This isn't homework. I'm trying to find a formula (or collection of them) that will help answer the following problem.
A tube is in a bowl of water much like a straw in a glass. The pressure above the water in the tube is the same as the pressure above the water in the bowl (atmospheric). My tube has a piston in it though and as I draw the piston upwards in the tube it begins to lower the pressure in the tube above the water. As it does so, the atmospheric pressure above the water in the bowl forces water up the tube in an attempt to equalize the pressure difference the piston attempted to create. The column of water in the tube rises and comes to rest a distance above the surface of the water in the bowl. At this point, the weight mg of the column of water is held in place by the pressure on the water surface around the tube in the bowl. But that same weight prevents the water in the tube from rising high enough to return the pressure difference between inside and outside of the tube to zero. That is, there would still be a slightly lower pressure in the tube above the water column than outside of the tube.
I'm thinking that the pressure in the tube is P - mg where P is atmospheric pressure, but this can't be the correct form as the units aren't the same.
Any help would be appreciated.