Autoclave gravity calculation

How would you calculate the original gravity of pressure canned wort (250*F at 15 min) based on a pre-boil gravity? I can take a reading next time I open a jar, but I'd like to know the science behind it. Since there is little to no evaporation with the lids on, does that mean there is no gravity change at all?. Thanks in advance.

No liquid loss = no gravity change. The science is that the specific gravity depends on the concentration, by weight, of the stuff dissolved in the wort pretending that it is all sucrose. If there is no loss of liquid then then percentage extract by weight does not change and, thus, neither does the specific gravity.

Excellent. That is what I imagined, but it's good to know.

A followup Q: Since I am collecting my wort from a mini all-grain batch instead of DME, the wort will be going into the jars at around 170*F. I expect there is a wort shrinkage factor here. Not the anticipated 4% from boiling to ~70*F. I believe wort density is similar to water, so the density change from 170*F to 70*F is 2.4%. Although there is no evaporation, this density change would raise the gravity by .001. Is this incorrect? If the gravity is measured by evaporation alone, it would seem to be.

One of the reasons advanced brewers use the Plato system is that the concentration of a wort in °P does not change with temperature. 100 grams of extract in 1000 grams of wort is always 10 °P whatever the temperature. The density, and thus the specific gravity do, of course, change with temperature. That is why it is necessary to give both the temperature of the sample and of the reference water when specific gravity is reported thus: 1.040 20/20 means that the weight of the sample at 20 °C divided by the weight of an equal volume of water at 20 °C is 1.040.

Worts expand and contract at about the same rate as water. For example, water at 80 °C has density 97.31% of water at 20 °C. 10 °P wort (or, more precisely, a 10 % w/w solution of sucrose) at 80 °C has density 97.35% that of the same solution at 20 °C and for a 20 °P 'wort' the ratio is 97.28%.