Choked Fire


 
J

JakeBrown

TVWBB Member
What is the best way to use the Heatermeter to ramp the temperature down from 350ºF to 225ºF, without choking the fire?

On this cook I had the pit start at 350ºF to get a good complete fire going; I want a lot of smoking coals. Then I want to bring the pit to 225ºF for the cook.
The problem I experienced is that once I get the pit to 350ºF and activate the Heatermeter, it shuts the damper closed, and then my fire goes out.

In the configuration I don't see an option for the minimum damper open (to prevent choking). I also don't see an option to shutdown the fire (total choke).

2nd cook with the Heatermeter, and still learning.
Jake
 
Yeah, the heatermeter doesn't know that it's choking oxygen from the fire, it just knows that in order to go from 350 to 225, it needs to put the brakes on, and hard!

You'd want to use something like the Super Ramp Down script to gently ease the pit temp down.

This will work:

Code: 
#!/bin/sh

NEWSP=$((sp-2))
NEWAL=$((al_thresh-2))
lmclient LMST,sp,$NEWSP
al_set $NEWAL
 
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I recently got my RotoDamper3 all set up, and I'm wondering about choking the fire also. Let's say I'm just doing a low and slow cook at 250 degrees. The way I understand it, the damper opens to whatever percentage the fan is running at. So if I reach my set temp of 250 degrees and the fan goes off, the damper closes all the way up as well. Is that going to choke my fire out? Or does that only happen when you are trying to drop the temp by 100 degrees or so?

I was thinking another option is to just set the minimum value for the pulse on the server motor so that the damper always remains just a hair open. But that might not be necessary.
 
From what I was able to tell from my cook is the damper/fan is at 0% when above the setpoint temperature. The damper/fan doesn't open back up until below the setpoint.
 
Think of the HeaterMeter as the gas pedal in a car. Once you get up to the desired speed, you still need to keep the pedal down a certain amount to maintain that speed. That's what you should also see on the HM. Since the HM has no brake, if you suddenly want to do a drastic change in temp ie: go from 350F to 250F, all it can do is completely take its foot off the gas pedal and hope for the best.

You shouldn't need to add any workarounds, such as re-calibrating the servo damper so that it's never fully closed, even when the HM thinks it's closed.

It just takes a little bit of understanding that the HM is only doing what it thinks is best to control the situation. it's just a PID controller that happens to be controlling a fire, and if a fire is starved of oxygen, then it will go out and there's no way to restart it. This is drastically different from something like a heating element in a water tank, where the controller is free to turn the element on and off as much as required to maintain the water temperature.
 
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I need some help understanding the super ramp down script...
It seems like the script will lower the pit temp and alarm by 2ºF every time the alarm is triggered.
So if my pit is 350ºF, the parameter inputs would be:
sp= 350
al_thresh=350

Then after alarm is triggered it would be:
sp= 348
al_thresh=348

...and this repeats every time the alarm is triggered.

And it keeps doing this until when? I want it to be 225, but isn't the setpoint 350 and getting changed every time the alarm is triggered? Or do I need to modify the script to exit out of this script once 225 is reached?

I think the script won't change the damper opening; the damper will be 0% for the same length of time without the script. It has no reason to open the damper.

I think the solution is going to have to be:
1) Don't bring the pit temp to a high setpoint. It will take too long to bring it down, and you risk choking the fire.
2) There needs to be a minimum damper opening.
ChrisMueller's idea of adjusting the servo minimum value is one idea. But I prefer not to mess around with the tuning; I still want Heatermeter to know how to close the damper.
I would like a minimum damper opening setting.
I would also like macro buttons: one for ramping down; another for shutdown.

Perhaps I can modify the ramp down script to manually control the damper opening. Would be nice if I can control the damper opening and fan separately too (I think this can be accomplished using smin).
 
You can always just set your servo range so that the damper is never fully closed. HeaterMeter doesn't have an "OFF" mode so there's no way to shutdown the fire if you do this, you'll have to go out and close the vents manually. I question doing this though because always having some air going through is going to give you less control maintaining temperature.

When I am running at my setpoint (generally 225F), there's always _some_ percentage of HeaterMeter output. If temperature is maintaned with 0% output, then HeaterMeter isn't controlling the grill at all, natural airflow is.

