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TB - Ovens - PID Controllers

On my oven I am using an Robert Shaw infinite switch (110V) and a digital meat thermometer to check the temp. Does anyone have any ideas about  controlling the infinite switch via a thermocouple to hold the temp steady. How does everyone else control their oven temp. My infinite switch will turn the oven on and off based  upon the internal thermostat in the switch, which is not dependent on the internal oven temp, so,  in a well insulated oven, the temp keeps getting added to and not decreased.  (Mark Bolan)

    Myself, M-D, and a few others are using PID controllers which read the internal temperature of the oven from a thermocouple.  The PID controller is a computer controlled thermostat, accurate usually 1 degree F.  It uses fuzzy logic to maintain the temperature.  They can be purchased through (M-D can sing the praises...) McMaster-Carr.  (Mark Wendt)


For those of you that have PID temp controllers on your ovens, how are they wired. Are they wired directly to the power source and then on to the mica strip, or is there other secret electronics involved?  (Mark Bolan)

    It depends on what type of controller you have.  M-D and I own the solid state relay output, which has a 12 - 32 VDC output, designed to drive a solid state relay, which in turn drives the elements.  I haven't seen a PID controller in the size we use that has the capability to be able to handle the current directly of the heating elements.  I'm using the 1/16 size controller, which I believe is the same size as M-D's.  The current going through the relay is fairly high, depending on the wattage of your heating elements.  I use two 850 watt elements, which gives 1700 watts, or approximately 17 amps.  The little controllers just don't have the power circuitry to be able to handle that much current.  The other reason I like the relay type, is that it allows me to use to isolated voltage sources, so that I'm not putting that much strain on a single circuit.  I have the heating elements hooked up to a circuit that's rated at 25 amps, and the PID controller and fan motor hooked up to a 15 amp circuit.  Even after 2 or more hours of running the oven, the cords for each circuit are nice and cool.  No heat in the cords, no over-amping the circuits, no popped circuit breakers or worse.  (Mark Wendt)

      Yeah, what Mark said. My entire power source comes through one 30 amp breaker, and I've never yet tripped the breaker, nor heated up the power cord, despite having run the oven continuously, all day long. I'm running 200W less than is Mark, though. There's one other Lister of whom I'm aware using a PID. Perhaps he will enlighten us as to his particular application?  (Martin-Darrell)

        The only reason I ran a separate circuit was because of the high amperage of the two elements.  Someone running a single element such as yours should have no  problems using  a single  circuit.  Like M-D, I wired the controller and fan motor together through a single switch, so that I don't forget to turn on the fan when the controller is running, pumping current to the relay, and activating the heating elements.  (Mark Wendt)

        I used a PID controller on the convection oven I built a few months ago. (Some of you saw the demo at SRG.)  The controller I used was a 1/32 DIN running 2000 Watts through a solid-state relay.  I had the fan motor wired through the same switch as the controller and elements, like Mark's, so you couldn't forget to turn the fan on.  The whole thing ran on a single power cord through a 20A circuit.  I ran the calculations on it during the design process, so I wouldn't end up with more current-draw than a common 120v breaker would carry.  The open-coil heating elements draw 16A and the motor drew about 1.8A, so a 20A breaker handled it just fine.  (Tim Preusch)

    I let the PID controller turn a relay on/off.  The relay is between the power source and the mica strip.  My PID controller was marginal to handle the mica strip amperage.   (Bob McElvain)

      I think that would be true of any small PID controller. They are all made

      to control the process through a relay, rather than directly through the controller. I chose one with a DC output for the solid state relay (SSR).  A mechanical relay is too slow to allow the controller to adequately compensate for temperature changes, plus it wears out, whereas an electronic relay can handle whatever the controller throws at it, and has a much longer useful life.  (Martin-Darrell)

      I'm in the dark here, what is a PID control?  (Danny Twang)

        Proportional-Integral-Derivative controller. Each aspect of that has a portion of the control in the process. The Proportion is the error between set point and actual temp multiplied by a gain. In effect it is an adjustable amplifier. Integral is essentially the time aspect of the equation, and Derivative is the rate of change. To adequately tune one of these manually is some type of miracle. The best thing to do is buy one that has an auto-tune feature, with fuzzy logic, and let it figure out what needs to be done. You'll chase your tail for days on end otherwise. You'll learn a lot, but I don’t think its worth it.  (Martin-Darrell)

