I found that it took a period of time for the humdity to change once water was injected into the incubator so I wrote a small progrma for the mindstorms Brick to drive the syringe only about a 1ml then it would wait a few minutes and check the status of the humdity sensor. If the humidty was still low it would inject another ml into the incubator and the process would repeat.
an opto sensor was used to detect when the syringe had reached the end of travel.
To keep track of the position of the syinge at start up the Birck would drive the syringe till the optosensor detected the end of travel and then draw the syringe back a predetermined distance based on the conts from the rotation sensor.
This also allowed for the syringe to be refilled a "T" piece in the fluid line and a couple of one way valves allowed the water to go out into the incubator and also be drawn in from a resiviour allowing for extended automated opperation of the system.