On 10/15/2015 6:13 AM, Art Todesco wrote:

Sorry, I don't have any specifics on your oven. But, about 14 years ago we
bought a Dacor built in oven. We always suspected control problems. When I
checked, there was indeed control problems. Set at 350, it would over shoot and
then undershoot 350. Calling Dacor was absolutely no help. They just kept
repeating the words on the script that industry standard allows a 25 degree

There is a whole *science* to designing good control loops. Unfortunately,
it *is* a science and many people implementing them aren't formally schooled
in that science. How you measure, where you measure, how/where you
"control/actuate", etc.

At which location in the oven do you want to have control of the temperature?
E.g., most ovens have adjustable racks that aren't there *just* to allow
you to accommodate different sized foodstuffs! The location of each wrt the
upper and lower heating elements (and, which elements are actually in use)
determines the actual conditions *at* each rack location. Placing an *extra*
something above or below can alter the heat available at a particular location.

(e.g., I bake two sheets of cookies at a time -- *knowing* that the upper and
lower sheets experience different heat profiles that must be reflected in my
baking routine)

Legacy "on/off" control systems react differently than proportional
controls (imagine trying to maintain a desired speed on the road
when your only method of speed control is to engage or disengage
the clutch with the engine running at a fixed RPMs behind a fixed
gearbox).

Is there a "transport delay" (i.e., inherent "lag") in the measurement or
control sides? I.e., if you're measuring the temperature of water
coming out of a hose *at* the hose end -- but controlling it at the
*source* end -- you can see how easily the temperature will fluctuate
(because the control doesn't know the temperature of the water until
it has traveled down the length of the hose).

Our furnace, for example, "coasts" after the demand for heat has been
removed. The furnace designers realized there is still a lot of heat
trapped *in* the furnace and lets the blower run for another minute
or so to extract that heat (and cool the furnace down). *But*, the
thermostat has already decided that the house was "warm enough" when
it stopped calling for heat!

variation. Well this one was a little worse, say maybe 30 or so. BTW, 25
degrees is ok for baking large mass things, however for small things like
cookies, etc. it means the difference between undercooked and overcooked. We

What you cook in/on also plays a role. E.g., glass "saves" you ~25F.

lived with it for 5 years. I called Dacor again and they said they have a new
controller unit (the whole front panel) which holds the temperature much
tighter. They offered it for free, but the catch is, that I had to have it done
by a so called professional and pay them. So, it cost me $100 to get what
should have been there in the 1st place. With the new control panel, it was
significantly better, but not as good as I would have liked it. Now there was a
top to bottom swing of about 14 degrees. I now have a Whirlpool in a different
house and have never measured the temperature because the baking results have
been very good.

The tighter the control (i.e., less hysteresis) the more control action
and the more "work" the controller has to do.

I'd wager most temperature control loops in appliances, HVAC thermostats,
etc. are simple bang-bang controls: temperature too low? turn on heat.
temperature too high? turn off heat. Very little "anticipatory"
(derivative) control so the system is almost always underdamped
(oscillates)

Note the same sorts of problems are present in freezers.

There are techniques that can be used to improve the control. But,
usually, customers aren't concerned enough nor willing to *pay* for them!
They'd rather be enamored with the appearance of the stainless or the
brand name on the front than the actual *performance*!