Auber PID controllers question

[JimmyB11]

I installed a PID controller last year on my oven. I bought it to stop the high cycles of the oven thermostat. It seems to work but I have a question. When new, I ran it to "teach" the controller about the new oven. I start the oven and let it warm up for 30 mins. and when I start heat treating it is bang on 375 deg. plus or minus one.

When the strips go in, the temp drops quite a bit, 50 degrees or so but then it wants to run up to 400 or so deg. I want it NEVER to exceed the target temp ( or maybe 5 degrees). Is it functioning correctly of did I have unrealistic expectations. Perhaps it was not set up properly I don't know. Any advice from PID users is appreciated.

[eotr]

JimmyB11

The first thing I would try is "teaching" the PID with a set of strips and turning off the element when you reach 375.

You actually have two problems.

First - The temperature drop when inserting strips is normal unless you have already warmed them to 375. If your oven temperature is dropping 50 degrees I would suspect that your oven has a relatively low thermal mass. You can increase the thermal mass of the oven by using heavy gauge galvanized steel sheet for the internal oven shell. Some folks use aluminum but it is a poor choice due to it's low thermal mass.

Second - When you insert the strips the air surrounding them cools until the air and the surface of the strips are at equilibrium. If the air is not circulating the element will be at or near the set temperature of 375. When the PID senses 325 degrees it turns on the element, which is already at 375, until the air around the strips and surrounding air reach 375. The only way for the system to accomplish that is to raise the element temperature well above the set point. Then when the air and strips are at 375 the PID should turn off the element. Unfortunately the element is still well above the set point temperature. The only place for the stored element heat to go is into the oven air and strips - hence you will reach an equilibrated temperature significantly higher than the set point. The solution to this problem is to change the element to the highest possible wattage with the lowest possible thermal mass.

I built a closed loop two chamber circulating oven a number of years ago. It has been working well for me. The heating element is in one chamber and air is circulated through that chamber and into the "blanks" chamber, then back around into the element chamber.

I just tempered 6 blanks this morning so the numbers are fresh. I put all six blanks in at once.
Set Point - 375
Temp after inserting blanks - 362
Time to return to set point - 4 minutes
Temperature overshoot - 382 degrees

Normally I just turn off the element when the temperature reaches 371 or 372 and the remaining heat in the element raised the temperature to 375 +/-1. I left the element on this time to check the temperature overshoot. The element has a relatively high thermal mass, which accounts for the overshoot. I would have preferred a lower thermal mass/higher power element but I used what was available at the time.

Ideally you would want....
1 - Oven shell with high thermal mass to reduce the "insert" temperature drop
2 - Element with extremely high wattage and low thermal mass so that when it turns off there is little excess heat stored in the system
3 - Circulating fan

I think your best solution is to add a circulating fan. Otherwise you are asking the PID to control a system that is probably not controllable to the precision that you require.

[JimmyB11]

Excellent lucid response, it's possible I am not smart enough but you have given me some great insight. The strips were darker than I wanted but not burnt, maybe salvageable but it was a prototype so no biggy, lots of bamboo in the shop.

[JimmyB11]

What type of fan could work in such a harsh environment?

[eotr]

The fan is mounted inside the oven but the motor is mounted outside with a shaft through the oven body. I'll post some photos later this week.

[Don Andersen]

Jimmy,

Brent is bang on with the oven design. I've used a similar oven for many years. The circulating fan I now use is made for the exhaust gas on a natural gas home style furnace. The plumbers we've used for years provided me with a fan from a used furnace for free.
See: http://www.bamboorodmaking.com/html/and ... _oven.html
For a series of pictures.

Regards,


Don

[JimmyB11]

Don, I have oven envy, that is serious step up from my Cattanach design that has worked for 18 yrs but lacks the control and ability to hold target temps for our needs, back to the drawing board.

[WIHEXROD]

Don,
I have been working on that same oven design. Do you have any pictures from the inside that would show the way the inside fan duct work looks?
Scott

[eotr]

Scott

Here are some photos of my oven. Note that I used a seperate fan and motor so had to make an external motor mount. If you use a furnace exhaust fan like Don's you can mount it directly on the end of the outer shell.

