The sensor would need to be part of the bottle.
Liquid left in bottle is bottle capacity minus removed content, but since you only want to know what is in bottle you don't need to know its capacity
Weigh bottle when empty We
Weigh bottle when part full Wp
Liquid in bottle Wl = (Wp - We)
So use a weigh scale as your primary measurement device
If you want volume you need to know liquid density
Or use a pencil to mark a line on the bottle and catch whoever is drinking your tipple
Set the water on this sensor, remove water to the level you wish to take action at and take a reading. Then write code to take action when that reading is reached.
FlexiForce Pressure Sensor - 25lbs (1" area) - SEN-11207 - SparkFun Electronics?
Good luck!
Or this one to work in direct contact with the water itself.
Arduino420:
Or this one to work in direct contact with the water itself.
That seems to have a pretty restrictive temperature range?
10.Operating Temperature: 10 ? -30 ?
Yeah he did not say what temp. Besides, until the whole rest of the planet gives up on that Metric crap, I would have not know what he meant anyway. 