get a lab thermo, they are really cheap and accurate. Then match the correction to the lab thermo.

Just a side note on the Auber RTD. I took mine apart, why? I always take things apart just because I like to see what makes them tick. The way the RTD is constructed is basically the pt sensor is soldered to the connector and the sensor is gooped up with some white thermal grease and shoved in the tube. I don't know if this is SOP for pt100 probes, I don't know what the thermal conductivity of the grease is, but my gut feeling is it's not quite as good as the metal.

you get what you pay for.
These are first class