Fluid level sensor

Claim 1

An apparatus for indicating the level of fluid in a tank, the apparatus comprising, in combination: a casing disposed at the bottom of the tank and having opposed surfaces; first and second diaphragms carried by the opposed surfaces of the casing, the casing and the first and second diaphragms forming an atmospheric chamber isolated from the fluid, the external surfaces of the chamber being exposed to the fluid pressure at the bottom of the tank, the fluid exerting a force equal to A 1 *P f on the first diaphragm and an opposing force A 2 *P f on the second diaphragm, where A 1 is the area of the first diaphragm, A 2 is the area of the second diaphragm and P f is the fluid pressure per unit area at the bottom of the tank and which is proportional to the level of the fluid in the tank; a valve including a moveable valve member and a valve seat having an area A 3 ; means for transmitting the net force P f (A 1 -A 2 ) exerted by the fluid on the first and second diaphragms to the valve member in a sense tending to seat the valve member against the valve seat; an output pressure line coupled between the pressure gage and the valve seat, the output pressure line being vented to the tank when the valve member is unseated from the valve seat; a fluid pump for pumping fluid under pressure from the tank; a fluid flow restrictor; means for coupling the liquid under pressure to the output pressure line through the fluid flow restrictor to pressurize the output pressure line, the fluid pressure in the output pressure line generating a force equal to P g *A 3 on the valve member tending to unseat the valve member from the valve seat, where P g is equal to the pressure per unit area of the fluid in the output pressure line, the valve member being moved by the forces acting thereon to modulate the pressure P g in the output pressure line to a value substantially equal to P f (A 1 -A 2 )/A 3 ; and a pressure gage responsive to the fluid pressure in the output pressure line for providing a measure of the fluid level in the tank.

Claim 2

An apparatus for indicating the level of fluid in a tank, the apparatus comprising, in combination: a casing disposed at the bottom of the tank and having opposed surfaces; first and second diaphragms carried by the opposed surfaces of the casing, the casing and the first and second diaphragms forming an atmospheric chamber isolated from the fluid, the external surfaces of the chamber being exposed to the fluid pressure at the bottom of the tank, the fluid exerting a force equal to A 1 *P f on the first diaphragm and an opposing force A 2 *P f on the second diaphragm, where A 1 is the area of the first diaphragm, A 2 is the area of the second diaphragm and P f is the fluid pressure per unit area at the bottom of the tank and which is proportional to the level of the fluid in the tank; a valve including a moveable valve member and a valve seat having an area A 3 ; means for transmitting the net force P f (A 1 -A 2 ) exerted by the fluid on the first and second diaphragms to the valve member in a sense tending to seat the valve member against the valve seat; an output pressure line coupled between the pressure gage and the valve seat, the output pressure line being vented to the tank when the valve member is unseated from the valve seat; a fluid pump for pumping fluid under pressure from the tank; a fluid flow restrictor; means for coupling the liquid under pressure to the output pressure line through the fluid flow restrictor to pressurize the output pressure line, the fluid pressure in the output pressure line generating a force equal to P g *A 3 on the valve member tending to unseat the valve member from the valve seat, where P g is equal to the pressure per unit area of the fluid in the output pressure line, the valve member being moved by the forces acting thereon to modulate the pressure P g in the output pressure line to a value substantially equal to P f (A 1 -A 2 )/A 3 ; and a pressure gage responsive to the fluid pressure in the output pressure line for providing a measure of the fluid level in the tank.