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Pellet Temperature Controls - my thoughts.

banjo

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Joined Jun 18, 2012
Pellet Smoker Temperature Control - For Low Temperatures

I decided to write this after buying a new Pit Boss 820D, and experiencing some difficulties (temperature swings) in controlling the temperature. The things I'm discussing here could apply generally to all pellet smokers with temperature controllers.

This is going to be a long article, so sorry! But there's a lot of information here that I'm wanting to convey to you.

I want to give you a little insight to my background before delving into the subject, because my background provides me with some insight into controlling temperature in BBQ Smokers.

For the first five (5) years after college (Electronics), I worked as an Instrumentation and Controls (I&C) Technician (process control) at a nuclear power plant in Southport, NC.

I then moved over to software, where I worked for about a year, programming in Fortran, but for process control.

Later, I accepted a promotion to Nuclear Startup Engineer, specializing in process control and computer systems.

In total, I spent about 10 years in I&C, process control

I got interested in smoking BBQ around 1986. But no-one I knew, knew anything about it. So I had to learn on my own; per-Internet days.

My first "pullable" BBQ pork was when my fire in my New Brunsfield smoker ran away and hit 450; charred, but the middle pulled. I thought I had hit the "Holy Grail" of smoking BBQ.

Over a long period, I kept backing off my temperature and increasing my time: temperate and time => energy into the meat, and it gradually got better.

But I was growing tired of all-nighters, so I decided to automate.

Using an Arduinio, I created a control, using a thermocouple, to control the air-inlet damper. Success! I rigged this up to everything, and all worked well (BGE too).

But I was having trouble getting enough smoke into my meat; consistent temperatures with proper oxygen gives low smoke. Think old-style carburetor here.

I finally decided to give 'pellet smokers' a go.

So I bought a Pit Boss 820D. I did a lot of research, and there were a lot of complaints on all manufacturers with respect to temperature swings; noted and made my purchase, with the idea I could fix any problem in that arena.

Last night I used it for the first time.

At a set-point of 200, and P4, using PB pellets, I had big swings in temperature. I tried all of the different P settings over a 7 hour period (make a single change, then sit back and see), I tried all of the P settings while keeping the set-point to 200.

Well, my temperatures, on any given single test, had a range from 180 to 330. Pardon my French, but that aint' no kinda control!

So I delved deeper into this 'P' thing and how the control was being performed.

--P Setting Explained--
This is a Ratio setting. It controls the time the auger is on, to the time the auger is off. The only variable here, given by the change in P, is the length of time the auger is off.

Per the Pit Boss manual, the P settings are as follows:
-P0 Auger On 18 seconds, Auger Off 55 Seconds. Total time per cycle: 1 minute, 13 seconds. On duty: 25% of 1:13. On Duty Cycle Per Hour: 24.7%
-P1 18 Seconds, 70 Seconds. Cycle Period: 1 minute, 28 seconds. On duty: 20% of 1:28 On Duty Cycle per Hour: 20.5%
-P2 18 Seconds, 85 Seconds. Cycle Period: 1 minute, 43 seconds. On duty: 17.5% of 1:43 On Duty Cycle per Hour: 17.5%
-P3 18 Seconds, 100 Seconds. Cycle Period: 1 minute, 58 seconds. On duty: 15% of 1:58 On Duty Cycle per Hour: 15.25%
-P4 18 Seconds, 115 Seconds. Cycle Period: 2 minutes, 13 seconds. On duty: 13.5% of 2:13 On Duty Cycle per Hour: 13.5%
-P5 18 Seconds, 130 Seconds. Cycle Period: 2 minutes, 28 seconds. On duty: 12% of 2:28 On Duty Cycle per Hour: 12.2%
-P6 18 Seconds, 140 Seconds. Cycle Period: 2 minutes, 38 seconds. On duty: 11% of 2:38 On Duty Cycle per Hour: 11.4%
-P7 18 Seconds, 150 Seconds. Cycle Period: 2 minutes, 48 seconds. On duty: 11% of 2:48. On Duty Cycle per Hour: 10.7%

If we have a 'steady state' external environment (e.g., no wind gusts, temperature fairly constant), then the on-duty cycle will accurately control the temperature...almost.

The reason for the 'almost' has to do with the: A) temperature of the meat, and B)moisture in the meat.

As the temperature of the meat increases, the more moisture is 'sweated' from the meat, and this will lower the temperature of the smoke chamber somewhat.

This will change up until the time the meat has lost most of its moisture, at which point the temperature of the smoke chamber will begin to increase, as will the temperature of the meat.

--Smoke--
Us older smoker-guys have the benefit that younger smoker-guys don't have: we grew up around carburetors instead of fuel injection.

