A PID Question

[thedrayman]

Today I fired-up 'Big Berth' my 200L copper to check both elements where working and there where no leaks. I half-filled her with 100L of water and I set my N2006P PID to Manual Mode and 100% of the total heater power. I set my alarm settings to 100c so that I could switch one of the elements off at that point however I've found that the PV reading (from the PT100 sensor) never goes above 98. I lifted the lid and noted that there was a rolling boil going on at 98 so it looks like the elements are working okay. It's just that, when it comes to a Brewday, after initially adding the hops, I don't really want to have to lift the heavy lid of the pot to check I have a rolling boil.

Anyone have any ideas about my PID settings?

Although I believe the Manual Setting overrides the PID function I have the PID outy setting as #2 (Relay J1, J2 as alarm output; SSR PID non-contact control output; used in maintaining a constant temperature (SV=Target temp)).

I'm wondering if it should be set at #3 (Relay J1, J2 as alarm output; SSR for Limit control output; SV=Control value.)?

Any help would be greatly appreciated.

 

[chastuck]

This may be a daft question, but have you checked the accuracy of the sensor against an external thermometer, such as a thermopen? I have found that quite often my sensors are one or two degrees out from the actual temperature and I've adjusted the offset in the PID accordingly.

 

[Aleman]

There will be an 'Offset' that you can apply that will bring your PID into line with a reference device . . unfortunately is does vary depending on your PID.

 

[thedrayman]

Thanks chastuck and aleman, I'll take a look at the instruction leaflet today for the Offset settings and give it a go. :thumb:

 

[Cactus]

Iirc my sensors needed adjustments up to 2c, once calibrated with a thermapen though they haven't shifted.

Thermapen is superb will be great once the BBQ is out.

 

[thedrayman]

I don't have a Thermapen but I do have an electronic thermometer which I can use as a reference against the PT100 sensor. That'll be my job for the day on a dreary, drizzly Sunday. Thanks chaps.

 

[thedrayman]

Did a double-check with a second thermometer and the sensor was a couple of degrees out, adjusted the Offset and everything is sweet now. Thanks again chaps, I love this place (even in its' new guise....so-far).

 

[Fil]

an easy diy 2 point calibration for any thermometer is to use distilled water and freeze some, so u can mix a 60/40 mix of ice and water for a 0C reference, after a few mins standing mix well and test the temp just under the surface of floating ice.. (dont touch ice with the probe tip that'l be colder)

for a 100C test boil the distilled water and when boiling test the temp of the liquid surface, as thats the point water at 99.9C turns to steam at 100C

if your geographic elevation is significantly higher or lower than sea level it may be worth googling the impact it will have on tests, especially if using digital devices with 2 or more decimals

in fact google the whole proceedure, its late and im running on a failing memory.. but u get the jist