Evaporation Rates & Hops Utilization

[lauterdaysaint]

Evaporation Rates:
I'm new to the BN forums and all grain brewing (first batch is fermenting now), and planning for my second batch right now. However, I have a few questions about testing boil off rates before I go out and start brewing without first dialing in my efficiency. Unfortunately, during my first batch I didn't take measurements of post-boil volume and was unable to calculate the boil off rate of my 32 quart aluminum turkey fryer boil kettle. Before starting my second all grain batch, I wanted to do a test to determine my boil off rate, but wanted to run this by some experience first.

Here's the (very simple) test: Fill my kettle with just water and boil it at a couple of different burner settings to determine how much boils off in an hour. (I might just do a half hour to save on time and propane.)

My only question: is there anything that would lead anyone to believe the boil off rate would change once the specific gravity is increased above plain water from the (un)fermentable sugars?

Hop Utilization:
I also wanted to calculate the estimated hop utilization, but was wondering which specific gravity should be used in the Rager formula. Is it the pre or post boil specific gravity?

Also, it seems to me that the utilization would change based on the volume of the wort, which would be a function of the evaporation rate and time; and that the utilization would change depending on these factors, not simply either the pre or post boil gravity. Is it standard, for simplicities sake, to use either the pre or post boil SG for the IBU estimation? Or does the Rager formula (and some of the other methods) somehow take this into account?

Let me know if any information is needed to answer these questions. I'd appreciate any help figuring this out.

[Bugeater]

Using just water for determining your boil off rate is just fine. Any difference in using wort is so small that it is not worth even considering.

As far as what gravity to use for the hop utilization, it is all based on the preboil gravity.

Don't let yourself fall into the trap of overthinking things at this point. For your 32 quart pot, using 1 gallon per hour for boil off rate is a great place to start. This is generally with a good rolling boil. You don't want a boil so strong it is splashing around inside the pot but you definitely want it more than just gentle movement.

Hop utilization is something I have never bothered to figure out in nearly 7 years of all grain brewing. I let my brewing software take care of that. You do need to keep in mind that utilization is lower as the gravity increases. Your preboil gravity will help you figure out if you need to add or decrease hops to allow for the change in utilization. Again, if you use brewing software you just adjust the bittering hops to keep the reported ibu's the same. There is no reason to mess with figuring out the exact utilization rate. You have plenty of other stuff to figure out. If you want to get really geeky with the math, you should wait until you get all your basic processes figured out to the point that all you need to do is fine tune those 38 point beers to turn them into 40 point beers.

Wayne

[lauterdaysaint]

Thanks Wayne, I appreciate your response.