Just got a response from Miso. Here are the facts on the LS3:
First, the sensor measures the internal pressure as soon as it's turned on. This is the reference pressure. Assume that the reference pressure is atmospheric, i.e. 1000 mbar. To get the blinking green light, one must reduce the internal pressure by 200 mbar, i.e. go from 1000 mbar to 800 mbar by pumping down. Once at blinking green, an increase in pressure of only 5 mbar will trigger the alternating colors blinking. This is a very small change and will be achieved by the introduction of about 6 ml of water per liter of internal housing volume. My SWAG (sophisticated wild-ass guess) is that the typical housing volume with dome port is 1.5 to 2.0 liters, less with flat port. I don't plan to fill my housing with water to test this estimate.
Also from Miso: He has a new product, the LS4 with temp. comp and other features that are very nice. He is offering the LS4 circuit board to current owners of LS3 for 50 euros. The LS4 board is a direct replacement for the LS3 board. The other nice feature is that it allows you to turn the sensor light off and retain the reference set point for the future. With the LS3, when you turn it off, you lose the reference set point and must equalize the housing before turning the LS3 back on. Then you have to pump it down again. The LS4 will extend time between battery changes. PM me if you want more info.