Rotisserie roasting (electric oven)

[Wushoes]

Re: Rotisserie roasting (electric oven)

Good work. Dont forget that when youre actually roasting, therell be a temperatue drop when the milo tin, and the mass of beans start to absorb heat (heat flows from hot to cold masses). Obviously, the more beans, the longer the lag time until the ambient air around the thermocouple regains original temperature.

Have fun logging.

-Robusto

Thermal equilibrium...gotcha

I finally found a satisfactory setting. Oven ramped up to full temp with both top and bottom elements on and then using only the bottom elements to heat.

I am able to regulate the temp quite satisfactorily by either or a combination of slightly opening the door and/or using the thermostat on the oven.

YAY! Now I know my oven can get the temps required and some.....what this means is that I can overheat the oven...load the beans....wait for the temp drop to stablise and control the oven manually to the temperatures I want.

What my graph shows me is that in the initial heating stage with both top and bottom elements on (with significant overshoot of the temps I need to roast) I can load the drum in there, turn off the top elements....wait for the temps to drop and control the temperatures from there, rather than letting the temperatures stabilise and then opening the oven door to only let the temperatures drop for a 2nd time.

Roasting here I come...oh I got my Coffee Snobs starter pack today!

[robusto]

Re: Rotisserie roasting (electric oven)

Good work. Dont forget that when youre actually roasting, therell be a temperatue drop when the milo tin, and the mass of beans start to absorb heat (heat flows from hot to cold masses). Obviously, the more beans, the longer the lag time until the ambient air around the thermocouple regains original temperature.

Have fun logging.

-Robusto

[Wushoes]

Re: Rotisserie roasting (electric oven)

Lots of playing around with trying to get stable temps.

This graph shows me operating the oven with top and bottom elements on. However the temps werent very stable as the thermostat kept on turning on and off the bottom element.

Temp hovers around 200 deg C mark...never rises due to the transient nature of the thermostat controlling the elements.

[Wushoes]

Re: Rotisserie roasting (electric oven)

Heres a pic of inside. You can see 4 elements total. 2 on top and 2 on bottom. The red dot is the hole I drilled to feed the thermocouple through.

Still waiting on my green beans from Coffee Snobs....itching to roast

[Wushoes]

Re: Rotisserie roasting (electric oven)

Heres a list of the things I have done so far after I bought my oven.

1) Rewired the oven so that the rotisserie motor runs all the time, rather than just on "rotisserie" mode with only the top elements on.
2) Drilled a hole through the back of the oven so that a thermocouple can fit through to measure and datalog oven temperatures during roasts.

Things to do:

1) Get my milo tin cut up and bend in flaps for agitation. Rivet in flywire.
2) Get a professionally made SS drum made.
3) Change the synchronous AC motor. 2.5 RPM seems a tad too slow for my liking.

In the photo below:

Autoranging and datalogging DMM and thermocouple, Warranty voided Mistral oven with unmodded Milo tin, and digital kitchen timer.

[Wushoes]

Re: Rotisserie roasting (electric oven)

You are right...it was the heat shrink that started to burn...even though the fibreglass was turning a tan colours.

Here is a graph I have compiled of my oven temps with a probe type TC...I had to drill a hole in to the oven and shove it in from the back....dead centre on the back....but didnt reach right in to the centre of the oven....so I got less reliable results than the wire type TC....but I got readings that would be off by a some degrees none the less..

The oven is operated with all 4 elements on. Where the temp drops down at 10 minutes was when I opened the oven door fully for 30 seconds to simulate me putting in the roasting drum. 30 seconds seemed a bit zealous....and the temp dropped by atleast 50 degrees. I could have loaded the drum in less than 10....giving me a drop of less than 30 degrees.

I will go buy another TC and take off the heat shrink and protect it with some copper tubing.

I will next try just the bottom 2 elements on and see how that differs.

Cheers.

[JavaB]

Re: Rotisserie roasting (electric oven)

My thermocouple was http://www.jaycar.com.au/products_uploaded/productLarge_5275.jpg; QM1284 from Jaycar. The website says it is rated for over 250 deg C. Maybe its not the junction that gave in...maybe long the wire somewhere it just gave in...because I had half of it in the oven...not just the tip....as it was the wire type...not probe.

The thermocouple itself will take a lot more than 250C, the fibreglass insulation around the two wires is also good for a lot more than 250C.... but right at the junction they have used heatshrink tubing as insulation (and to prevent the end of the fibreglass fraying..... that melts at about 250C.....

