My PID yo-yo's up and down about 20 degree's

[mozeppa]

is there any adjustments that can be made to stop it from cooling to 10 degrees below my target temp to 10 degrees above?

for example i'd like it to kick "on" when the temp drops 3 degrees...and kick "off" when 3 dgrees too hot.
is this possible?

any instruction that helps is much appreciated.

thanks , mike

[dragon813gt]

What PID is it? Does it have an autotune function and have you used it? They learn as they go. And there will be some variation depending on the level of lead in the pot.

[bangerjim]

That is where the Rate & Reset (Integral & Derivative) math functions come in.

AT should take care of it.

[mozeppa]

its a 90 lb pot ...it does it whether there is 90 pounds in it or 5 pounds and any where in between.

it was made by the guy banned from here.

i shoot guns and make boolits....no idea what Rate & Reset (Integral & Derivative) math functions are.:veryconfu

[dragon813gt]

Set temp, hit the AT button. Let it run the cycle which is going to take about twenty minutes once the pot reaches set temp. From that point on it learns and adjusts.

[edctexas]

A large pot will definitely act and need slightly different tuning between full and near empty. All pots on a PID will tend to do this, but here is why. When you apply heat to the mass of lead it has a heat capacity which you must charge with the heat energy to raise its temperature. If you know any electronics. it like trying to charge a capacitor thru a resistor. As the pot empties, the capacity to hold heat decreases. It your PID has the autotune (AT) function you should tune it a stable temp with the pot near full (75-80 lbs). Write down the P I D t settings. Now when the pot is near empty (10lb) do an AT and compare the settings. You can compromise by setting in between settings. It is also a good idea to know what the settings are anyway.

The heater is like a battery whose voltage is controlled by the PID. The resistor is the Thermal resistance between the heating element and the lead. The lead in the pot is the capacitor in the electrical equivalent circuit. There are other elements like the loss of heat from the lead and pot and the loss of heat from extracting lead. Lets ignore those. The P in PID stands for proportional, that to say is the heat going in is proportional to the difference in the desired temp and the lead temp. The "I" term means integral. The integral function averages the heat versus time applied to the pot. Generally the I term is to cancel the thermal mass time constant. The "D" term means derivative. This is the rate of change of temperature. In some PIDs there is a "t" term which works with the "I" term. The "t" stands for time. Now that we are all confused, I'll jump out of here, but you probably can only get very good regulation for one range of PID settings. You could also jack up the available heat input. But thats another story!

Ed C