Fluctuating Evaporation Rate

[scruff311]

Hi,

I've been frustrated by my two most recent brews where i ended up with > 1/2 gallon more wort than expected, but obviously short of my intended O.G. Both of these brews were 90 minute boils (mostly Pilsner malt grists), where I usually boil for 60 minutes and my volumes are more on target at the end. It seems like I am not getting the same evaporation rate on the 90 minute boils for some reason. I don't believe it makes any physical sense. The weather was cool and rainy on one brew and warm and sunny on the other, so I can't draw a conclusion based on the atmospheric conditions. Does this happen to anyone else? Is 18% the accepted standard for evaporation rate?

Thanks!

 

[hottpeper13]

When I first went all grain I read somewhere to always do the same boil time. Since pils needs 90 min. I always boil 90 min. 18% is a bit high,mine is between 12 and 15 % for the 90 min. thats starting with 13.5 for 10.5 in the fermenter. That's also with a normal amount of hops ~6-8 oz.

 

[chickypad]

Use rate as gal/hr not %. The amount you boil off should not be affected by how much liquid you start with in the pot, but the % will change.

 

[Stillraining]

Do you use a propane burner by chance.?.... I find It is sort of difficult to get the exact rolling boil batch to batch with perfect consistency, so this is where most of your discrepancy lies if so. Boil off rate is greatly affected by air to fluid surface exchange...a slightly more violent boil will boil off quite a bit more water and vice versa. And the same burner setting on a warm day compared to a cool one will produce a more violent boil and vice versa. Wind is also a factor messing with your flame to pot interface. I just have to look at the boil and guess its about the same as always...and guessing isn't very scientific..

I never complain when I end up with more beer though...only less.. I could really care less about a couple tenths of a percent ABV either way...I just want the volume.

 

[scruff311]

Use rate as gal/hr not %.

Yea I agree, I don't know why I stated 18%. I've had 1.66 gal/hr plugged into Beersmith for a while and it was reliable for 60 minute boils. Seems the 90 minute boils were more like 1.37 gal/hr.

Do you use a propane burner by chance.?.

Yes I do, and I hear your point about consistency with the burner. What's weird though is my evaporation was pretty reliable on previous 60 minute boils, it was just the last 2 90 minute boils that were wildly off. The last one missed the target OG by 10 points! It was an IPA so I was afraid of the bittering being too much.

 

[kwadric]

If you have an accurate way to measure kettle volume then you can top up with some fresh water. When you add the water depends on you process and how much water you need to add. The room temp water will lower the kettle from a boil. Heck, you could even top up in the fermentor like some extract brewers .