The pellets are being burned in a 'combustion' chamber. There's a fuel:air mixture involved; too much fuel (or fuel not hot enough) and too little air will cause smoke.

One of the main goals, as far as I'm concerned, with controlling the temperature of a pellet smoker, is the production of smoke. The ability to control the temperature, as with an industrial controller such as a PID controller, if connected to the auger and fan, will cause a situation almost like that of fuel-injection: no (or not much) smoke.

So, in my opinion, the PID controller (and I've worked with literally thousands of them in nuclear power plants) is the most efficient available....but efficiency isn't what I'm mainly after: for me, it temperature and smoke.

So, I don't think, that I want a PID.

The temperature control on most pellet smokers have a difficulty in controlling temperature while producing smoke. They get around this problem by dumping raw fuel into the combustion chamber and heat it. It first starts producing smoke, but very little heat. Then it gets hot enough to produce a flame and the combustible products in the smoke are consumed, and it burns clear; this is when the heat is produced and the temperature is rising. Then the fuel burns out. That completes that cycle; the next cycle begins when more fuel is added to the combustion chamber, heated hot, and the smoke/heat cycle starts continues.

So an important element of all of this is to let the fire more or less go out before adding new fuel, thus producing smoke when new fuel is added and heated to the combustion point. If we have a flame and add more fuel, then pretty much all of the smoke will be consumed, if there's enough air.

Smoke is fuel heated to combustion temperatures bu un-burned. In a carburator, this typically is due to too lean (not enough air) to support combustion. Dirty tailpipes and smoke coming out the tailpipe.

So, this is the situation facing pellet smoker manufacturers: how to control the temperature while producing smoke.

One way to produce smoke is to heat the fuel to combustion temperature, then cut back on the air, but this can lead to creosote on the meat. So you want some good ol' blue smoke but not the yellowish-gray stuff.

I think, for higher temperatures cooking, their temperature controller probably works fine. But for low temperatures (e.g., 200 and below), they get into a problem. That's where the P control comes in.

If we have low temperatures, and low changing external environmental factors, (such as wind, gusts, temperature, humidity) then a P control works fine. Find the P for the given conditions that day, and it will provide superior capabilities.

--P Setting Temperature Swings--
Have a look at the duty cycles given above. Note that the auger time on is CONSTANT for all P settings; the only thing that varies is the auger time off. A lower duty cycle corresponds to a longer time off. This longer time off gives heat time to decay to a lower temperature in the smoke chamber - air leaks, carry off to exhaust, cooling through metal walls to outside. So the longer the period off, the lower the temperature will become - the more it will move downwards from the maximum heat generated from the pellet fire after they burn out. So this is the temperature swing in P mode; the longer the time off, the more the temperature will decay from max, the more the swing. But this does control the average temperature.

I'm interested in maximum temperature and average temperature when I'm cooking. A few small spikes aren't really going to make much difference in a nine pound slab of meat, unless they are applied for a while!

So the maximum temperature shouldn't really increase over a previous cycle while in a P only setting mode; it's on for 18 seconds regardless, and except for some slight variance in the number of pellets that are transferred into the combustion pot, the maximum will be the same each burn cycle (allowing for meat moisture, and outside environment variables).

The minimum temperature however, will change with decreasing on-duty cycles, so the average temperature will change along with that. So the higher the P setting, the longer the time off for the auger, the longer the duty cycle, the less heating cycles, the lower the average temperature, and the lower the minimum temperature, while the maximum temperature should stay about the same.

--What is a PID--
A PID controller stands for: Proportional, Integral, Derivative. When you are trying to control a process, such as liquid level into a tank that is having liquid withdrawn at different rates, then the PID is a critical component for success.

Proportional, in this case, gives a percentage increase in inlet of the liquid for a percentage decrease in the tank.

Integral takes into account the average over time.

Derivative takes into account the rate of change in the liquid level; if the level is dropping fast, then we need to add additional inlet volume, more than the integral mode will supply; otherwise, the tank volume level will be 'chasing' the setpoin.

None of these conditions are really associated with controlling a pellet smoker.

Think about that a moment....(sounds of Jepordy playing)...our smokers are cooking in a relatively constant environment. The temperature may change, but it's usually somewhat consistent through out the day. Same for the humidity. Wind is really the big issue hear, and maybe rain as well, as it both can extract heat through the walls, resulting in lower temperatures, lower maxes, lower averages.

But wind gusts: that's where the real problem lies for a proportional duty cycle controller like we have. It can't really track with a lot of wind gusts; that's where the PID (Derivative mode) shines.

