Need PID tuning help with graph (WSM)


 
PaulFisher

PaulFisher

TVWBB Super Fan
Hey everyone,

I'm having continuous problems with holding a steady temp on my WSM. It's an 18.5" with the top vent open 25%. All other vents are closed except for the fan that goes in one vent. Not sure what to change to prevent the waves as seen below. Any help is appreciated.

bbq3.jpg
 
What are your PID settings? Is this graph typical or was it a one time deal? How do you have your coals laid out and lit?
 
Tom Kole said:
What are your PID settings? Is this graph typical or was it a one time deal? How do you have your coals laid out and lit?
Click to expand...

I believe my PID settings are default except for the last one which I believe I changed to 8. I think I may have changed the other one to .003. The graph above is typical just about everytime I cook. Coals are laid out using the minion method. The reason I changed the PID settings from Default was because I was seeing similar behavior.
 
It looks to me like most of your temperature gain occurs after the blower turns off, indicating that it's somehow generating more heat than the pit probe is registering. In your graph you can see that the temperature quickly turns around as it crosses the set point, then slowly starts to climb as and the fan starts backing off. As the fan speed trails off the temperature takes a sharp upward turn. The fan was actually interfering with the apparent temperature reading. This is the sort of thing that happens when the blower CFM is too much for the smoker and you see a sort of cooling effect caused by the fan, then when it turns off, the stoked coals now burn extra hot for a while.

You can try cutting the blower "max speed" to 50-75%. Adjusting the D constant up a little (10-12?) might help a little but it would be masking the issue and might end up working against you if set to high, where it would prevent the temperature from ever going below the setpoint.
 
Bryan Mayland said:
It looks to me like most of your temperature gain occurs after the blower turns off, indicating that it's somehow generating more heat than the pit probe is registering. In your graph you can see that the temperature quickly turns around as it crosses the set point, then slowly starts to climb as and the fan starts backing off. As the fan speed trails off the temperature takes a sharp upward turn. The fan was actually interfering with the apparent temperature reading. This is the sort of thing that happens when the blower CFM is too much for the smoker and you see a sort of cooling effect caused by the fan, then when it turns off, the stoked coals now burn extra hot for a while.

You can try cutting the blower "max speed" to 50-75%. Adjusting the D constant up a little (10-12?) might help a little but it would be masking the issue and might end up working against you if set to high, where it would prevent the temperature from ever going below the setpoint.
Click to expand...

Thanks Bryan, in my original picture I believe I had the max fan setting at 30%. Would there be any benefit to me raising that or would that just make it worse?
 
What I like to do when setting up the hm or learning to run the hm. Start your wsm the same way you always start it and let it get up to temp without the blower on the smoker. Try to get a stable temp at a lower temperature than you usually run your wsm at. Once it's stable, make a mark on the air inlet and outlet if not fully open. Then slowly close the inlet until the temperature drops down at least 10 to 20 and make another mark at that point. Now you can install the blower on to the wsm and set the pit temp to the point about 30 above the temperature it's at now. Put the blower at a very low max speed and slowly raise the max speed until you see the temp rises. If you see when the blower is on and the template does not change and the temperature lowers, then the blower max speed is to high and needs to be lowered until you see the temperature rise again
 
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