The last batch of beer is about half drunk so it's time to make some more. I brewed it up Saturday and by last night it was already fermenting very well with a thick krausen. Unfortunately it's a little weak and watery, like the last batch. This is due to my sparging efficency, I'm not extracting as much sugar as I should be. The following formula will come in handy in the future:

(Gallons•Specific Gravity/Pounds of Malt)/Maximum Extraction=Efficiency

(5.15 Gallons•40/8.8)/36=65%

That's a pretty bad percentage, so in the future I'll just need to add more barley to make my beer stronger.