I am trying to determine if it is possible to "physically" determine the volume of liquid (such as water) in a non-solid container like a fluid bladder like this or this?
Assume that it is not possible to keep accurate track of the difference between how much fluid is placed in the bladder and how much is taken out, and that a "physical" measurement must be made (where "physical" can include using a sensor, or using some sort of actual measurement of the dimensions of the bag to compute the volume).