heater control for curing oven

GREAT explanation Richard. That's exactly the way I understood it too
(and why I am trying to be pretty careful with my ramp rates). If you
ramp up too fast then it could cure before it has a chance to get rid
of the voids. I'm not exactly sure what happens if you ramp up too
slow but it must be a bad thing

Also, if I use a bladder I'm assuming I still have to use breather
cloth so that the air has a chance to go somewhere? If so how do you
keep the prepreg from "losing" some of the resin to the breather cloth?
Should I use a peel-ply that breathes the trapped air but still
doesn't allow the resin to get lost to the breather cloth? Is there a
peel ply that is stretchy enough to expand when the bladder expands?

Good discussion folks, we're all learning lots.

Joe

On Wed, 21 Sep 2005 17:49:49 -0500, "Dan, U.S. Air Force, retired"
wrote:



47 bucks delivered with the thermocouple! Does it do PID? Heck of a deal it looks like...


I'd be able to answer if I knew what PID is.

Dan, U.S. Air Force, retired

in control system parlance PID is proportion, integral and derivative.

they are used when the state of the process being monitored cant be
directly measured but must be inferred or calculated as the integral
of some calculation. one that comes to mind is the inference of a flow
rate from the upstream pressure and downstream pressure differential
of a fluid flow through a known diameter aperture plate.

Stealth Pilot

Should I be considering this method in building my fuel tanks? It's my
understanding that the higher cure temp, the higher delaminating temp.
Or am I reading too much into this?
Lou

In article ,
Stealth Pilot wrote:



in control system parlance PID is proportion, integral and derivative.

they are used when the state of the process being monitored cant be
directly measured but must be inferred or calculated as the integral
of some calculation. one that comes to mind is the inference of a flow
rate from the upstream pressure and downstream pressure differential
of a fluid flow through a known diameter aperture plate.

Stealth Pilot

PID control algorithms may well be used in conjunction with indirect
process monitoring, but that isn't my understanding of what PID means.

PROPORTIONAL --

You want to heat a pot of water to 170 degrees F with an electric
heating element. If you turn off the burner at 170, the water temp will
coast to 200. So you decide to scale back the power, beginning at 150.
The difference between target temperature ("set point") and the
beginning of scaling back power is called the "proportioning band."
Typically, the power to the burner is cycled off and on. 9 seconds on, 1
second off. Then 8 seconds on, 2 seconds off, etc. Even when the
temperature is above the target, you still get a couple of seconds of ON
time and 8 seconds of OFF time, just to minimize PIO -- Pilot Induced
Oscillation.

Now, you've got less initial overshoot, and smaller oscillations around
the set point. (An ON/OFF controller is always shutting off too late and
coming back on too late, because it can't anticipate and correct for lag
time in the system.)

INTEGRAL --

But, proportioning controllers tend to stabilize below set point. How do
you fix that? With manual reset, or, automatic reset, calculated
mathematically as an integral. Integral compensates for droop before it
exists.

DERIVATIVE --

We're still stuck with that pesky initial overshoot, the largest
magnitude deviation from set point, before the oscillations dampen out
to an acceptable level. (Tempering of chocolate requires very minimal
deviation, and the process requires many hours. Overheating and then
cooling a chocolate bar destroys the temper, and thereby the texture, so
don't do it!) The derivative function, obviously, watches the RATE of
change as the process temperature enters the proportioning band and
begins to approach the set point. To anthropomorphize, "where am I,
where am I going, and how fast am I getting there, and how quickly can I
react?" You start pulling the power back on downwind, at the latest,
planning a carefully controlled deceleration to stall speed two inches
off the ground. As things change, you keep fine tuning things.

Sorry for waxing pedantic on this one.

In article . com,
wrote:

GREAT explanation Richard. That's exactly the way I understood it too
(and why I am trying to be pretty careful with my ramp rates). If you
ramp up too fast then it could cure before it has a chance to get rid
of the voids. I'm not exactly sure what happens if you ramp up too
slow but it must be a bad thing


The more I think about this, and based on what Richard and others have
said, the more I think that you don't need a sophisticated controller,
particularly in light of your financial constraints. Maybe a
well-insulated box, (oven) with the correct size heater, would work
fine. You might have to experiment a bit with heater size to fine tune
it. And, you could put in three small heaters with external switches.
Use all three for heat up, two for holding temp, and one for
decelerating the down ramp. A bit of experimenting with different
wattages might do the trick nicely.

"Smitty Two" wrote in message news
In article ,
Stealth Pilot wrote:


Sorry for waxing pedantic on this one.

Spoken (written) very clearly, no apologies needed....

On Thu, 29 Sep 2005 18:32:03 -0700, Smitty Two
wrote:

In article ,
Stealth Pilot wrote:




Sorry for waxing pedantic on this one.

so why didnt you step in and answer the guy's question?

In article ,
Stealth Pilot wrote:

On Thu, 29 Sep 2005 18:32:03 -0700, Smitty Two
wrote:

In article ,
Stealth Pilot wrote:




Sorry for waxing pedantic on this one.

so why didnt you step in and answer the guy's question?

I'm not positive which question you're talking about, because I don't
remember all the details of our little saga. Recumbent bike guy came in
here looking for temperature control info for his custom carbon fiber
bicycle. Someone suggested sitting and watching, with a thermometer and
a switch, so I brought up the shortcomings of ON/OFF controllers and
suggested he might need a PID controller. (Based on subsequent
discussion, I've reconsidered that, and said so.)

I think it was Dan who had an inexpensive controller from ebay. He was
asked whether it was a PID type, and by looking at the picture, I could
tell it wasn't, and I said so. When Dan said he didn't know what a PID
controller was, I didn't interpret that as him asking for a detailed
explanation. I thought I did answer the relevant question, which was,
"is the ebay model such a beast?"

Which brings me to my best attempt at trying to answer your question: I
actually thought that the nitty gritty details of what PID controllers
do was outside the scope of RAH and beyond the participants' interest.
But since your explanation seemed incomplete, I wanted to offer a little
clarification in case anyone was still actually reading this thread!