Abstract: Borehole pressure is measured with an open-ended chamber positioned in the borehole and in fluid communication with a surface pressure source through a microtube. The chamber contains a ball float that seats in the upper end of the chamber to seal the microtube to entry of borehole fluids. The ball is made of a material having a density greater than that of the borehole fluids in order that the float can withstand high pressures. Partial support to compensate for the high density material of the float is provided by a spring attached between the chamber and the float ball.