hydrometer question

[jeysiin]

i'm still having trouble getting consistant hydrometer readings for my OG. FG is typically fine and consistant, but i get something like a .005-.015 swing on my OG. I *know* that the problem is that i'm not getting the wort mixed with the tapwater (i do partial boil extract batches, at the moment.) can someone look at the following equation, and tell me if this should work (not to replace getting it sufficiently mixed, just as a reference to make sure that it is)

(SG of Wort)*(# of gallons after boil) + (# gallons tapwater)
-------------------------------------------------------------
(gallons in batch)

for example, if i have 2 gallons of 1.15 wort, and fill with 3 gallons of water, it'd look like this:
(1.15(2)+3)/5= 1.060
does that sound right? i'm NOT very good at math, and things that make sense in my head are often wrong, so i wanted to double check.

of course, i'd be extra careful to make sure that everything coming in contact with the wort is sanitized.

 

[FlyingHorse]

What you want is total gravity points divided by total volume. The added water doesn't contribute any gravity, so it goes in the denominator only:

Wort SG * Boil volume
---------------------------
Total batch volume

The math is easier if you drop the 1.xxx and just use the 3 digits -- in other words, for 1.050, use 50.

Example: 2 gallons of 1.150 OG wort diluted with 3 gallons water gives you

2 * 150 300
-------- = ----- = 60, or 1.060
5 5

Not sure how you got the right answer with the wrong setup in your example, but this should make the calculation easier.

 

[Funkenjaeger]

Not sure how you got the right answer with the wrong setup in your example, but this should make the calculation easier.

There was nothing wrong with his math. Your equations are essentially the same, but since water has a SG of 1.000, in his equation it needs to be in the numerator, but if you only look at the fractional part it has a value of zero, so it gets dropped from the numerator, resulting in your equation.

Your equation is simpler for this case, his is closer to being the general case which could handle adding something other than water, if you so desired.