Maintaining oven temperature

P

PaulBC

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Dec 31, 2014
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My homebrew oven i built is 240v, with a 17 ohm element. This works out to 3388watts, and the oven is about 14x9x9 inches. It heats up to 1050 Celsius in about 10 minutes or so.
My problem is the temp goes above and below my target temperature by about 3-4 degrees Celsius (for whatever reason my PID only allows 1200 degrees, whether it be C or F, so I have to use C to get the temps I want. also I'm in Canada)
I have never used a commercial HT oven, so dont know how well they maintain their temps
I figure I am probably losing too much heat, but was wondering if anyone has had any luck with a second element, one that is a higher resistance, to maintain the temps. I figure that my element gets too hot that why it overshoots the temp I want. I could alternately change the element in there now for a higher resistance one, lowering the wattage.

The door fits fairly snug and not a lot of heat is lost really, I put my hand around it and do not feel too much.

How much should I worry about that difference in temperature?
 
First thought is that your not letting it set at your desired temp long enough. 3-4C is not a huge jump up and down but you may be able to get it better.

How long have you left it on a temp for and had it jumping temp? I ask because if it only takes ten minutes to get up to temp the whole oven may not be at equilibrium try setting it to your temp and leaving it at that temp

for 45mins and see if it is still swing temp.

DR...
 
Thanks for the response. I will give it a good long run tomorrow, see what happens after 45 minutes. There is an auto tune function that when enabled, dials it in a bit better but it only runs for 20 seconds then its 'tuned' i guess, but fluctuated worse then when it was tuning.
The instructions are pretty terrible
 
I've got two different Chinese PID controllers. One came with significantly better instructions than the other. I also ran into the same 1200 degree limit -- weird, eh?

It may help to set the P, I, and D values manually. I haven't done this since my oven only fluctuates a degree or two at 1500 F. I also wonder if at that temperature the errors in the system add up enough to make the temperature reading fluctuate by that much.

My oven has a 4.5" x 4.5" x 15" cavity and draws about 1500 W peak. It heats up significantly slower than yours (about 70 minutes to 1500 F) but has no trouble holding there. I do find that it will settle about 10 degrees low, which is easy to compensate for and probably due to the PID tune.
 
Questions:

1/ What is the controller? Do you have a link to the manual?

2/ What is the thermocouple? Type obviously, but also diameter of wire, is it sheathed or Mineral Insulated, Outside Diameter of the assembly?

3/ What are the controller settings? Particularly the output cycle time? Are you switching an SSR, or a relay/contactor?

Thermocouple details are important because the thermocouple needs to be the fastest-responding thing in the chamber. If it is insulated, the tendency will be to heat the outside until the heat soak to the inside reaches setpoint, then switch off, then still have heat soaking to the inside (the measuring junction). The inside temperature will remain higher than setpoint until the outside temperature drops below the setpoint by a big enough margin to drive the heat flow to the outside.

The output cycle time is the time period over which the output switches; if the controller is giving 50% output for example, With a 30-second output cycle time, it will be on for 15 seconds and off for 15 seconds. with a 2-second output cycle, it will be 1 second on, one second off. The best the controller can do is to hold the average temperature over the time period so shorter cycle times generally give tighter control. On SSRs, 2-seconds is usually OK. On relays/contactors (contactors are basically just high-current relays), longer cycle times are usually necessary: 5 seconds is probably a good compromise between relay life and cycle time for a hobby HT oven IME.

If you've bought a cheap controller online, the manual is likely to be pretty sparse: good manuals are expensive. Usually, they are just good enough for someone familiar with controllers to use. If it's your first experience of a controller, you'll find the learning curve is steep. My advice is never to buy a controller until you've read the manual online. This gives you a fair chance of working out whether you'll be able to make it work, and gives you a link you can put in a "help" email to give the guys who know controllers but have never used one of yours, a chance of giving useful advice.
 
Is this the manual?

http://www.sanyoutech.com/UploadProductPic/200991715182749860.pdf

If so, it doesn't seem to give output cycle times specifically, but the CtL parameter at the bottom of the first column on page 2 might do it. I'd check to see what the value is and try changing it up or down by 1 at a time, to see what effect, if any, it has.

As far as thermocouples go, I'd really recommend you speak to Omega. Make a cup of tea/coffee, have a pad and pen handy and expect to spend some time on the phone; their technical guys/gals really know their stuff.

Personally, I tend to use Mineral Insulated Type N thermocouples with grounded junctions (the TA4 does not seem to support type N, so you will need to use type K) and transition joint to a metre of trailing lead, which will connect straight into the controller terminals.

http://www.omega.com/pptst/KMTXL_NMTXL.html

Something like a KMTXL-125-G-12 would be my choice for a controller that does not support type N: The Omegaclad XL sheath will give the best long-term stability you'll get from a type K, and will allow a grounded junction to give fast response. 1/8" diameter is stiff enough to keep it from sagging and 12" length should be enough for most HT kilns.

Interference can be an issue though, when using grounded junctions. I am in England and use thermocouples a lot in my day job, so have ways of overcoming the grounded junction issues. However, I can't speak for wiring codes and the like where you are: one of the reasons I advise you to speak to Omega.

If you forge, I'd strongly recommend ordering a KHXL-14U-RSC-24 at the same time.

Used with a TM902C readout bought off ebay, it lets you set up a forge to a measured temperature, rather than an eyeballed estimate. The TM902C only reads in degC, so you may need a calculator handy, but all those I've tried read to 1370 degC or thereabouts (about 2500 degF) and are as accurate as any British, American or German readout I've tried when tested against the works calibrator. At around $5-6 delivered, the degC-only limitation seems forgivable.
 
I've got that same MyPin TA4 controlling my oven. It cycles at 1 second period with the default settings.

Having thought about your problem statement I wonder if you've just got too much wattage for that size oven. Do you have room to run a 2nd coil in series? This would give you around 1500 watts. This would give you a similar effect as cutting the cycle period in half. I guess running the coil off 120 VAC would have the same effect (you'll want to do it off a 20 A circuit though).

I searched and searched for info on this controller months ago and found this third-party manual for the TA4 written by someone who speaks English:

http://zhyla.net/files/mypin_ta4_manual_good_one.pdf

It was enlightening to me since I had no idea what a PID controller was at the time.
 
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thanks for the responses. I have a ton of elements, and oddly enough the one i have in there now failed. i will try a 26ohm one that i have, bringing my wattage down.

I do not forge i just HT, i will call omega and have them sort me out
 
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