help with temp fluctuations / PID tuning? - WSM 18.5"

Hey guys,

I'm sorry for yet another tuning thread, but I could really use some ideas here.

I smoked ribs on Saturday, and I fought my stupid WSM all day on the temp. I love the HeaterMeter, but it seems like about half the time I end up with fluctuations like this:



My rib smoke before that (last Sept) was more reasonable, but still not as smooth as I'd like:



Any ideas where I should start to diagnose this problem? I'm hoping to do an overnight smoke on Friday, so I really need to fix this.

Thanks!
Andrew

Are the big dips lid openings? Your graphs don't show anything cyclical, not consistently low/high, almost random large movements in temp? Are you sure your pit probe isn't flaky or something? If you run a couple probes do they all show the same thing? Could wind be a factor? The HM works best when the rest of the vents and openings on the grill are sealed tight...

Yes - the big dips are when the lid is opened. But then the fan kicks into high gear and drives the temp WAY up high.

On the first graph, I had all the bottom vents closed at the beginning except the HeaterMeter vent. It was opened about halfway. But it just couldn't get enough air into the smoker.

Maybe part of my problem is peeking too often to test the meat. But for ribs, I'm not sure of another option...

What are your PID and Blower settings? The HM fan should be able to provide plenty of air to stoke a WSM...

B 4
P 2
I 0.01
D 15

fan: pulse, min 0%, max 25%

I'm wondering if maybe it's partially my blower connection method - I'm using a sink drain that just fits over the vent tabs. Maybe I should drill a hole and use pipe fittings for a better blower connection.

I think a lot of people use the "dog dish" method to connect to a WSM, search the forum or internet for info on that...

On my more genaric bullet smoker I built what I called an "air burner", which is a series of pipes with small holes drilled into it that I connect the fan to. It kinda ended up looking like a gas burner so I called it the "air burner", 'cause it just did air delivery... I used copper pipe and fittings (1/2" I think), drilled small holes so the blower made jets of air shoot out, put that into my burn plate. That thing burns rock solid!

Here's a pic, I posted more about it in other threads and a couple other people have built nice versions of it on their own...

1. If you peek, it makes a big change in temp that the HM will try to recover. If it starts adding air & the pit can recover naturally, you get a big overshoot. You need to either make your lid detect more sensitive, or manually set lid open mode when you peek. That gives the PID time to 'learn' what is going on & not give an erroneous output (too much air). I don't think it's your problem, but I don't see it kicking in all that often.
2. The control system looks like it's doing what it should, but your pit temp isn't responding linearly (the cooling at ~12:45 when the output has been at 100% for an hour...). Get your airflow figured out & the PID will work much better (at about any P/I/D settings)

Your 2nd graph looks pretty good, don't get caught up in how smooth the graph is. Meat don't care & I've never took a bite & said 'Damn, there was a 20* spike 1 hour into this cook, I'll have to toss this'.
The first is either airflow, wind, crappy charcoal, you peeking or a combination of all. I don't have a WSM, but, ignoring charcoal & wind, I suspect you have it choked too far down (dropping off quick at 0% output) + a bit much fan (the cooling at 12:45 & overshoot). However, the bounce from 12:45-1 is weird, I wouldn't rule out anything. Meat probe temp might tell us something (stall, quick rise, etc). Or you might have a wonky pit probe.

Andy Snider said:

You need to either make your lid detect more sensitive, or manually set lid open mode when you peek.

Click to expand...

Thank you for the help guys!

I should check my Lid Open settings. I think it's set at 25% or something like that. It sounds like I have that setting all screwed up. And based on my first graph, the HM didn't detect any of the lid openings until the last one at about 16:30.

What's the standard Lid Open setting, or what's a good average number to use?

Setting your lid mode % you need to set it high enough so it wont trigger if you pit temp fluctuates a bit (without the lid opening). Typically I run mine around 10%, but keep in mind, 10% of 175F is 17.5F but 10% of 500F is 50F, so it kinda depends on the temp you are running in the pit. To get an idea if this is even an issue just go ahead and trigger the lid mode manually (either with the HM button push to the left or clicking the Lid indicator in the upper left of the HM web page (while logged in)).

I can't stress enough that the whole HM concept is to let the HM control the air flow, and so you should have the rest of your vents closed off completely and the entire grill sealed up as best as possible so the majority of the air feeding your fire is coming from the HM. On my bullet smoker I used aluminum tape (available in the heating cooling isle in Home Depot) to close off the other vents and seal over where the HM blower connects to the grill. From there you can tweek the blower and PID settings to tune the HM to best control your pit.

RalphTrimble said:

you should have the rest of your vents closed off completely and the entire grill sealed up as best as possible so the majority of the air feeding your fire is coming from the HM.

Click to expand...

I can definitely seal the bottom vents and improve the fan connection. I might even look at a gasket for the two body connections and the door. I know a lot of air gets in via those gaps.

Does your bullet smoker have a top vent? I'm assuming that one stays open, right?

My bullet is a cheap-o unit that doesn't have a top vent, it leaks like crazy so it doesn't really need one. You would be doing good to put nomex gasket on the door, I don't think you should need to do the body and lid but it couldn't hurt. Generally you want to leave the top vent as wide open as possible, and close it down only if you have overshoot issues and have a hard time keeping the pit temp low. If overshoot is a persistent problem then you might look into some sort of damper like a gravity flap, the ping pong ball valve, or a servo valve like the roto damper. These dampers are able to choke down the air flow through the HM blower that continues when the blower is turned off, which in certain conditions might cause overshoot. Though on my bullet I use the "air burner" and don't have any problem with overshoot so I don't run a damper on the bullet...

