Boil off relative to volume

[Ged]

Guys..Some of the CChurch boys did a wee brew today - James and Dylan ( thanks for your company and help) and discovered that our boil off was ridiculously high..Out of 21.75 wort we boiled off approx 7 litres during the process. Started from cold ( took about 30 minutes to get to boiling) ( and then a further 60 minutes on a rolling boil) - a total of approx 90 minutes from sticking the lautered wort onto the burner to flame out.

Temps seemed stable at around 97.5 to 98.5c according to my thermometer and the wort was definitely boiling away...This kind of screwed up a few calcs but the question that's bugging me is why was it so high - around 30 odd percent boil off...I tried it again with water and from cold to flame out - approx 95 mins again i lost approx 8 litres to boil off on a starter of 21.75...

It it only down to the amount of gas I'm sticking through the burner i.e would we still maintain a boil with a lot less gas? I'm using a 3 burner campmaster ring and they are all on full for the entire period.

Thanks for any opinions...Suffice to say we had to do a fair bit of 'topping up' but everything else looks like it worked.

Thanks

Ged (and the boys)

[Nick]

He should loose the same amount on every boil regardless of the brew size.
The water loss is a function of boiling time and the surface area of your kettle.

Small factors like humidity and altitude come it and this assumes you have the lid off the whole time.

[Rusty Chch]

A thread on AHB relating to this very topic. Some good information for you here Ged.

[Ged]

Cheers Dylan...