Boil-off rate versus test run

I am new to AG brewing (two batches so far). I thought I would be slick and boil several gallons of water and compare before and after weights to arrive at my rate of evaporation. This way I would know in advance before brewing a batch of beer.

A couple weeks later on brew day I determined that the evaporation rate of boiling wort is quite a bit different. At least I now know my boil-off rate, but I am curious to know why it would be different. I did boil for the same 90 minute period so boiling time was not the issue.

My guess is that there are compounds in the wort that boil off more quickly or more slowly than water. It could because I was measuring weights before and after and wort and water don't weigh the same for each volumes. Could it also be the boiling temperature for water and wort are not quite the same?

Just curious...

Greg

I would say just do a brew and see. You'll probably figure a lot of things the first time round. Use it as a test run and don't worry about it too much.
Just do your math based on average standard figures and you won't go far wrong.

It doesn't really mater if you end up with 4.5 or 5 gallon.
Don't forget you'll have absorption by the hops to contend with as well.