# finding the opening to cook chamber.................

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#### bob1961

##### Smoking Fanatic
Original poster
I read a few threads asking how to find the square inch area for this opening, so I ran my numbers for the opening i'll need with this circle calc....

http://www.1728.org/circsect.htm

I made a 1/4 scale drawing of a 30" dia tank only using a 1/4 of that circle, then drew squares that equals square inches, counted them and doubled that figure to get total square inch area it covers....I used what I have on the drawing in that calc and I came up with a different number then the calc ??....the segment area with that calc gave me 100.64 square inches, where on my drawing I came up with 126 square inches for the same numbers I put into it ??....

my drawing showing 6" height x 24" across = 126 sq in and 7" x 25" = 151 sq in....

not sure why I got two different numbers ??....if people are using the numbers from this calc there opening looks to be too small....

Bob, have you check your numbers using the calulator at www.feldoncentral.com? Run your numbers on that calculator and compair them with the numbers you got from the other one. A number of smokers that are on this site have been built using feldon for running the numbers.

that link showed up as a blue fish tank picture, I use that calc for the rest of my numbers....this is what I got using my link saved....

http://www.feldoncentral.com/bbqcalculator.html

the link I posted does not come out with same numbers that I get using paper and graph grid....

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Bob, morning..... Feldon's has some inherent faults.... The big one is using the firebox to calculate all the parameters....
Using the circle calculator, as you did, use the radius and segment height to figure the numbers.... The radius is a constant and the segment height will be used for the FB/CC opening height and the height of the RF plate.... plug in different segment heights until the "segment area" comes out close to the FB/CC opening in the calculator below..... Chord AB will be the width of the RF plate...
I have adjusted a calculator based on Feldon's and making adjustments to compensate for problems folks have run into using it...
All of these calculations are based on the volume of the CC...

Some of the numbers should be cubic inches.... like the stack volume and FB volume... others are square inches like the FB/CC opening.... If you have any questions, PM me.....

Many folks have used this readjusted calculator and are very happy with the results......

.......click on image to enlarge ........

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how did you account for partial squares? Maybe you over gues-ta-mated?  Just for fun, draw out the full half circle, not just the quarter, bet you come up with another number

I guess I don't understand what you are doing, as I don't get 126 when I count the 6 inch high squares. I count 43 full squares, and 11 partial squares, plus a few little slivers that I didn't bother to count. Even counting all the partials as full, only gets me to 54, or 108 when you double it, and that is doing some accounting that I ~know~ is going to end up with a high estimate. So I guess you gotta explain exactly how you are coming up with your numbers, because I don't understand

There is one additional way to measure the opening.... Draw a full scale of the opening.... Draw a 10 x 10 sq. in. on the same piece of paper.... Cut out the full scale drawing.... weigh it in grams.... weigh the 100 sq. in. drawing... calculate the opening....

Dave

Sorry Bob-for some reason the "bbqcalculator.html" didn't paste when I posted the link. Pretty fish though, don'tcha think? :biggrin:

yeah there were Thumbs Up ....yeah I got different numbers just now :th_crybaby2: not sure how or what I did ??....I got 102 for 6" and 127 for 7" height, weird....

I guess I don't understand what you are doing, as I don't get 126 when I count the 6 inch high squares. I count 43 full squares, and 11 partial squares, plus a few little slivers that I didn't bother to count. Even counting all the partials as full, only gets me to 54, or 108 when you double it, and that is doing some accounting that I ~know~ is going to end up with a high estimate. So I guess you gotta explain exactly how you are coming up with your numbers, because I don't understand

with my drafting back round work I kind of have an eye for looking at partial's, then seeing which two fit and counted 1 for every time I seen a new square....

Yeah I've got a decent eye for that as well, but I didn't even try to since I was coming up with a smaller number anyway. Sounds like you went all googly-eyed when you were counting the first time and got it correct now.

I do kind of the same think, but skip any squares less than 1/3rd, that gives me room for error and the thickness of the metal. Never hurts to go a little big, Plus, if you slant your reverse flow plate, if you go to small here, it will be worst at the other end,....right?

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