The oven is two simple boxes with a divider that separates the heat chamber from the bamboo chamber. If I were to build it again I would make the inner box and divider out of 16Ga. galvanized steel to increase the thermal mass and reduce the temperature drop when inserting the blanks.

If I recall correctly I had all the sheet metal work made for around $200 at a local HVAC shop.

[Don Andersen]

Guys,

The fan I used looked similar to this one
http://www.deyparts.net/product/furnace ... 079/474597
It is very important that you place a diffuser in front of the fan. This is to breakup the air flow (homogenize the air) so you get a completely mixed air flow. No hot spots!

Regards,

Don

[FlyfisherX]

I have a simliar setup with likely the same controller. I have not fired it up. I also use a seperate heating chamber, but a smaller fan. I think you should be able to change the control characteristic for the controller, by adjusting the timing for the system how long it turns on the heater. It will take longer to get to the right temperature, but you avoid the overshoot.

If you don't have the 1 pager manual anymore, you should be able to download it.

Andreas

[fishbum]

I use a Chromolox 16xx controller and could not be happier with the way my oven operates. No fans, no double chambers just two pieces of furnace pipe with fiberglass insulation between them and an element salvaged from the broiler of an old stove. I do get overshoot on warm up but after it reaches the set temperature it stays rock solid. Put in the cane and get a 50 -100 degree drop, start the time when the oven recovers. I sometimes have six sets of strips on MD fixtures and the large drop is expected. It never over shoots coming back to the set point.

Jerry

[WIHEXROD]

Thanks for the pics Brent. I have all the items to make one of these, thanks to Mark Wendt. Just need to put it together. I didn't like the idea of having a motor with pulleys. Using the exhaust gas fan setup makes sense to me. We had our furnace replaced last week and the contractor says he can drop off the old fan.
Now I just will have to get it done and figure out the wiring:(
Scott

[eotr]

I just came across some photos from about 10 years ago that reminded me why I built my circulating oven.





7 minutes at 375F, but I think there was a hot spot

[T4R11]

The design I use was suggested by a forum member (Dbatt - a due credit) and has been fantastic. It consists of 6"x60x1/8" aluminum pipe, a Briskheat 2"x20" heat tape, Briskheat Digital PID controller, two duct cap ends with vents, wrapped twice with insulation surrounded by wood box and thermometers at each end. Metal lathe as a base to hold the cane. A little drop in temperature when you put the cane in then comes back very quickly. Temp is exact at both ends. Works great!

[satbuilder]

The oven I'm building is very similar to Mark Wendt's "Mother of all ovens". It is similar to Don's, and Brent's, and was highlighted in Powerfibers Vol 11. I have looked at the center divider and am modifying it to an insulated double wall for greater thermal mass and to isolate the heat source a little more from the cane chamber.

Recently I had to restring an auxiliary heater unit on my home heat pump. Those little insulators that come with the restring kit are perfect for isolating the heaters.

It's going to be a fun winter project. I've only had all the parts for 3 or 4 years, about time I got started on it.

Kurt

[Mike McGuire]

I think some of strange behavior of controllers relates to not understanding what is happening with the bamboo in the oven. Before it can come to heat treating temperature, the water in it has to be boiled off, and that takes quite a bit of heat. It only takes one calorie to raise one gram of water one degree Celsius, but it takes 512 calories to vaporize it. With my oven setup, the PID controller stabilizes the temperature of the oven wall. I put the bamboo strips in with a thermocouple attached to them in the middle to measure their temperature. What I find is that the oven wall recovers within 5 minutes or so, but the bamboo temperature as measured by the thermocouple may take half an hour or more to reach the oven wall temp. Steam is coming out the vents of the oven while this is going on. My practice then is to set the oven wall to 300 degrees--a temperature which is below where heat treating effect occurs, but sufficient to drive out the moisture. When the bamboo has reached 300 and no more steam is seen, I set the wall temp to 350 degrees, my treatment temperature. It gets there in about 5 minutes with the bamboo temp tracking it closely with no more water to vaporize. The overshoot is negligible. I hold the bamboo at 350 for half an hour and then remove it from the oven.

The problem with "teaching" the controller with bamboo strips in the oven is that you have varying effective heat capacity as the moisture in the strips boils off. The controller is likely designed for constant heat capacity.

Mike

[satbuilder]

You could also try placing your bound strips into a cold oven. You won't overshoot that way.

Kurt