@JakeBrown: you could always just open up your vents a little during the 100F cooldown period before you cook to provide just enough air to keep the fire alive. However, the easier solution is to not get the fire so hot on startup. Get it to 250F then stir the coals to spread them out if that's what you're actually trying to do.
 
JakeBrown said:
I would like a minimum damper opening setting.
I would also like macro buttons: one for ramping down; another for shutdown.

Perhaps I can modify the ramp down script to manually control the damper opening. Would be nice if I can control the damper opening and fan separately too
Click to expand...
I will say that the first two will never happen because there's no "I am running" versus "Turn the fire off" modes in HeaterMeter. HeaterMeter always tries to maintain the temperature set, and having a whole separate set of parameters for two modes and having a way to switch between them is too complicated.

Yes you can control the servo and the blower independently from a script (although the fan will NEVER run if the output is 0%). And yes you definitely need the code from the beginning of the "Super Ramp Down" script to make it stop ramping down once it gets to temp. You can put the servo limits change in at the end there (and a limits change to leave it open a bit in the other part.
 
I think this should work for a pit ramp down script.

Code: 
#!/bin/sh
#
TARGET="250"
#
# This should be placed the in the "Pit Low" alarm script
# area, as it's going to be triggered as the temperature is
# lowered.

# You ramped the bbq up to 350, now you want the HM to bring it down to 250
# Step 1) Change your setpoint to 250 ( or anything lower than 350 )
# Step 2) Change your Pit Low Threshold to 348 ( assuming your
# current pit temp is at 350 right now). As soon as you hit 348F, the
# ramp down will begin.

if [ "$al_thresh" -gt "$TARGET" ]
then    
NEWSP=$((al_thresh - 2))
NEWAL=$((al_thresh - 2))
        if [ "$NEWSP" -lt "$TARGET" ]
        then
        NEWSP=$TARGET 
        NEWAL=$((TARGET - 10 )
        fi
lmclient LMST,sp,$NEWSP
al_set $NEWAL
fi
 
A couple notes on the choking issue.... First, the assumption that the HM will always completely close the servo valve when the temp is above the setpoint is false.... It's a function of the PID settings and the rate of temperature change on your pit probe that determine how aggressively the HM reacts. Look at the graphs I posted on page 40 of the RD thread for examples of the HM keeping the damper open above the setpoint. Also, if/when the HM does close the servo damper fully, a brief period of shut-off will not completely put out the fire. Unless you have MAJOR overshoot and the HM chokes the fire for an extended period you shouldn't have to worry about putting out the fire. If you do run into that situation then start a smaller fire so you have less overshoot, or close the lid on the grill soon after lighting it so the fire doesn't build up too big and cause extended overshoot...
 
Bryan,

On the Alarm Script Recipes wiki page, the "Configuration Variables" of "pall0, pall1, pall2, pall3 : Alarm low triggers" are those the same as settiing the "alarm low threshold" value on a probe?

I was thinking of creating a script that would invoke super ramp down in the event that the pit alarm high threshold was reached, which would then change the probe low threshold until temps were back to normal.
 
Thanks, though it doesn't seem to actually do anything for me after I tested it.

root@HM424:~# lmclient LMST,pall3,80
OK

root@HM424:~# lmclient | tr "{," "\n"

<snip>
"n":" Ambient"
"c":89.3
"dph":1.60
"a":
"l":-40
"h":-190
"r":null}}]}
 
Oh yeah I misunderstood. Those can't be set, they are read-only. To set it, it needs to be in the same format that came from Heatermeter, which is al,x,x,x,x,x,x,x,x like al_set does:
Code: 
al_set() {
  local AL="LMST,al,"
  local PREMAIN=$al_probe
  local VAL=$1

  while [ $PREMAIN -gt 0 ] ; do
    AL="${AL},,"
    PREMAIN=$((PREMAIN-1))
  done
    
  [ "$al_type" == "H" ] && AL="${AL},"
  lmclient ${AL}${1}
}
You could just `al_type="L"; al_set newval` to use the existing function (optionally changing al_probe as well)
 
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