Calculating efficiency in boiler

Quick question -- I don't take the SG of what ends up in my boil kettle because by the time my sparge is done I'm pretty much at a boil. I do take a gravity reading of the first and final runnings however.

Can I average the first and final runnings to get the efficiency into the boiler?

It seems to add up in Beersmith -- for example I plan for and get 80% efficiency. My first runnings are 1.053 and my final runnings are 1.025. The average is 1.039 -- I put this into Beersmith and it comes out with exactly 80% eff into the boiler.....so that's got to be right, correct?

If you're batch sparging (and I assume you are, given those numbers) AND if your two runnings are the same volume, it's close enough.

Why not just take a sample at the start of your boil, let it cool, and get a gravity reading?

Bike N Brew said:

If you're batch sparging (and I assume you are, given those numbers) AND if your two runnings are the same volume, it's close enough.

Why not just take a sample at the start of your boil, let it cool, and get a gravity reading?

Click to expand...

Actually I'm fly sparging -- the first reading is the very first wort out of the MLT (after vorlauf) and the last reading is right when I have collected my full boil volume.

I guess I could pull a sample from the boil kettle and keep it in a glass container until it had cooled....I was just curious about the math.

With fly sparging, the gravity drop is theoretically linear (or close) but I wouldn't want to chance it. If preboil is something you want to record, stir the wort in the kettle and pull a sample. It's the only accurate way assuming you have a method of measuring volume as well.

I'll have to start putting that sample aside like you guys are suggesting and compare it to the first/final runnings avg.....This is just curiosity on my part -- I actually don't worry about boiler eff.