Self-compensating low velocity trip or alarm device

Claim 1

A VELOCITY MEASURING DEVICE, A VERTICAL PIPE THROUGH WHICH A FLUID IS FLOWING, A RESTRICTION IN SAID PIPE FOR DEVELOPING A DIFFERENDE IN PRESSURE IN SAID FLUID HAVING A DEFINITE RELATION TO THE VELOCITY OF THE FLUID, A FIRST PRESSURE TAP BELOW AND ON THE DOWNSTREAM SIDE OF THE RESTRICTION, A SECOND PRESSURE TAP ABOVE AND ON THE RESTRICSIDE OF SAID RESTRICTION, THE VERTICAL DISTANCE BETWEEN SAID FIRST AND SECOND PRESSURE TAPS BEING SUCH THAT WHEN THE FLUID FLOWING THROUGH THE PIPE IS AT A PREDETERMINED VELOCITY, THE PRESSURE DIFFERENTIAL BETWEEN SAID FIRST AND SECOND TAPS IS ZERO, REGARDLESS OF THE TEMPERATURE OR DENSITY OF THE FLUID FLOWING, PRESSURE RESPONSIVE MEANS CONNEDTED TO THE FIRST AND SECOND PRESSURE TAPS FOR COMPARING THE PRESSURE EXISTING AT SAID FIRST AND SECOND PRESSURE TAPS, AND NORMALLY DE-ENERGIZED INDICATING MEANS CONNECTED TO THE PRESSURE RESPONSIVE MEANS, SAID INDICATING MEANS BEING CONNECTED TO THE PRESSURE RESPONSIVE MEANS IN SUCH A MANNER THAT IT IS ENERGIZED WHEN THE PRESSURE EXISTING AT SAID FIRST AND SECOND PRESSURE TAPS ARE EQUAL.