Looking for an addition feature...


 
M

Marc Strickland

TVWBB Member
Problem
For people with really efficient grills like Primo, BGE any of the ceramics for that matter. Running servo only. If you overshoot your target temp on start up, say you want 225 and you hit 260. The controller will shut the vent completely until you get near the target. Unfortunately this can in a lot of cases put out your charcoal.

Is there a way to make sure the vent cannot be completely closed for more than a set amount of time regardless of temp? ie. If the temp is 260, vent is closed waiting to get near 225, temp is dropping but has not yet reached target. If vent has been closed for 10 min. open to 50% for 1 min. then close again and continue waiting to reach target... Or something like that. Looking for a way to fix an overshoot without putting out the charcoal.

Thoughts?
 
Treat the cause, not the symptom. You'll be better off tuning your PID settings to avoid startup overshoot rather than playing servo open/close games to keep the fire alive while it's coming down.
 
Have you actually experienced this problem? I find it is actually quite hard to fully extinguish the charcoal to the point it won't come back. It shouldn't happen in the time it takes to recover from a 10-20 degree overshoot.
 
Steve_M hit the nail on the head. You need to better tune your PID so you don't have such an overshoot. However, if you are getting a huge initial overshoot that you can't control with your PID, then you are starting with much too large of a fire in your pit to begin with.
 
The overshoot was my problem. I started the grill up without the controller running. So the overshot was self induced. My overshot was 35 degrees and when I started the controller my grill dropped to about 228 ish before it ever opened the vent again. Then it just kept dropping. I let it get to 199 before I disconnected the servo and manually ran the vents. I opened the top and bottom vents fully for 5 to 6 min. (waited until I saw the temp start to climb to about 204) after that I had to go so I shut the top vent back down to about 1/4 in. open and hooked the servo back up. The controller was then able to stabilize the temp just fine. If I wouldn't have opened the vents manually I don't think it would have come back.

A couple of problems,
1. I should have let the controller do the start up.
2. On my Primo there is really no way to restart the charcoal if it were to go out, I'd have to remove the food, the food grates, the drip pan, the ceramic "D" plates, the "D" plate holders, then re-light then replace everything. And when you have 24 lbs of pork on it, this is not something you'd like to do.
3. I don't want to play with the PID settings since once I'm at temp this thing can hold my grill for over 24 hours +- 3 degrees.

I don't have a blower hooked up, I'm thinking if I make an adaptor to hold the blower and set it to only run at 100% vent opening that might fix an issue like this as well. Was just hoping to stay with servo only. And.. Next time let the controller do the start up instead of me doing it manually.
 
I think part of the problem is that we're using a PID controller with a relatively complex environment. Most things being PID controlled are a little more stable ie: turning on/off an electrical element, or controlling the flow of liquids, gasses, etc. In these environments, it's safe to assume that whatever is being controlled will always be available when the PID is making adjustments. Need less heat, turn down or off the element, need more heat, turn on the heating element. This isn't the case when it comes to a bbq pit.

On my kamado joe, I've experienced 2 different types of situations where the controller wasn't able to deal with things properly. The first one is the one being described by Marc, in that an overshoot in temps causes the PID output to sit at 0 for so long that the pit is starved of oxygen and becomes extinguished. I've also experienced the situation of the PID trying to bring the fire back to life after it was pretty much out, but the fan was blowing on the pit so hard that it was essentially blowing it out rather than stoking it. Of course, this happened while I was away from the pit, so I had to manually override things with the pit meter app, setting the blower max to 0 then back to 50, then back to 0, etc.

Something else to remember is that it shouldn't take too much time to bring a ceramic cooker back within range during startup, even with an overshoot, as the ceramic mass probably hasn't had enough time to be fully heat soaked. This becomes a much harder issue to deal with after a couple of hours.

You could also try and combat this with an alarm script. You could modify something like the Super Ramp Down script to lower your pit temp to 2 degrees below the current temp, stepping it down by 2 degrees each time until the desired temp is reached.
 
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I guess you could also do that ramp down in reverse. To remove the overshoot in the first place. Maybe have it set the initial set point to 10-15 degrees below desired and then do the same kind of ramp up to bring it to temperature.
 
You could also try changing your max startup fan speed to something lower. This is what my dad does to prevent overshoot (50% startup max), although I would say you'd probably want to just increase your PID D term or decreasing your P term so heatermeter starts cutting back the power sooner.
 
Interesting, I use a primo - servo only, overshoot has really never been a problem for me. The only time I had an issue was once I came inside to check something and left the lid open..... so much of the charcoal was going at that point it created issues for the first hour or so that the primo was lit. I ended up just delaying my cook a bit.
 
So, I am going to stick with my thought that this is not really a problem, but the solution is already in the software.

I tested full shutdown on my grill last night. After 40 minutes of total shut down, there was enough latent heat in the charcoal to reignite. I think you need to way way overshoot and have an extremely air tight grill and HM sets up for this to be an issue. You should fix the overshoot with the PID settings and how you startup the grill.

That said, you can simply change the setpoint for the servo at 0% to leave a very slight opening so it won't completely snuff the fire out.
 
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