Is boil off linear?

[aeviaanah]

If I lose 1gal at 1hr will i lose .5gal at 30min?

 

[h22lude]

Yes

 

[pricelessbrewing]

Yes-ish, not enough of an "ish" to care about.

 

[mabrungard]

What you are describing is a constant loss rate. For most kettles, that should be an appropriate estimate. Boiling evaporation is dependent upon the exchange with the atmosphere. That exchange is dependent upon the surface area of the liquid, the humidity of the atmosphere, and the amount of movement between the near-surface steam layer and the rest of the atmosphere. Since you are likely dealing with a constant liquid surface area (straight-sided kettle) and the wind speed is the same throughout the boil, the evaporation loss rate should be constant. If your loss rate is 1 gal per hour, a 30 min boil should result in 1/2 gal loss.