10Assuming the liquid level is equal to or greater than x. Otherwise, the pressure difference between Pbottom and Pmiddle will depend on liquid density and liquid height. However, this condition is easy to check: the level computer simply checks to see if Pmiddle and Ptop are unequal. If so, then the computer knows the liquid level exceeds x and it is safe to calculate density. If not, and Pmiddle registers the same as Ptop, the computer knows those two transmitters are both registering gas pressure only, and it knows to stop calculating density.