Boil-off issues

Up until recently, my AG beers were always coming up short in volume. I'd be shooting for 5 gallons, and end up with 4.5 gallons, or sometimes even 4! This was because I was brewing outside during the dry Michigan winters, and losing quite a bit to evaporation. Of course, my gravity often was higher than anticipated.

On my second to last batch, I hit both gravity and volume (5 gallons) for the first time ever. However, I had to set my evaporation rate at almost 20%. That seemed pretty high.

Yesterday, I brewed a brown ale, and was shooting for 5 gallons again. However, I wound up with a little over 5.5, even after boiling for almost 90 minutes. As such my gravity was about 14 points lower than I'd hoped for.

My bazooka screen (homemade) has an extended pick-up tube, so I don't leave more than a cup or two of wort in my kettle. Also, I find the boil-off percentage calculation in BeerSmith a little cumbersome. Shouldn't the boil-off volume be roughly the same if I were to do a 5-gallon batch as a 10-gallon batch? Wouldn't an "X Volume Per Hour" boil-off calculation make more sense?

At this point, I realize I'm rambling I'm just wondering how everyone deals with wort losses during and after the boil. Thanks for any feedback!

Colin,
 Unfortunately boil off volume can vary quite a bit.  It is usually pretty steady for a given setup and volume, but if you change from a 5 gallon batch to a 10 gallon batch the differences can be quite large!  Losses are usually higher for smaller batches.

 Evaporation depends on the size of the pot (particularly the surface area of the exposed wort during the boil) as well as the strength of the boil.  For example a tall narrow pot will probably lose less than a short fat one.  Though a normal range might be 5-15%, some people lose 20% or even higher on small batches.  

 The easiest way to accurately determine the percentage is to measure your volume at the start of the boil, and then measure the volume remaining after one hour.  You can then calculate the percent as:

  evap_rate = 100 x (original_volume - post_boil_volume) / original_volume

If you do this over a few batches you will likely find that your results are pretty consistent.  Note however that if you change from a 5 gallon batch to a 10 gallon batch you will need to perform the measurement again, because you have a much larger volume and the same amount of exposed surface area (for a given pot).

Cheers!
Brad

If you do this over a few batches you will likely find that your results are pretty consistent.  Note however that if you change from a 5 gallon batch to a 10 gallon batch you will need to perform the measurement again, because you have a much larger volume and the same amount of exposed surface area (for a given pot).

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Well, that's the problem I'm running into. As the climate has changed--from less humid to more humid, and from cold to somewhat less cold--the boil-off value has changed quite a bit. In the winter, I'd sometime lose 2 or more gallons... yesterday, I think I only lost a little over a gallon or so.

I agree that the kettle shape makes a difference. My original kettle (7.5 gallon) was very tall versus the width, and when it was full enough for a batch, the wort depth was about twice the width. Now, I'm using a converted keg, which has roughly the same shape as my original kettle, but I'm only doing 5 gallon batches at this time. I try to keep the boil rolling, by not crazy rolling.

My problem with the "percentage" figure is this: let's say you assume 10% boil-off. In a 5 gallon batch, that's .5 gallons. However, in a 10 gallon batch, that's 1 gallon. That obviously doesn't make much sense, because as you said, more volume boiled would likely lead to less boil-off. Of course, I can make separate setups (actually, I have) for 5 and 10 gallon batches, with independent boil-off percentages--that might be the only solution.

I think what I may do, however, is have the program calculate my losses in a different way. What I'm going to try to do is set the evaporation rate to 0%, and then use the "Losses to Trub and Chiller" field to set up how much wort I'll leave behind. Based on yesterday's boil, I should be able to come close through some tweaking.

Thanks for listening to me babble

Colin,
 Only recommendation I can offer is to measure the actual volume going into the boil and out of the boil.  There should not be a huge variation (certainly not 10%) from batch to batch.  I have notches cut on a stick that I use to monitor the volume.

 The only other advice I can offer is don't worry too much - everyone has small variations from batch to batch even when making the same brew.  I've had all kinds of things happen at various stages and in 99.9% of the cases the beer just tastes great anyways!

Cheers!

Brad

I think that on my next batch, I'll have adjusted the boil-off/wort loss value enough to compensate for my real losses. And I'm not so much worried about brewing a weaker or stronger beer than I'd anticipated--I've long since dropped the notion that beer is so important that I need to worry about it More than anything, I just want to hit close to 5 gallons, finished, because I primary in 6.5 gallon carboys, but secondary or keg in 5 gallon vessels. So, shooting for any more than 5 gallons is just a waste of raw materials. No worries anymore!

Thanks for the feedback Brad!

I think there are two big variables, one the size of the patch and also what you are brewing with. I use to use gas (turkey cooker) now that I have went with direct electric water heater elements I get more boil off and had to set beersmith at 20% which it takes and compensates no problem.

I've been tweaking the boil-off percentage after each batch.  After enough trial and error, I'll get it.  I don't think you can figure out a consistant percent.  More gravity(more sugar)can raise the temp of boiling liquid, thus more evap.  Leaf hops vs pellet should affect it also due to surface agitation.  And who can set their burner to the exact same flame every time?
Mike

Add preboiled and cooled water to the primary or secondary fermeter to compensate for low volume/high OG.

For low gravity, here's my suggestion:  take a gravity reading at a known volume before you start your boil and determine the volume to hit your taget OG. If, in the last 20 minutes of boil, it looks like your volume isn't dropping fast enough, crank that burner all the way up. Or boil for a longer time if you haven't already added flavor hops.