Why is boil off expressed as % per hour?

Those of us who use brewing software are used to expressing our boil-off rates in terms of percent per hour. Why do we do this? It seems to me that for a given setup, the boil-off rate will be fairly consistent, regardless of the volume under boil.

Example: With my current rig (keggle plus propane burner), I boil off about a gallon and a half in an hour with a vigorous boil. For a 7 gallon initial volume, that's about 22% per hour. But, for the second hour of boil, again boiling off 1.5 gallons, the rate would be nearly 46% per hour.

Similarly, for a 11 gallon batch with a starting volume of 13.5 gallons, the initial boil off rate would have to be set to 19% per hour.

Given that the rate needs to be adjusted for initial volume, why do we do it this way instead of simply expressing boiloff as a volume per hour?

Or am I thinking about this wrong? Anyone? Beersmith, you here?

I have gotten used to BeerSmith and have not looked to change the seetings I have but, IIRC, it is possible to change the way BS represents boil off as a volume (gallons) per hour.

If I am correct, the seeting would be under prerferences, units.

I may be thinking of ProMash tho'.

I can't confirm either way as I don;t have either in front of me now.

BTP does it by volume per hour and it's a superior method IMHO.

Bobby_M said:

BTP does it by volume per hour and it's a superior method IMHO.

Click to expand...

Yep, BTP is using the correct method. Beersmith uses an incorrect method, and it has been brought up on their forums for many years. No change yet, though. I suspect Brad is just following the accepted standard (which is incorrect) to be consistent with the brewing literature.

Boil volume indeed has little or no effect on the rate of evaporative water loss. The key factors are: surface area of boil, flow rate of air, relative humidity of air, wort concentration. The volume of the boil is not one of them.

A good thing to know with Beersmith is that if you always keep your vessel and boil times consistent, the evap. rate calculation works. But if you lengthen (or shorten) your boil time, the estimated rates of evaporative loss will be incorrect.

^^^^^^
+1234

I know my gear well enough to know what it will do, regardless of what software tells me. I just ignore the boil volume calculations (in beersmith, I deselect 'calculate boil volume automatically'), and go with what I know works. It just always seemed 'off' to me.