Pompeii heat up?

[david s]

Re: Pompeii heat up?

Thanks for the detailed input, Ken and Lee. It's interesting that after the initial temp drop the dome and floor coast at nearly identical rates. This statistic suggests that the oven hastruly equalized and that the dome will always remain hotter than the floor since heat rises.
John

"The higher the heat, the greater the heat loss". That's why the cool down is not a straight line graph. Given time the floor and dome temps will equalize, providing the floor is adequately insulated or does not contain dampness.

[Aegis]

Re: Pompeii heat up?

There are a lot of variables from oven to oven.

Brick chemistry, manufacturer of the brick, how they are cut, type of mortar used, oven shape, ambient outside temp & humidity, humidity of the insulation, type of insulation, door design, heat breaks...

Whew! I don't think you can pick any one variable to explain differences in ovens .

Hi Ken, You are correct with the number of variables that are contained in each oven build and data collection. And to compare a 5 degree difference would be impossible for sure. Most of the variable could be discarded when trying to explain a 50 degree or more difference.
An example would be using thermocouples to measure temp. Unless you have them calibrated and referenced to an ice bath they will not be accurate. So why do I use them without an ice bath reference? Simple the difference maybe 5 degrees between displayed temp and actual temp. But I am looking at relative temps. If my temp read 87 degrees on a day that is actually 83 degrees big whup! I am not recording daily temps, but pizza oven temps at 700,800 or 1000 degrees If I read 1005 degrees is it going to matter to me and the burnt pizzas if the temp was really 1000F or 1005 F? Now if the temp difference was 50 degrees and I am trying to bake bread and the oven is at 375 F and I want at least 425 F (which is what the meter reads) for some oven spring then this is a problem.

I would say the basic design of the Pompeii oven cannot be compared to a barrel vault when discussing changing number of heat breaks, but Pompeii to Pompeii oven with approx. same mass and insulation. Then anything that is a significant difference could be attributed to more or less heat breaks.
I do wish I had more heat breaks in the landing/vent area.
Hope I didn't ramble too much.
btw Ken I love your build! GREAT Job!

[Ken524]

Re: Pompeii heat up?

There are a lot of variables from oven to oven.

Brick chemistry, manufacturer of the brick, how they are cut, type of mortar used, oven shape, ambient outside temp & humidity, humidity of the insulation, type of insulation, door design, heat breaks...

Whew! I don't think you can pick any one variable to explain differences in ovens .

[Lburou]

Re: Pompeii heat up?

Ken, how much difference does a heat break make in these temperature curves? I have three....That could be some of the difference between our numbers rather than insulation, yes?

[Ken524]

Re: Pompeii heat up?

Ken, how thick is your floor insulation?

Chris, I have 2" of refractory board under the floor.

[Aegis]

Re: Pompeii heat up?

As soon as the monsoons stop here in the northeast, I'll fire my oven up and take recodings on heat up, saturation, equalization and cool down. I will use four of my thermocouples for these measurements. One is located in the center of the hearth floor about 1/2" from the top surface. Second is under that same hearth brick but above the foamglas. These two readings will give me hearth temps and when they are close to equal saturation of the hearth. I have a similar set up on the right side of the dome which will give me dome temps and saturation of the dome. I will intentionally fire the oven until both get saturated and then let the equalization take place followed by the cool down. Unfortunately this won't happen until next weekend. Too much rain and no roof! Ugh! Need to finish chimney and put the last couple of trusses up.

[Lburou]

Re: Pompeii heat up?

And my cool down numbers.

http://www.fornobravo.com/forum/f8/heat-transfer-metrics-under-door-heat-13372.html

I also noted Ken's floor and dome curves and how they parallel. The question that had was what would the cool down of Ken’s oven have looked like with another hour or so of saturation. Would the floor have been closer to the dome, more like the seperation further down the cool down curve.

It’s also interesting that my cool down is so close to Lee’s numbers.

My numbers.
850 500 400 250 - ? - ? -and 108F 7 days later

Lee’s numbers starting at 9am the following days
500 337 250 184…

Chris

Yes Chris that is interesting....We have a trend here

[SCChris]

Re: Pompeii heat up?

My dome has 6 inches of insulation plus vermiculite filling the enclosure and 2 inches of rigid insulation under the floor.

Ken, how thick is your floor insulation?

Thanks

Chris

[SCChris]

Re: Pompeii heat up?

And my cool down numbers.

http://www.fornobravo.com/forum/f8/heat-transfer-metrics-under-door-heat-13372.html

I also noted Ken's floor and dome curves and how they parallel. The question that had was what would the cool down of Ken’s oven have looked like with another hour or so of saturation. Would the floor have been closer to the dome, more like the seperation further down the cool down curve.

It’s also interesting that my cool down is so close to Lee’s numbers.

My numbers.
850 500 400 250 - ? - ? -and 108F 7 days later

Lee’s numbers starting at 9am the following days
500 337 250 184…

Chris

[GianniFocaccia]

Re: Pompeii heat up?

