How does the max fan speed work?


 
N

NickH

New member
I finally used my HeaterMeter for an actual smoke on my WSM! I smoked 9 pounds of chicken breasts and it worked great. I was a bit confused, though, about one thing. I had set the "Max Fan Speed" to 30 as I thought I would just try low to minimize the chance of ash blowing around, and if that wasn't enough fan speed to get things hot enough, I'd bump it up.

But during the smoke the fan showed that it went to 100% several times. For example, I increased the temperature set point from 250 to 275 and the blower went to 100% for 15 minutes.

So can someone explain to me how this is supposed to work? Does the graph show 100% fan speed but really it was only doing 30% speed? Or are there cases where HeaterMeter ignores the max fan speed setting? Thanks,

Nick
 
I believe max speed in the config is really max power ie: 0-100% of 0-12 volts. If you see 100% fan on the graph, that means 100% of the 30% you defined in the config. The 100% on the graph is related the the percentage of fan needed based on the BPID variables. So if you see 100% fan for 15 minutes, your 30% config limit is probably too low.
 
The HM will always show you the fan speed on a scale of 0-100, but if you have set the MAX fan speed to 30% when the HM says the fan is running at 100% it will only be running at 30% of it's potential speed.

Try it out, make the setpoint on the HM really high so the fan will be blowing at 100%, then change the MAX fan speed from 30% to 100% and notice the difference in how much it blows....
 
Confirming what everyone else has said. The speed you see on the web interface really is "PID output", so 100% in your case is 30%. The fan speed is scaled between 0% and MAX. I keep forgetting to change the text on the web display but I'm not sure what to call it yet.
 
So if MAX=30%, and "PID OUTPUT"= 50%, then the fan power will be 15%? I was under the impression that the MAX just clipped values above 30, but it sounds like I was wrong and that it's actually changing the gain.
 
A few versions ago it just clipped but when the servo was added the output wasn't clipped any more because it seemed unintuitive. If you saw 30%, 50% or 100% and the fan ran the same speed (because max was 30%) you'd be wondering what happened, right?

It would also make your PID constants only work from 0% to 30%. You can get the same behavior as the old clipping, if you just triple your pid values because what would previously be 30% is now 100%. At that 100%, your fan output will now be 30% again.
 
Thanks everyone. I was thinking that this might be what was happening, but it is good to hear exactly how it works. I will probably bump my max speed up to 50 or something the next time I smoke and see how that goes. I didn't notice any ash at 30%.
 
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