Increased sensitivity and versatility over other models, mercury requirement is reduced to 20-30cc. A control point within 1mm Hg can be fine-tuned by adjusting the height of the pump-out tubulation. The Cartesian driver float drops away from pump-out orifice on an increas- ing system pressure and reseals it when the reference pressure set within large bulb is reattained. Reference pressure is easily set. The spherical joint allows the attitude of the orifice to be adjusted to accomodate a small continuous leak without hunting of the float.