If all the dips are due to you peeking, 1, peek less & 2, manually activate lid open mode before you peek.
The PID control needs time & data to cancel out the natural fluctuations of the system. Garbage in, garbage out. You're causing the PID to react to what it thinks is a natural fluctuation, but it's not, so you get big bounces. lid open mode tells the PID to relax & not add air since you've added air by opening the lid.

If you're peeking often, you'll go nuts trying to tune your PID settings. Stock should be close. I usually start 30-50* below desired cook temp, then increase once it hits the 'peak' of overshoot. You then have a closer to stable system to let the PID catch up (or rather, let the fire catch up to the PID).

From the HM wiki:
There's an explanation of what changing each parameter will do to your pit.
https://github.com/CapnBry/HeaterMeter/wiki/PID-Tuning

For this advice, keep in mind I have trouble running at 225* because the fan lets in too much air naturally...I need a servo. I have a servo and a 3D printer, just haven't re-wired my 4.0 to use it or built a 4.2 (got all the components except the PCB). Most of the time I BBQ at 275 w/fan or 225-230* w/o fan (HM just monitors)

I would:
1. mount fan, close all other vents. seal if necessary.
2. light fire
3. keep fan unplugged from HM, but put the pit probe in.
4. see where it settles down to. hopefully a little less than what you intend to cook at. let it get as steady as possible. No peeking
5. Plug fan in & set to cook temp. You might need to modify the min/max fan settings here - min = as low as the fan will run, max=probably close the the 25% you have now.
6 leave it alone for an hour. No peeking.
7. see if graph is holding steady or at least getting steadier - the PID might take a while to work. You can show which PID terms are affecting the output on the graph, but I can't remember how to turn it on w/o a keyboard (look in config).
8 if you have more fluctuation than w/o the HM fan, reset your B/P/I/D to stock & read the link above. Trial & error until you're happy. Or too drunk, then try again tomorrow.

Remember, when empty you will have faster response than if you have food in there, so don't slow it down too much to get a pretty, smooth graph to show off on the internets. You'll have a really long cook the next time.....you could stick a fire brick or heavy pan in to slow it down a bit.

My guess is that your P is low, your D is high & B needs to go away. But that could be way off if you have air leaks or something else.
The defaults in 20140630B and above are: (Posted August 2014)
B = 0
P = 4
I = 0.02
D = 5

Andy Snider said:

For this advice, keep in mind I have trouble running at 225* because the fan lets in too much air naturally...I need a servo. I have a servo and a 3D printer, just haven't re-wired my 4.0 to use it or built a 4.2 (got all the components except the PCB). Most of the time I BBQ at 275 w/fan or 225-230* w/o fan (HM just monitors)

Click to expand...

To use a servo on the HMv4.0 all you need to do is solder wires to Gnd, 5V, and the SPD pin on the ATMega and connect them to the servo, it's not a big surgery... You can add a CAT5 jack to the HM but don't have to, you can add just the three wires for the servo. Don't share the GND from the blower with servo on the HMv4.0 though, 'cause the ground is pulsed rather than the 12V on the blower on that HM version.

Of coarse you could go the full distance and change the blower circuit to match the HMv4.1 or better yet (and more complicated) the HMv4.2, which would enable you to match the CAT5 pinout to the current HM version exactly, but you don't HAVE to do that, you could just add the three wires for the servo, that's how it all started out (on the HMV4.0)....

Thank you guys for all the help. I've got a solid plan on where to go from here. I don't have time for a dry run before my smoke on Friday night. But I should be able to start a couple hours early to work on my temp control before putting the meat on. And I'm doing pulled pork, so I'm not terribly worried if the temp struggles a bit.

I'll be sure to update this thread with what I do and the results.

Pulled pork was a success!

TL;DR - I needed to close the WSM vents better and remove the "B" PID variable.

I followed Andy's advice above and dialed in the smoker much better. For someone's future benefit, here's what I did:

1. Lit the smoker and assembled it (1 full ring of charcoal, 20 lit briquettes on top - top vent 100% open, 2 bottom vents closed, 3rd bottom vent 75% open with fan connected on it
2. Connected HM, but left the fan unplugged
3. Temp rose to about 158 and then started falling, got to about 149 about 45 mins after start
4. Plugged in the fan 45 mins after start
5. Temp didn't want to rise, so I put foil tape all around the fan connection to the smoker
6. Temp began rising
7. Temp overshot pretty bad (20:15)
8. Temp finally got close to setpoint of 225 (20:49)
9. Eliminated the "B" variable in the config settings (changed from 4 to 0) at 20:49
10. Closed top vent to 75% open at 21:12

The temp got pretty darn steady from there, so I put the pork on right about 22:00 (the first lid open line).

The fluctuations between there and the second lid open line were due to periodically adding wood.

Then it burned rock solid all night without me touching. Success!!

(The odd graph after 10:39 was due to me ramping up the temp to try to hurry it along for my deadline.)

To make a better seal in the future, I think I'm going to drill a hole in my WSM for a pipe fitting and attach the fan that way. Then it will be simple to attach / detach the fan, and there shouldn't be any air leaks. But I'll still have the vents free for use without the HM. (I smoke turkeys with a recipe that has the WSM run at full blast with no heat control needed.)