# Boil Off Rates With Different Pot Diameters



## Bribie G (8/11/10)

I'm getting frustrating results with my new Crown Urn. After a lot of brews in the Birko I hadn't done precise calcs, I just KNEW what the initial strike water level SHOULD look like for either a sixty minute or ninety minute boil and hit it spot on every time, but with the Crown I'm frustratingly getting an extra litre or two. I even filled the Birko to familiar level and transferred to the crown. However that still didn't work.





As you can see the urns are different 'aspect ratios' for the same volume of 40L and in the case of the Crown obviously it will boil off less per minute than the Birko because there's fewer water molecules exposed to the air above so less escape as steam. You wouldn't think it would make all that much difference, but it's knocking my OG's off, sometimes as much as five or more gravity points. That's as far as my physics go.

So I'm going to go right back to basics and recalculate the whole lot with loss to trub, boil off and loss to damp grain, and should get my initial strike liquor volume nailed. I really want to get this happening as I'm going to all kegging and want to calculate pretty precisely for a 21L brew length in the kettle, to yield just a keg plus losses.

Does anyone know a boil off-formula that includes the radius or diameter of the 'orifice' of a pot?


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## MarkBastard (8/11/10)

Shouldn't you just put 1 litre or so less water in the Crown?

That's how I did it. First brew was 22L so the next one I just put 1L less strike water in.


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## WarmBeer (8/11/10)

Any calculation is just a mathematical model, and while it will give a good indication of boil-off, it can be modified by any number of factors, including rate of boil, wort gravity and amount of "foaming" during the boil. You're probably better to develop a ratio based on experimentation and experience.

Without wanting to teach you how to suck eggs, Bribie, just fill 'er up with water, bring to the boil, measure the depth, boil for an hour, then measure the depth again.

If you do the same experiment with both your Birko and your Crown, you'll be able to develop a ratio of boil-off between ol' faithful and the new urn.


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## Bribie G (8/11/10)

I'm doing a single batch brew shortly today and I think I might just go to the other extreme, put in a low - looking amount of strike liquor just enough that it annoys the shyte out of me and at the very end of the process have a few litres of boiling water ready in the Birko to top up if necessary, won't do any harm. And start keeping a list of measurements. Then put some tape on my sight tube when I've hit the good level. :icon_cheers: 

I might even refine it later to thin bands of tape for "4k dough in, 4.5k dough in, 5k dough in, 5.5k dough in, 6k dough in" etc etc. Or make up some sort of heatproof stick on gauge that can sit behind the sight tube. One for the Crown and one for the Birko. Then I'll produce them and sell them and buy Craftbrewer and be rich I tell you rich . :blink: :blink:


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## mccuaigm (8/11/10)

Didn't PP's calculator spreadsheet take that stuff into account?

http://www.biabrewer.info/viewtopic.php?f=25&t=189


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## Bribie G (8/11/10)

goldy said:


> Didn't PP's calculator spreadsheet take that stuff into account?
> 
> http://www.biabrewer.info/viewtopic.php?f=25&t=189


Thanks Goldy I'll have a look at that right now.


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## WarmBeer (8/11/10)

goldy said:


> Didn't PP's calculator spreadsheet take that stuff into account?
> 
> http://www.biabrewer.info/viewtopic.php?f=25&t=189



That is true, but it takes a relatively simple model based on pot diameter. Whilst this is partly correct, greater boil vigour will provide a greater boil-off rate due to increased turbulence/surface area of the water.

Thinking about it in layman's terms (and that's all I am, I'm not a scientist), if you're putting more energy,in the form of heat, into the boil, and the boiling temperature of wort is a constant ceiling, the extra energy needs to go somewhere. So that energy must be dissipated in the form of evaporation.


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## speedie (10/11/10)

Bribie g 

Are the two urns the same kilowatt rating in element power

If you have access to an amp draw meter check and see if they pull the same power

Maybe the new one isnt as grunty as the old one

There fore it would evaporate more over a given time frame


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## chadjaja (11/11/10)

I'm thinking after yesterdays brewday on a warmer day that I'll take off the camping matt next time. I have a 40L Birko and the boil off rate is far too high, much higher than the same recipes in the colder months.


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## asis (11/11/10)

Ditch the tape bribie and just use a sharpie style permanent marker. Our hlt sight glass has marks for every litre, and can hit our volumes every time.


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