Level indicators for liquids

Apparatus for indicating the level of liquid in a receptacle, which comprises:

• a. a first U-tube having a horizontal base, a pair of vertical arms at opposite ends of the base and a plurality of vertical branches rising from said base, the base, arms and branches being in fluid communication and containing a dense indicating liquid, and means for adjusting the liquid level in said branches;
• b. means for transmitting hydrostatic pressure from the receptable to the surface of said dense liquid in one of said arms of said first U-tube;
• c. a second U-tube, conduit means for providing fluid communication between said other arm of said first U-tube and an arm of said second U-tube, the second U-tube being of smaller diameter than said first U-tube, and a less dense liquid extending from the surface of the dense indicating liquid in said other arm of said first U-tube through said conduit means and into said second U-tube, the height of the less dense liquid in the other arm of the second U-tube being an indication of the liquid level in said receptacle;
• d. a third U-tube of larger diameter than said second U-tube and containing a less dense liquid, second conduit means for providing fluid communication between the other arm of said second U-tube and an arm of the third U-tube, there being a volume of air between the liquids in said second and third U-tubes such that any expansion or contraction in the indicating liquid contained in said other arm of the first U-tube as a result of a change in ambient temperature will produce a proportionate change in volume of the air to cause porportionally an opposing head of liquid in the other arm of the third U-tube in compensation therefor;
• and
• e. conduit means for connecting the free arm of the third U-tube and the branches to the atmosphere when the receptacle is selected to be open to the atmosphere or to the top of the receptacle when the receptacle is subject to a pressure different from atmospheric.