          In the immortal words of Charlie Brown:  ARRRRRRRGGGGGGGGHHHHHHHHHHHHH!!!!!!!!!!!  (John Channer)

            Actually, John, that's a similar reaction to trying to tune one of the things manually -- only it's much worse, more drawn out, more excruciating.  (Martin-Darrell)

        PID is a TLA for Proportional, Integral, and Derivative, the three numeric methods of control available to the unit. In short, an electronic vs a mechanical controller.  (Larry Blan)


I am building an oven and despite my best efforts to abort the process, the stainless steel stove pipe arrived today. I am considering installing a PID temperature controller and would like feedback on what others have used. Model names and numbers are appreciated.

I will be using a mica strip heating element and not currently considering convection. It is my thought that the heavily insulated steel pipe (pipe for coal fired furnaces) will absorb heat and radiate to level the heat across the strips. I may install a stainless (1/4 inch sheet) to separate the chambers    (George Wood)

    You can find the Watlow 965 series PID controllers on eBay all the time.  The specific ones to look for are the one's with "3CC0" in the middle grouping of the model number.  That means they are 110/240VAC powered, can use any kind of thermocouple, and have switched DC output so you can use a solid state relay.  (Mark Wendt)

    After many attempts and trying everything suggested on this list for the last 12 years, I would strongly suggest you build a convection oven.  This is the only true way to get consistent temps throughout the oven.  (Scott Grady)

      You might want to take a look at the bamboo toaster.

      I also use an oven of similar design with similar temperature uniformity. It is not necessary to go to the expense and complexity of of the circulating convection design, although an oven for treating dozens of rods simultaneously would be easier to design as a circulating convection oven.  (Larry Lohkamp)

      I agree with you 100%.  I wasn't going to say much about it, since ovens can tend to be almost a religious belief, but there's no way in the world you're going to get the temperature control of a pure closed loop convection oven in any other kind of oven, whether it's a re-fitted heat strip oven, a heat gun oven, or any other kind of oven that isn't a true closed loop convection oven.  The mechanics and the physics of the oven just won't allow it to happen.  (Mark Wendt)

        I know it is conventional ( U.S.) use of the word "convection" in kitchen ovens that are under forced air circulation from an internal fan.  This has always puzzled me as convection is the movement of air due to differences in densities of the air without mechanical assistance. I presume you and Scott both mean fan driven circulation when you say "convection"?  (Dave Burley)

          I suppose if you want to be very technical, geeky and pedantic about it, you should call them "forced" or "mechanical" convection ovens, but fan assisted ovens have been called convection ovens for as long as I can remember, both for cooking food and in the industrial setting.  (Mark Wendt)

            Well I am very technical, geeky, pedantic AND older, so I remember when these ovens were first offered to the public they were called "forced convection" ovens which also makes no sense.

            Here's a wikipedia comment on this subject.

            "The word convection actually refers to the natural circulation such as that caused by temperature differences and all ovens have these natural convection currents that vary with food placement and the position of the heating elements. Thus, strictly speaking, it is a bit of a misnomer to refer to an oven where the circulation is forced by a fan as a "convection oven" however one might construe the name to be a shortening of "forced convection oven."

            Which is what happened.

            Frankly, I don't care what you call it, especially since this is a convention in the US, it just may not be in other countries and to certain newbies using various heating strips without forced air.

            I agree with you and Scott, just trying to be clear for all parties.  (Dave Burley)

          This is interesting, I've never really thought about what is true convection for bamboo ovens, other than the heat source is separated by whatever means from the cane rack, allowing for the cane to be treated by circulating the heated air around it vs the heat strip under the cane, which we then call radiant heat.

          Now having said that, conventional or convection is not strictly a U.S. term or marketing strategy. We just replaced the range in our house last week. It is a Bosch dual fuel model with gas cooktop and electric "convection" oven. Bosch calls this "Genuine European Convection" and is trade marked on their web site and in the user manual. Next to this description is what they call "Advanced Circulation System" or "ACS",  another trademark.

