Another PID tuning post

[Dave Robson]

I'm sorry but I've read the wiki and the other posts on PID tuning and I still don't get it. I just did my first overnight low and slow and this is what i got:

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I have a King-griller, Heatermeter 4.0 and Ralph's rotodamper. Initial settings were B=4 (can't find any reference for what this is), P=3, I=.005, D=5.
From the graph, PU=3600, using a KU=3 I calculated B=4??, P=1.7, I=.002, D=1350 (I left it at 5) and now the temperature is peaking around the set temperature and dropping down about 25 degrees before heading back up. Which setting do I change?

Thanks.

 

[Dave Robson]

Live graph is at http://64.231.224.237/luci for some reason it's not resolving my web address but that's another issue.

 

[Steve_M]

I'd change B to 0 and reduce I from .005 to .003

 

[D Peart]

and I would bump D to 10. It tries to predict what is happening and heads it off. On your graph your fan lags the temperature, increasing D will tighten up the response.

david

 

[Dave Robson]

Thanks for the suggestions, I'll do another test roast today and try them out.

Dave!

 

[Dave Robson]

So I'm confused. I made the changes suggested and lit the charcoal using alcohol soaked cotton and the temperature came up nicely and stabilized. After I put the chicken on at around 15:00 the temperature started cycling widely, almost 50 degrees. I bumped the temperature up so I could eat by 19:00 and the temperature still swung quite a bit. After I took the chicken off I left it lit and sure enough it stabilized and held steady until I shut it down 3 hours later. I don't get why it would be stable when there's nothing cooking but when there's meat in there it won't stabilize.

 

[John Bostwick]

The meat cools the smoker a bit, acts like a heat sink. What size blower are you using?

 

[RalphTrimble]

Where are you placing the pit probe?

 

[RalphTrimble]

Also, the char griller kamado is kind of a tough one to regulate, likes to overshoot temps very easily. I recommend for low and slow cooking that you set your blower to be "On at Max Only", which helps in a couple ways. First, it lowers the air flow a bit... When the damper clamps down to fight overshoot the air flow drops down to next to nothing, then when the temp drops below the setpoint and the HM kicks in, if you have the blower enabled it causes a good deal of air flow which accelerates the cooling quite a bit and can make the pit temp drop dramatically. The blower stokes the fire rapidly to compensate for the drop and then you get overshoot again and the cycle continues. If you let the damper do it's thing (and leave the fan On at Max Only) the air flow is less dramatic and has less of a cooling effect on the grill and helps avoid getting stuck in this cycle.

 

[Dave Robson]

I'm using the blower specified in the BOM.

The pit probe is center rear on top of the grill held with the bracket it came with.

Thanks Ralph, I've set the "On at Max Only" and see how my next cook goes.

I just got a roll of Nomex gasket in the mail so I can seal everything better and hopefully that will help.

 

[RalphTrimble]

While I would tend to tell you the Char Griller Kamado has a reputation for being leaky, you seem to have no problem tamping down the fire with the roto damper, seeing how you have those deep dips in temperature... So I don't think leaks are your problem, I think you need to get your PID and other settings dialed in and perhaps get used to the grill and get a sense of what the right size fire for your desired cook temperature is....

That said, some tips on sealing up this grill. The first place to start is the ash pan. Remove it from the grill and look around the edge where the round part meets the flat part that mates with the bottom of the grill body. Due to the steep angle of the press there often times you see cracks around this edge. Also look behind the handles, cracks often appear there for the same reason. Then make sure the ash pan is getting pulled up to the body of the grill tightly enough to mate with and compress the gasket all the way around the pan a bit. If not, loosen the clamps on the body of the grill and push them upward and tighten them back down, loosen the latches on the ash pan and push them down and tighten back up. If it is still not tight you can wrap something around the clamp arm or bend them a little so it pulls the ash pan tighter to the body of the grill to make the seal. Also look at the vent, some were not sealed to the pan at all, others are sealed pretty good with some kind of high temp goop...

A good way to see where you may have leak issues is to get a smokey fire going, make the HM blow at 100% and then close your vent and lid, look around your grill and see where smoke is coming out, that is where air will be sneaking in... You kinda have to do this on a calm day, cause wind will make it hard to see exactly where the smoke is coming from....

EDIT
Just to be sure, check that your roto damper is actually going from fully closed to fully open when the HM is going from 0-100%. One time my servo settings got screwed up on the HM and my grill was doing all sorts of strange things, cause it wasn't opening and closing the way the HM was telling it to, so of coarse the effect on the fire was not as expected either. What's a heater meter to do in that scenario other than over compensate....