Question about behavior


 
A

Aaron Morris

TVWBB Member
Ok,

So I'm testing my Heatermeter with mini Mini WSM. This is what I'm seeing:


I'm running with 10% blower on all the time and 20% startup max. When I had it set at 225 degrees, it was running perfect, but going to 250 causes this sinusoidal. This is with a Rotodamper, 3/4" inlet to the side of the grill.

Should I try for just fan on at max only?

Thanks!
 
How is the top vent set? Sometimes tweaking the top vent will level things off...

I am 100% behind the "Max Only" setting on the blower, reason being, the blower pumping in cold air will accelerate the temperature drop in the pit... so the HM ramps up the air flow to compensate and then the pit rebounds into overshoot. Damper only control doesn't cause that cooling effect, so it is a more stable and perhaps slower to adjust. Typically I would say 10% Max on the blower shouldn't cause this, but perhaps being a mini WSM you need to dial it back even further or go with the "Max Only" setting.

All that said, some PID tuning should be able to level that off too.. You seem to have a pretty quick oscillation there with a period of about 15 minutes.... PID tuning isn't simple, but if you take a look at the WIKI section on PID tuning you will get some idea how to adjust out that oscillation.

EDIT: For some reason I am always happier with the old PID settings of 4, 3, .005, 5 as a starting point.
 
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Ok, so I changed it to the PID settings you mentioned as a base, and set it to Fan only on at 100%, with a fan max of 10% since the pit is small. Now it's looking a lot better, but the damped is running wide open a lot. Of course, there's only one 3/4" hole in the BBQ for input right now.

 
OK, better, but you are running too high on the damper alone...
Now try opening the top vent bit by bit and you should see the servo start to close off... Dial the top vent in so the HM is running about 35%, note that position as it should be a pretty good starting point for your top vent.
 
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Problem is that the top vent is 100% open. Not sure where to get more air from, unless I open up one of the bottom vents a little ore.
 
Hmm... I wonder if I should use a 3/4" inlet of each side of the smokey joe gold, and attach via a Y-tube to the damper. That way I have more air inlet to work with. I did notice however, that (I have several RD3's I'm playing with) that this RD3 won't open all the way up, like the servo won't travel as far as other ones I have.

Ah, adjusted pulse duration to 2400 max, and now the damper gets opened all the way, before it was only about 3/4 open. Hopefully that helps.
 
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OMG.. now I feel like an idiot. I was looking at the BBQ, thinking wow, I got 12 hours out of a half basket. I looked at the top lid and realized it was shut completely. Last night I closed all the bents to kill the fire before I left. I never did reopen it, so the damper has been fighting that the whole time. I guess I'll load up some more coal in the morning and retry everything using damper only fan at max and see what happens.
 
You do need to calibrate the RD3 so it will open/close fully, every servo is different, so calibrated SPD numbers will be different, but they should all be capable of rotating the full range of the RD3 valve.
It was seeming to me like a pit that is having the air flow choked off by the top vent, though you said it was completely open, I guess you were mistaken somehow?
At any rate, properly calibrate your RD3 and open your top vent and give it another try....
 
Yes, I had the top vent closed fully, rather than open fully. I was assuming it was fully open because it's been that way for the last few tests.. I forgot I had closed it fully when I was trying to kill the fire the other night. Doing another test with a half basket of coals. Top vent fully open, RD3 is fully open now with new servo settings. Fan is set to max only with 10% blowing, and 20% on startup. Side lit it for about 2 mins with a torch and after 15 mins it is almost up to 250, a nice even controlled ramp up. Hoping this works better this time!
 
Hmm.. still having osciallations.. I played with it for a bit, and now just have fan on 3% all the time. What should I be doing to smooth things out?
 
I wish I was a PID tuning expert, but honestly I am not. I do well enough to dial in the various grills I have, I typically don't have to stray far from the old stock BPID settings I had mentioned before to get a nice stable cook.
Log in on the HM page and then hit the P key, the PID popup should appear. Take a look at that and adjust your PID numbers to level off the bars on that display. You can read the section in the WIKI about PID tuning and see if you can make heads or tails of it. The problem I always have with my grills is the oscillation period is very long, making it hard to calculate the PID settings. You seem to have a fairly rapid cycle, so perhaps the PID tuning method described in the WIKI will work out easier for you.

BTW, can I assume you have the RD calibrated now so it goes fully open/closed and the top vent is open?
 
Getting a little better. Rotodamper does go fully open/closed. Top vent is fully open.

I've adjusted the settings to B=0, P=5, I=.005, D=14

 
I just noticed you said you have the blower on 3%.... This could cause you a problem, reason being...

If your blower MAX is set to 3%, then when your HM is at 30% your blower will only be reaching 1%. I've never run the blower art such a low percentage, but I would assume when set to only achieve 1% max in the first 30% of the HM range the blower will be going on and off quite a bit. When the blower goes from off to on the HM will give it a boost start to get the blower rolling, otherwise the blower may not start turning until the HM reaches a higher percentage. The boost start will get the blower rolling an from there the HM can keep it rolling at a very low speed (the HM has a hard time STARTING the blower at a low speed but can keep it rolling if it is already in motion)

If you have your blower speed set so the blower is going from 0% to 1% (or above) often you will get a puff puff puff effect from the blower and it will be delivering more air to your pit than the HM is calculating. Observe your blower to see if perhaps this is going on? Note the burst of air that happens when the HM goes from 0% to 1% (or above) so you can recognize it when you see it.

I haven't tried running the blower as low as 3%, I suspect it is not going to work very well if at all. I would suggest you bump that up to at least 10% (or more) or give another try using the damper only (blower on at Max Only) now that you have the top vent open.
 
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