Hi guys. Got a question on the Heatermeter. I've used it a few times now. My question is, if I set it at 225, when it hits 225, the fan is still at like 95%. I feel it should start backing off way before it hits my goal temp. When it's blowing at 95%, it over shoots the mark. I do not have a dampner on my set up. Will that help out? Using it on a Vision Classic B kamado grill. Still kind of new to the controlled temp game and kamado. I love the idea. Any help is appreciated. Thanks!
Need a little help.....
- Thread starter JLoring
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Enrico Brandizzi
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Can't help you J, but I surely can wish you
Welcome to the forum !
Welcome to the forum !
Bryan Mayland
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The "backing off" value is the "D" constant in the PID term. If you're still at 95% when it hits 225F then that means that it is approaching so slowly that either:
A) the "I" sum has built up so much that the the device is powering through your setpoint
B) the "D" constant is too low so the slow temperature change doesn't let it back off enough
Next time you're logged in to heatermeter on startup, press the "P" key on your keyboard to activate the "PID Internals" graph. It will show you the current proportions that the P, I, and D values are contributing to the output. When you hit your setpoint P will be 0%, your "I" will be a large value, and then the "D" should be a negative large value. If you want them to perfectly balance out, you can take the "I" value and divide it by the "delta T" value, which is the change in temperature, and set your "D" constant in settings to whatever that number is. For example, if your "I" is 95% and the "delta T" is 10, then set your D constant to 9.5.
You might find that that D value is too high for normal use, which would mean you'd want to decrease your "I" constant so it wouldn't build up so large a sum so fast. If you're running "Fan startup max" not at 100%, because that slows the initial approach to the setpoint, it could be allowing your I sum to get too large as well because the I sum is attempting to compensate for the grill not heating up quickly enough.
With the default constants on my BGE, my output goes to 0%-20% about 5-10 degrees below the setpoint because it is usually approaching so quickly that the "D" constant shuts the whole thing down well before it even gets there.
A) the "I" sum has built up so much that the the device is powering through your setpoint
B) the "D" constant is too low so the slow temperature change doesn't let it back off enough
Next time you're logged in to heatermeter on startup, press the "P" key on your keyboard to activate the "PID Internals" graph. It will show you the current proportions that the P, I, and D values are contributing to the output. When you hit your setpoint P will be 0%, your "I" will be a large value, and then the "D" should be a negative large value. If you want them to perfectly balance out, you can take the "I" value and divide it by the "delta T" value, which is the change in temperature, and set your "D" constant in settings to whatever that number is. For example, if your "I" is 95% and the "delta T" is 10, then set your D constant to 9.5.
You might find that that D value is too high for normal use, which would mean you'd want to decrease your "I" constant so it wouldn't build up so large a sum so fast. If you're running "Fan startup max" not at 100%, because that slows the initial approach to the setpoint, it could be allowing your I sum to get too large as well because the I sum is attempting to compensate for the grill not heating up quickly enough.
With the default constants on my BGE, my output goes to 0%-20% about 5-10 degrees below the setpoint because it is usually approaching so quickly that the "D" constant shuts the whole thing down well before it even gets there.
Bryan Mayland
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While typing that I just had an idea. The PID I sum should be constrained to the max startup fan speed on startup, to prevent people who have lowered their startup speed to the point that it generates these large I terms that can't be canceled out by the D value.
I quickly coded that in, but then I realized that a side effect of that would be that in normal operation, the I sum would be constrained by the normal max fan speed. If someone has a damper and a big fan and has set their max fan to 50%, then that means that the damper would also be limited to 50% maximum which obviously is the wrong thing to do. Of course, the easy and dumb thing to do would be to add another pair of "max I sum" settings to the heatermeter configuration to allow someone to control these. Easy because it requires no thought on my part but dumb because it shifts the burden of figuring out what these values do and how to use them properly to the user, who is already overwhelmed by the number of knobs they have to control.
Maybe I should make it constrain to the fan startup speed only on startup, otherwise 100%. That would work in the startup case, but I am concerned about tying two unrelated things together for only a portion of the HeaterMeter operation. This is why software development is so hard! Not because the coding part is hard, but figuring out what I want it to do is hard. :/
EDIT: Just posted AVR Firmware 20161018B which limits the PID I sum to "fan startup max" when in startup and lid recovery mode for people to try out.
I quickly coded that in, but then I realized that a side effect of that would be that in normal operation, the I sum would be constrained by the normal max fan speed. If someone has a damper and a big fan and has set their max fan to 50%, then that means that the damper would also be limited to 50% maximum which obviously is the wrong thing to do. Of course, the easy and dumb thing to do would be to add another pair of "max I sum" settings to the heatermeter configuration to allow someone to control these. Easy because it requires no thought on my part but dumb because it shifts the burden of figuring out what these values do and how to use them properly to the user, who is already overwhelmed by the number of knobs they have to control.
Maybe I should make it constrain to the fan startup speed only on startup, otherwise 100%. That would work in the startup case, but I am concerned about tying two unrelated things together for only a portion of the HeaterMeter operation. This is why software development is so hard! Not because the coding part is hard, but figuring out what I want it to do is hard. :/
EDIT: Just posted AVR Firmware 20161018B which limits the PID I sum to "fan startup max" when in startup and lid recovery mode for people to try out.
Last edited:
Bryan Mayland
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Just update the AVR firmware by going to LinkMeter -> AVR Firmware -> Online Repository and click the FLASH button next to the snapshot firmware.
Bryan Mayland
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I'm not sure this code is going to stay in. I've though about it and it creates a possibility that the PID output could be locked too low for the setpoint to be achieved. It gets close to the setpoint but then just stays there because the startup max is set too low. If you have your startup max set to 50% and P=5, when you get to 10 degrees from the setpoint the controller is at most going to give you 50% output (damper and fan combined) and it looks confusing because there's still output left to give but HeaterMeter won't give it any more. I'll leave it in for now but I might have to rethink it or remove it entirely in the future.
Bryan Mayland
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hehe Sorry to hear about your headache, hope I didn't contribute to it!
The problem is that we're taking the fan max and end up limiting the PID output max to that. If you have your "fan on above" set to 50% and "max startup speed" set to 50%, the blower will turn off completely once you're within 10 degrees of setpoint (with P=5) which effectively sets your "max fan startup speed" to 0%. If the fire has enough momentum to reach the setpoint you'll be OK, but if it doesn't it may not ever reach the setpoint because the fan will stay off. If your servo max is at 100%, the damper will remain half closed until the setpoint is reached as well, and I doubt it ever would in that condition.
The problem is that we're taking the fan max and end up limiting the PID output max to that. If you have your "fan on above" set to 50% and "max startup speed" set to 50%, the blower will turn off completely once you're within 10 degrees of setpoint (with P=5) which effectively sets your "max fan startup speed" to 0%. If the fire has enough momentum to reach the setpoint you'll be OK, but if it doesn't it may not ever reach the setpoint because the fan will stay off. If your servo max is at 100%, the damper will remain half closed until the setpoint is reached as well, and I doubt it ever would in that condition.
SteveCK
TVWBB Pro
I don't know and yet again maybe I'm looking at this wrong. But I don't see this as problematic. You mention that if the pit doesn't have the proper momentum at the start that is the only time this is a problem. This doesn't sound like something we should be changing the software over, seems to me that this is just a fundamental user error on the only part of the cook that is manually operated.