Struggling with a Stoker

[Robert J.M.]

Folks... I'm struggling with my July 2014 stoker controlling my 18" WSM! Please look at my log running StokerX on a Mac...

As you can see the stoker is not controlling very well and it never really has during log cooks regularly getting out of control with +- 10 degree swings or more. I am cooking in the South and it is very hot here right now. I started the cook @ 8:30pm, top vent full open lower vents closed (except the fan one of course) and it struggled during the early morning when there were no storms or significant wind changes (the WSM is in a corner on a small porch) just normal very slow temperature drops during the night (87 at 9pm-81 at 2 am). Since the early morning stuff was happening only 6-8 hours into the cook there were't any issues with low charcoal ash, etc. Then when the sun came up shining on it, it could not handle that at all. I thought maybe the water was empty but it was 1/2 full, I went ahead and added water at 7:30 am. My WSM has gaskets on the lid and door so it's pretty tight. I can move the smoker to an area shaded in the morning to help the situation but I am really disappointed the stoker doesn't seem controlling well and does not seem to be seeing the slope of the temp rise at all.

I have a few questions...
1. When StokerX is running what is in control of the PID logic, the MacOS StokerX App or the internal Stoker logic.
2. I have used commercial/industrial PID temperature controls before. I really don't understand why the Stoker isn't cutting off the fan before it hits the pit setpoint this is what's causing the issue really.
3. Is there somewhere I can read and understand the PID logic for the Stoker?

Does anyone have any suggestions?
I am going to post another thread about another issue that happened at the very end of the log above that has no bearing on this thread.

Thanks Robert

 

[KLee]

What kind of fuel are you using? How do you start your fire? Chimney starter, or wax starters? How much fuel did you start with? Was it completely full?

Where is you probe located? and how much food?

Full open is a bit much for 225. Try closing the top vent down to 25%.

The Stoker is controlling PID logic at all times. The large cooling trend while the blower is on would indicate a fire issue (too much ash, not enough fuel, incorrect probe placement)

 

[Robert J.M.]

Ok So..

Fuel - Kingsford Blue, Full to the top of the fire ring
Start - Chimney Starter, a layer of lit charcoal covering about 3/4 of the unlit then assembled with the meat. Beginning overshoot says thats too much I will use maybe 10 lit coals next time. I am not focusing on that as I am the problems 6 hours into the cook and then 9 hrs in when the really hot sun hit the WSM (it's 98 deg here now btw).
Probe - About 1/2 way between the center and edge of the grate, between the meat. The only reason it's not in the middle of the grate as there was not enough room so I angled the meat to create a space.
Meat - 2 Butts about 6 lbs each trimmed, 12 lb total
Ash/Charcoal - I didn't finally run out until now, 17 hours into the cook and nearly all of the ash was in the bottom.
Top Vent - I reset the top vent to 50% at 9:30 this morning still getting 10 deg fluctuations but it was getting better and took about 2 1/2 hours to get to about +- 5 deg control, when then I was out of charcoal.

Looking at the graph, after it pulses for a while and if the temp does not come up, the blower goes full on until it hits the setpoint, and only at the setpoint does it cut off. There is no way it can avoid an overshoot if the fan doesn't cut off before the setpoint. I understand it cannot learn how to smooth the graph unless there is some overshoot & undershoot but I think there is a point/time where the controller resets the PID in the sense that it "discards" the PID parameters it WAS using and starts over, and then begins to re-establish it's PID parameters. It seems the initial/after reset parameters are too aggressive.

 

[KLee]

The problem seems to stem from too much lit at the start and not enough time to stabilize. I always start my cooker at least an hr before i want to put the meat on in order to give it enough time to stabilize. This helps the coals settle in and heat up the cooker. I usually use about 1/3 large chimney which covers about 1/3 of the unlit if cooking at 225. Top vent closed down to about 25%.

The large spikes would also indicate too much fuel to start. Once KBB is lit, it takes off and goes until its gone. As the blower runs, it excites anything that is lit which causes the sharp spikes. I've never been a fan of KBB as I prefer the responsiveness of lump in my cooks. 225 is also hard to control no matter what PID or method you use. most cooker sweet spots are 240-275. My cooks usually ramp up 15 degrees when the sun comes up at 225, not as much when cooking at 275.

The PID doesn't discard the data until it is turned off. There is new software our which has improve the control algorithm. Check the website to see if you have the most current version.

 

[Robert J.M.]

I'm fairly sure I have the latest Firmware since I bought it July last year and the release notes say "June 8, 2013: Version 2.7.0.332" as the latest release.

You can see in the earlier graph, it got good control at about 10:45 pm a little more than 2 hrs in and then lost it about 12:30 am and then really lost it at about 2 am. This is the area I'm wondering about. And when you look when the sun came up, I still do not understand why it is blowing to the setpoint well into the session, there is no way you can avoid a big overshoot when that happens, why doesn't the fan anticipate this with either a hard anticipation point lower than the setpoint or by "seeing" the steep upslope of the temperature.

As you can see from the newest graph, it's getting control but it's taking along time. I have also learned that when you add charcoal you must step-up the setpoint up otherwise it will do the blow-to-the-setpoint thing and WAY overshoot, I've stepped it up to 230 now and controlling to +-5 deg

Thanks for helping out!

Robert

 

[Robert J.M.]

Here's the final graph, total cook a little over 20 hours. I had one butt on the stoker and the other on a Maverick (I use the Maverick for overnight alarms). The Stoker Butt hit 190 at 4 pm Maverick Butt was 187 then stayed at 189 for 45 min. As you can see I started getting out of control about 4:45 pm and decided to take them off, Stoker probe butt 194, Maverick 189. With Thermapen, temp was 190 almost every where on Maverick probe butt and about 195 on Stoker butt. Again we have this situation where it was controlling fine then got wonky, seems the stoker logic is fine until something causes it to drop more than "expected" then stoker overcompensates. I am still running, I was hungry so I put some sausage on, set at 250.

Robert

 

[Amir]

One key parameter is the location of the pit probe. If it is too exposed to the flow of the blower, it can cause wide variations.

 

[Robert J.M.]

The WSM is so big it seems unlikely that the location would be a big deal unless it was at the edge on the lower grate. Anyway here's where mine was.

 

[Chad M.]

I'm not sure exactly why, but I've had much better stability with my Stoker once I ditched the water pan for a foil-covered terracotta plant saucer. A 14" saucer works well but you have to add some spacers inside the smoker on the lower bracket bolts to push the brackets in a little bit. That helps seat the saucer better and reduces the risk of it falling through. I think the saucer is a much more stable heat sink than a bowl of water that's constantly changing due to evaporation.

I put my pit probe in a similar location, maybe a little more toward the center of the grate, but I don't think that's the issue for you. And I've had bad spikes with the sun so I try and keep the smoker with an umbrella or something. Like KLee, I too try to let the temp settle in for a while (at least 30 minutes) before adding food and I use about 1/3 chimney on top of a full ring of KBB.