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Whenever I run my Autotune cycle, it seems that the third set of values is way off compared to the first 2 sets. Should I average all 3 values or should I just stick with the two closest values?

I ran this today on MAX's little brother. I noticed that after the initial heatup, and after each round of new settings (except the last) the min and max temps achieved by using that set of numbers is displayed. Every time I ran it the first set was the worst (highest range) and with each iteration they got better. If the next set wasn't an improvement it was only off a little on either the min or max. I'm not sure averaging the numbers is supporting the process. I wonder if we are not meant to accept the last set as the best.

It's really supposed to be a way to get the numbers in the right ballpark, it's not magic, and it won't provide optimal values.
You want to look for the set with the least temperature variation and start there.
The autotune values are particularly poor with aggressive heaters, like the 40W cartridge settings, and you'll likely have to resort to manually tweaking the settings to get something good.

My problem is that I don't fully grasp the effect of the changes and what numbers do what of the three.

For instance.

Almost always, my actual temp will sit 2c above my requested temp. So when I hit print it will stall and wait for the 2c drop that never actually comes. I either have to set the temp lower before starting my print so that it will reach the operating temp on warm up and proceed. Or I have to physically blow on the hotend to make it drop the 2c so that the print will commence.

Kind of annoying.

I have this isuue with a number of hotends and configs, but yes, always with the heater carts.

Flateric wrote:My problem is that I don't fully grasp the effect of the changes and what numbers do what of the three.

For instance.

Almost always, my actual temp will sit 2c above my requested temp. So when I hit print it will stall and wait for the 2c drop that never actually comes. I either have to set the temp lower before starting my print so that it will reach the operating temp on warm up and proceed. Or I have to physically blow on the hotend to make it drop the 2c so that the print will commence.

Kind of annoying.

I have this isuue with a number of hotends and configs, but yes, always with the heater carts.

The quick version,
P is the basic power, so bigger numbers heat more, and will generally increase initial overshoot.
Increasing D will make the slope of the curve more dominant in the result, bigger numbers result in less initial overshoot, and reduced oscillation.
I is usually used to tune out the oscillation in the steady state.

NONE of these numbers are independent, changing one will result in needing to change the others.

There are additional issues with reprap firmwares, for example Marlin hard codes the point at which it reduces the heating rate from 100% on to I think 80% of the target, the result of which being that with a heater cartridge it's not uncommon to see a 30 degree overshoot which is close to impossible to tune out without changing the code.
In Repetier, there are two additional variables PID Max and PID Min that dictate the maximum and minimum heater values.

Flateric wrote:My problem is that I don't fully grasp the effect of the changes and what numbers do what of the three.

For instance.

Almost always, my actual temp will sit 2c above my requested temp. So when I hit print it will stall and wait for the 2c drop that never actually comes. I either have to set the temp lower before starting my print so that it will reach the operating temp on warm up and proceed. Or I have to physically blow on the hotend to make it drop the 2c so that the print will commence.

Kind of annoying.

I have this issue with a number of hotends and configs, but yes, always with the heater carts.

I just ran into a similar issue with junior today. I was in the process of getting a modified Magmametal hotend working with Repetier. After PID autotune my hotend sat at 212 degrees after asking for 200. I lowered the PID_min to 10 and voilà, the issue went away. I believe, much like Poly said in a roundabout way, it is the lowest value the board will send the heater to warm it up. If it is to high, the firmware is not capable of getting any lower.