          To me, it's just a fan that drives the price of the product up. However, my wife will give you a different take on it.   (Tom Vagell)

        Technically, it is the area of heat transfer that you all are discussing. When engineers look at heat transfer we deal with conduction, convection, and radiation. The technical issue with "bamboo cooking ovens" is that they work on either "Forced Convection" or "Free Convection". Forced convection requires some form of air movement within the oven by an external source such as a fan or blower, while a free convection oven uses the natural flow of air due to thermal density differences to create a natural air circulation within the oven. Radiation is an insignificant heat transfer medium at the oven temperatures at which we cook our bamboo.

        The physical design or geometry of either type of oven is critical to achieve a uniform temperature distribution along an oven length. Ovens with heat guns or blowers use the forced convection principle and air circulation to achieve this temperature distribution. An oven that is closed and uses an external energy source to the enclosure heats the bamboo strips based on a natural convection principle. Most ovens available today use some form of forced convection and controller to maintain a  set point temperature within the oven.

        I might mention that if one uses the MD heat fixtures sold by Harry Boyd, one is employing both the conduction and convection heat transfer principles to heat treat their bamboo. Here, the bamboo strips are tightly held to the aluminum fixture and receive conduction through the aluminum strips on two sides (assumes a 3 sided strip) while the enamel outside surface is being heated by the convection principle. The use of these fixtures provides improved uniform heating to each strip in my opinion.

        I hope this little explanation helps those discussing various oven types.  (Frank Paul)

          Thanks for expounding Frank. I use both - a closed-loop forced convection and M-D's fixtures. Being one of M-D's original guinea pigs, er, uh, beta testers... I built my convection oven based on his recommendations and with the fixtures sitting  in a PVC tube in my shop!  (Mark Wendt)

        I agree, you can maintain very tight parameters with PID Loop control, 0.01f. But why is that necessary when you can get close, with a high quality heat gun with rheostat control?

        One thing I can't figure out about the convection ovens is, where does the moisture go? I can't see where the fresh air intake is, nor the moist air exhaust duct. Almost like an air handler in a large building? Maybe add a DX coil to dehumidify? My oven runs on 100% outside air and maintains temperature close to set point at high, heat treating temps and almost dead on for lower temps for heat setting the epoxy.

        Don't take me wrong, cool equipment like these interest me! I can see a production rod shop having the convection oven, CNC mill, and a 4 string binder, but will anyone be able to tell the difference in rods that were bound by hand, heat treated in an iron pipe, and hand planed on a set of steel forms?

        Does every shop need three lathes and a milling machine, NO! But they're nice to have! I'm running out of room to work! LOL But I need all of them to build my $400 milling cutter!  (David Dziadosz)

          Because it's not a closed loop.  With a heat gun, you're introducing outside air, which has to be heated by the element of the gun before it can be sent over the strips.  The moisture is absorbed by the heated air.  Occasionally, with a closed loop oven, you'll need to open the door to let the moisture escape, then close it back up making it a closed loop.

          You don't need to be in a production shop to take advantage of the precision of a PID controlled convection oven.  Even non-production shops can take advantage of the repeatability of heat/moisture treatment that a convection oven gives over the typical strip element or heat gun oven.  It all depends on how much control and consistency "you" want to have over the process.  Myself, I'd rather have as much control over the entire process as I can, rather than leaving it to luck, or however the wind might be blowing that day.  You say "almost dead on."  I can honestly say, "exactly dead on."  There  is a difference.  (Mark Wendt)

            All that said, it seems to me that unless you have some means of measuring the temperature over the entire volume of the oven the temperature may still vary over the length of the strips.  The PID will maintain an accurate temperature where it's located, but does it sense the entire oven?  (Neil Savage)

              I did measure the length of the strip rack with 5 thermometers.  All varied less than 1 F the entire length.  (Mark Wendt)

              What you say is true unless the oven design accounts for the distribution of heat along the oven length. A number or years ago, a group of students (it was a senior undergraduate design project) and I designed and built an oven based on free/natural convection, a large thermal mass, and heat distribution along the oven length that maintains a uniform temperature (within about +_2 degrees F). The oven has a large 4 inch diameter iron pipe that is externally and uniformly wrapped around the pipe circumference with nichrome heating wire with about 1000 watt capability; the iron pipe provides the large thermal inertia. This means that the interior of the pipe maintains an almost constant temperature once the oven achieves the set point temperature of the controller. The oven does take about 30+ minutes to reach thermal equilibrium.

