Sometimes I collect more wort than my pot can hold and need to use 2 pots. But then I want to know what the combine SG is of the total wort so I can calculate efficiency.
I finally figured this out the other day, so here's a way to do it for anyone else that runs into this situation:
1. Measure SG and volume for each pot. For example, lets say I have pot A which is 5 gal at 1.068 and pot B with 2 gal at 1.40.
2. Convert both of these SG readings to ppg (points per gallon), by taking away the "1." of the reading (or subtract 1 and multiple by 1000). So we have pot A with 68 ppg and pot B with 40 ppg
3. Multiple each ppg value by the volume in gallons to get gravity points. Pot A is 340 gravity points and pot B is 80 gravity points.
4. Add the gravity points of each pot together. Pot A + pot B = 420 total gravity points
5. Divide total gravity points by the total volume in both pots. 420 points / 7 gal = 60 ppg
6. Convert back to SG (divide by 1000 and add 1). 1.060 is the combined specific gravity of both volumes.