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Model 351/352/353 Liquid Level Systems
Section 2
Section 2—Theory of Operation
General
A fluid column exerts a pressure proportional to its height and density, inde
-
pendent of the shape or volume of the containing vessel. A differential pres-
sure unit (DPU) can be calibrated to indicate the height of a fluid when the
fluid density is known. In tanks of given dimensions, the weight and volume
of contents can be measured by a DPU.
One or two sealed sensors are connected to the DPU by capillary tubing.
These sensors contain bellows which function as “isolation seals.” As pres-
sure on the sensor bellows varies due to changes in liquid level, the variations
are transmitted hydraulically through the tubing to the bellows of the DPU.
Model 351/Model S048 Sensors
The pressure deflection characteristics of the Model 351 (500-psi or 3,000-
psi) and Model S048 sensor bellows are shown in Figure 2.1.
Figure 2.1—Pressure deflection characteristics of the Model 351/352 sensor bellows
Model 353 Sensor
In the event of thermal expansion of the fill-liquid in the capillary tubing
and in the other instruments, the Model 353 sensor bellows can extend 1 in.
(25 mm) for 7.5 in.³ (123 cc) displacement. If leakage occurs in the capillary
tubing, the bel lows becomes compressed approximately 0.56 in. (14 mm) and