Thanks for the detailed input, Ken and Lee. It's interesting that after the initial temp drop the dome and floor coast at nearly identical rates. This statistic suggests that the oven hastruly equalized and that the dome will always remain hotter than the floor since heat rises. Lee, if your oven holds its heat this nicely I'm thinking I will add a minimal (1") amount of thermal mass to 'tighten' up the dome and call it a day. What I'm really curious about is how the 3.75" SS-topped firebrick floor will behave. Can't wait to try it out!
John

[Ken524]

Re: Pompeii heat up?

Based on your normal heat-up times, what does your coasting curve look like?

Here's my oven's cool down curve. This was done a few years back with a fairly new oven. I should do another one for fun.

My oven is a standard 42" Pompeii with custom cut bricks. 3"-4" ceramic blanket with 3"-5" vermiculite/Portland on top of that.

http://www.fornobravo.com/forum/f17/oven-temperature-graph-4123.html#post33220

Looks like Lee's is a tad better insulated than mine.

[Lburou]

Re: Pompeii heat up?

Hey Lee,

Question: Based on your normal heat-up times, what does your coasting curve look like? The reason I ask is in response to what John (Aegis) did to finish his dome, which is add 1.5" of thermal mass (firebrick chips). I'm trying to determine whether or not the added mass is going to provide a decent return in residual heat for the amount of extra wood needed to heat it. This assumes insulation that exceeds acceptable levels.
John

John, my dome is always hotter/warmer than the floor of the oven. 80 to 90 degrees F hotter at pizza temperature and 30 degrees warmer four days later -does my 3/4 inch of castable on the outside of the dome make that much difference, or, is everyone's dome that much hotter/warmer? I don't know, but suspect any dome built to the plans will be that way.

Pizza Temperature around noon w/floor temp around 750-800 F

• 1 PM floor temp after pizza around 720 F
• 9 PM floor temp= 600 F
• 9 AM floor temp= 500 F
• 9 PM floor temp= 380 F
• 9 AM floor temp= 337 F
• 9 PM floor temp= 280 F
• 9 AM floor temp= 250 F
• 9 PM floor temp= 210 F
• 9 AM floor temp= 184 F

[Aegis]

Re: Pompeii heat up?

Hi All,
I am guessing at this, but intuitively the extra mass in the dome doesn't affect pizza heat up times all that much. My reasoning is as follows, the first several inches will be close to high temp and the next will some temperature lower. The temp. differential between the two is now relatively small and the outside mass will not be sucking the heat out of the inner dome nearly as fast as it was when the dome was cold with a first fire. Add the insulation into the equation and the thermal transfer rate slows way down(no place for the heat to go but back into the dome) Kinda of a kin to using a water hose to fill up a sealed container, at first it flows in quickly then the pressure builds and pushes back on the water until the water stops.
Unless I had another pompeii built beside it without the extra mass this is only speculation and maybe wishful thinking! Just remember- if you don't think too good, don't think too much! lol

[GianniFocaccia]

Re: Pompeii heat up?

Don't forget that you can cook for three or four days after pizza. Two days after heating the oven to pizza temps, you could easily reheat it in 45 minutes

Hey Lee,

Question: Based on your normal heat-up times, what does your coasting curve look like? The reason I ask is in response to what John (Aegis) did to finish his dome, which is add 1.5" of thermal mass (firebrick chips). I'm trying to determine whether or not the added mass is going to provide a decent return in residual heat for the amount of extra wood needed to heat it. This assumes insulation that exceeds acceptable levels.
John

[Lburou]

Time to pizza temp isn't such a big deal!

....snip.... if it can't be ready for pizza in under an hour then I don't think we'll get sufficient usage....snip...

I must report the evolution of several ideas about choosing a design, making the oven and using it in the 'process' of our oven.

I went through all kinds of iterations for the design, beginning with a barrel oven and ultimately to the dome oven of the pompeii plans. I must say that we were uncomfortable with the reports of an hour and a half to get the oven ready to cook pizza. After all was said and done, I built the oven as close to the pompeii plans as I could -that meant no additional mass in the floor because of the extended heat up times, and very little added mass on the outside of the dome for the same reason.

Now, to the point of your question. I've found that the time to get the oven to pizza temperature and then to equalize isn't the hardship I had first envisioned, you soon get into a routine which will allow plenty of time for food prep while the oven is on its way to being ready to cook.

I can heat the oven to pizza temperature in one hour, but the whole experience is better when It gets time to soak up all that heat, which will allow similar cooking conditions from one time to the next. In short, you will almost surely not mind the 90 minutes it will take to heat the pompeii oven built in accordance with the plans.

Don't forget that you can cook for three or four days after pizza. Two days after heating the oven to pizza temps, you could easily reheat it in 45 minutes. Don't forget about that residual heat. It feels great to put a couple potatoes in there any time and have them cooked to supplement a meal otherwise cooked inside.

HTH

Insulate, insulate, insulate