              The oven is 6&1/2 feet long with an active 6 foot length that has uniform temperature. The oven ends have additional nichrome heating wire wraps (calculated in the design) to account for thermal end losses of the oven - the oven from a thermal design viewpoint is a double ended fin with higher energy losses on the ends. Thus the temperature distribution along the oven shows a local higher end temperature than does the active 6 foot oven section.  The oven uses a single thermal couple in the oven center for PID temperature control. The oven could use a series of thermocouples for temperature control but the initial test design evaluation showed a uniform temperature distribution in the oven center section, indicating that there was no need to use multiple thermocouples for PID operation.

              The bamboo strips are held in the MD fixtures and the fixture is supported by two end racks about the center of the iron pipe.

              I hope this helps explain my previous comments about oven design.  (Frank Paul)


I understand what a PID does - adjust to a specific heat need to provide a stable environment by computer control - but what I don't begin to understand is how to hook one up if I purchase it on eBay.    It   may   as   well  be  a  Pig  In  Distress  with  my electrical /computer skills!  I also have no idea what a relay is or its function.  I just know I've read one needs both a PID and a relay.  My question is do these come with directions for folk who can usually change light bulbs (the standard screw in variety, not the complex tube kind!) with a fair amount of success?  If not, how does one go about installing a PID?  Does it need adjusted after installation, and if so, how does one do that?  It seems that using such a device will afford the consistent temperatures I'm after.  I know this doesn't have to be rocket science, but I also know I don't want any more strips that are charred in the middle.  Would also be nice to have a relatively set regime and not have to step off the deep end with every new build.  (Don Hansen)

    The first thing is to get a PID controller that has either 5 - 12 DC Voltage outputs or logic outputs to drive the solid state relay.  There will normally be at least two input posts that will take your temperature sensing input (thermocouple), two input posts that will take your 110 VAC input power voltage (for the PID Controller) and at least two output posts that you hook your solid state relay (SSR) to.  The SSR is where you attach the power input that will drive your heater elements.  You run the SSR inline on one of the power wires - it acts like a light switch.  When the SSR is energized, it allows power through to the elements.  When the SSR is de-energized, it opens the circuit, shutting off power to the the heater elements.

    The PID controller will keep the SSR closed till it gets to a certain temperature point, then will start rapidly opening and closing the relay as it reaches the set temperature.  Once the set temp is reached, using it's computer mind, the PID controller will close and open the relay enough so that the temperature remains constant.

    Just think of the PID controller as a high tech thermostat, because that's basically all it is.  It uses PID logic to ramp up to the set temp, based on it's "learning" the oven environment.  (Mark Wendt)

    This has been an interesting string about oven design and heat treating philosophies.    I’m not a Control Engineer but I have had specialized training on Process Control and Furnace Design, along with many years of hands on experience with process heaters and boilers. 

    My feeling in general is that we are over thinking oven design and I don’t believe there is a practical difference (for us) between a heat gun and air circulation ovens. 

    I chose to build an oven that circulates oven air past the heating elements which are controlled by a proportional–integral–derivative controller (PID).  I used a PID controller because it was cheaper,   analogue control would have been adequate.    The main reason that I didn’t use a heat gun oven was because I use a long heat setting regime with Epoxy.     I believed that having the oven humming away for 4 hours would be less annoying than the heat gun oven. 

    If I had to do it again I would probably buy a heat gun oven from Mr. Boyd and save myself some work.   That said the air circulating oven works pretty well.  I see vapor escaping from the door seal when the strips are wet and it quickly dissipates as the strip becomes dry.    Understanding that the MD fixtures will act as a heat sink, I put the bound strips in at 200 degrees f during heat up, knowing from experience will take 15 minutes to reach 350 f.   I know that this is somewhat subjective but is directionally correct. 

    Lately I’ve experimented with heat treating at lower temperatures in a vacuum.  Using a heat resistant vacuum bag over the strips held in MD fixtures gives some interesting benefits.    At 275 degrees f and -20” I get the pleasant nutty cane cooked odor in 15 minutes.     Also the atmospheric pressure presses the strips into the fixture and they come nicely straight. I realize that more detailed work needs to be done.  (Ray Taylor)

      Sounds like a very interesting experiment.  How were you able to get the nutty odor from the vacuum bag though?  (Mark Wendt)


 

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