The P contro pretty much just set it like a single valve handle control; set-it-and-leave-it. Unless you have a lot of GUSTY WIND. Then a PID would provide superior performance. But for me, fortunately I can control the days I smoke BBQ, and I don't like to do it while in the wind. So I don't need a PID, nor do I think it's well suited for this job if our goal is to control temperature and produce some smoke.

--Conclusion--
If you are looking at low temperature smoking of a pellet smoker, then ignore the temperature setting dial, place it to 'smoke', and set your P. Lower P values give higher duty cycles. Find the P setting for your temperature for that day given that environment.

Run a few tests without any meat, with 1/2 hour for each test (this allows any changes to overcome various vendor duty cycle changes when P changed, and for the environment to stabilize). Set it up, get it going, sit back with a brew or two, a cigar or two, and record some temperatures. Also include the temperature, wind, and rain; your environment conditions. Those notes you just made will support you every time you want to cook on days like this; run other tests on alternative types of environments you are likely to encounter.

For me, on days like today, my P modes resulted in the following (theses are all averages - I plan on picking up peak/bottom values later:
P0 260 degrees
P1 240 degrees
P2 220 degrees
P3 180 degrees

I didn't go further with this because I really want to cook around 200, so P3 - 180 might be my sweet spot.

I plan on updating with values when I can observe.

I'm currently working on a ESP32 device that will possibly either be a replacement for the existing controller, or as an additional. I don't plan on selling these as I don't want to fool with the insurance necessary for liability, but it would be available for you to make your own, via GitHub. Using this, along with a thermocouple, I can do the following:
- Alarm to my phone via SMS Text on low and high temperatures, on web page also.
- Serve web page that shows graph of temperature along with current temperature and sounds alarms. You don't need an app if you've got a web page that is served! I'm currently doing this for both of my cigar humidors: temperature and humidity on a web page using the ESP32.

Let me know if you have any interest in this.

Sincerely,

Ben Bailey
 
Last edited:

rablack21

Newbie
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Joined Mar 22, 2018
This is interesting stuff, Ben. I think that I am seeing what you are seeing on my Pit Boss Copperhead 5.
 

tallbm

Master of the Pit
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Lots of interesting info Ben.

I can understand if you want to tinker and play around and do things a specific way. I just want to let you know that I'm pretty sure a company already has a replacement plug and play PID controller for pellet smokers like yours.

This solves the temp swing and other temp related issues not involving smoke.

For solving the smoke problem a lot of pellet guys seem to use the A-Maze-N Pellet Tube (AMNPT) for smoke generation.
This solves the issue with lack of smoke or length of time smoking.

With the PID controller and the AMNPT you would heat and smoke be independent of one another and you could control both very tightly. You would then also have a truly set and forget smoker setup for the length of time you could get smoke generated out of the AMNPT. Lets say it only goes 6 hours well you just fill it back up, start it, and put it back in for another 6 hours. That is still quite hassle free.

Anyhow, I understand if you want to tinker and try things, I do all the time. I just wanted to share that bit of info in case you wanted a nice simple and less time consuming approach to solving the problem. I hope this info helps some :)
 

banjo

Fire Starter
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Joined Jun 18, 2012
tallbm, thanks for the response! Yes, I'm very familiar with PID controllers. I mention them above. There's nothing that I'm doing that needs a PID controller. The P mode works well, but there seems to be a problem with the temperature control section of the code.

A very good controller for this situation would just be no setpoint, just a knob that changes the duration of the time off, which is what P mode (smoke) does. That would be the simplest; not even bother with the supposed temperature set- point. Once it's been adjusted for the 'standard' position, then only a slight change would be needed for environment change conditions.

I would like a easier control (rotating knob instead of goofy push-button they have now so you have to go all the way around to get back -1 from where the current P value is.

It's beyond me why they do it the way they (PB) do, because in my opinion, it's certainly a big fail as currently shipped.
 

airman

Newbie
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Joined Mar 27, 2017
Keep posting. This interest me. As someone in the trades. I work with a lot of different controls and voltages. This sounds very interesting.
 

bregent

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Just a few comments. First, not all pellet grills with simple timer controllers have a 'P' setting. The CampChef is one example but there are others. The CampChef uses a very simple algorithm that changes the duty cycle depending on whether the grill is above or below setpoint. It works well most of the time but there are plenty of instances where it does not.

Next, several PID controllers like the ones made by Roanoke for Memphis and RecTec will pulse the fan when the grill is at smoking temperatures. For Memphis, this occurs when the grill is below 300F. The result is a nearly constant production of smoke output, and no trace of creosote whatsoever. You can see in this video the level of smoke output.

My Memphis will put out smoke like that all the way up to 300F maintains a fairly tight temperature range.
 

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