I removed the heatshrink, and inserted the flexible probe inside a small copper tube which holds the fibreglass in place (and allows for easy placement). Belinda did a similar thing. Just the junction projects from the end of the tube - prevents the beans from beating the hell out of the junction as they rotate.

The two metal wires making up the thermocouple lead ( Nickel/Chromium for one and Alumel for the other) are quite thin and rather brittle by the way... flex them too many times and they will snap- so they need physical protection (not just at the junction).

[Wushoes]

Re: Rotisserie roasting (electric oven)

I was wanting to use a thermocouple and DMM to log roast profiles and things.

But I will get one of those freestanding ones.

My thermocouple was http://www.jaycar.com.au/products_uploaded/productLarge_5275.jpg; QM1284 from Jaycar. The website says it is rated for over 250 deg C. Maybe its not the junction that gave in...maybe long the wire somewhere it just gave in...because I had half of it in the oven...not just the tip....as it was the wire type...not probe. I have a probe type TC that was included with the DMM but I need to drill a hole in the oven to mount it.

[robusto]

Re: Rotisserie roasting (electric oven)

Too bad about the thermocouple.

But many housewares shops and barbecue shops sell an ideal thermometer for the application: it has a face like a clock and a long skewer-style sensor.

Drill a hole to push the skewer end into the oven, keeping the face on the outside.

OR

There is one, for about $10, which I use on my barbecue set up, without the long probe, and sits on a a built-in stainless steel stand. The entire unit has to be inside the oven.

-Robusto

[Wushoes]

Re: Rotisserie roasting (electric oven)

After getting the Digitech QM1538 DMM and a wire k-type thermocouple from Jaycar today, I measuring how long it takes the oven to get up to temp this is what I got.

0 seconds - room temp...~24 deg
1 minute - 82 deg
2 min - 150 deg
3 min - 200 deg
4 min - 228 deg
5 min - 245 deg

I pulled the thermocouple out after this because the nylon or whatever material that is insulating it started to turn brown and the looks like i burnt the end of it! Ive definitely destroyed it because now its not measuring temperatures. Oh well it was on $10, but it was rated for 250 deg plus. Maybe I can get a refund? :P

Opening the oven drops the heat dramatically....fell down...and fell down rapidly....to Id say 150 deg after opening the door for atleast 30 seconds. I cant be sure because I was pulling the thermocouple out of the oven. This would have major implications on starting a roast because I have to open the door to load the rotisserie rod and drum.

So now I know there is no problems about the temperatures reached inside the oven.....just a matter of getting a thermocouple that can stand the heat. The two dissimilar metals (chromel and alumel) are still connected at the tips...I wonder what made it fail.

[Wushoes]

Re: Rotisserie roasting (electric oven)

point taken...

cheers

dave

[JavaB]

Re: Rotisserie roasting (electric oven)

Taking that into consideration then...cant you just cut and splice the wire - no need to disconnect it at the welded point?

That is where I have a problem. People who have PRACTICAL electrical skills wouldnt even ask about soldering..... and they would know how to overcome the problem.

There is a big difference between knowing the theory and being a "practitioner" with electrical items- especially those made in China..... they use some "interesting" techniques..... which can challenge even the most experienced at times......

The only thing we want roasted is beans - not coffeesnob members......

So be VERY careful when doing any such mods.... and think carefully if you truly do know what you are doing - if not 110% sure, then dont attempt it.....

Ive seen skilled electricians electrocuted (not fatally) - one was even temporarily blinded by a flash when the device he was working on blew up in his face - and they were experienced tradesman.

[Thundergod]

Re: Rotisserie roasting (electric oven)

Taking that into consideration then...cant you just cut and splice the wire - no need to disconnect it at the welded point?

[Wushoes]

Re: Rotisserie roasting (electric oven)

Its all good....Ive done quite a bit of electrical engineering whilst studying mechanical engineering. I full well understand the implications of what I am doing. But I thank you for being so caring.

[bluedevilfish]

Re: Rotisserie roasting (electric oven)

Some wise advice from JavaB.  Burnt beans are one thing, but burnt coffeesnobs are an entirely different matter (much more difficult to replace).

Just because some of the elements seem cooler doesnt mean that the overall combined oven temperature will be too low.  Slow down, take a deep breath, wait till you get your starter pack of beans, and try your oven unmodified first.  They will crack and you will be happy. ;D

Sorry if this seems to be preaching but your zeal for modifying your as yet untried oven has me a little nervous (safety first is a good thing).  

Matthew