E-Zine November 2008
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Hydrostatic level measurement systems are based upon physical properties that are related to the density of the material. These systems are typically applied to liquid or slurry levels, but other levels may be measured in some processes. Hydrostatic level measurement is based on the principle that increasing fluid level will increase the pressure exerted by the fluid. Hydrostatic sensors are typically used to measure the fluid pressure near the bottom (and sometimes the top) of the vessel. This information is used to infer the height of the fluid that is dependent upon knowledge of the specific gravity of the fluid.
Displacer hydrostatic level sensors are based upon the principle that submerging a displacer (float) in a fluid causes its weight to change due to buoyancy. Therefore, increasing level increases the submergence of the displacer and causes its weight to decrease. This information is used to infer the height of the fluid that is dependent upon knowledge of the specific gravity of the fluid. Displacer sensors may be located directly in the vessel or connected to the vessel with external piping.
A contact radar level measurement sensor is typically a probe that becomes covered by the material as the level changes. The transmitter sends a radar frequency pulse down the probe that is reflected back to the transmitter at the surface of the material. The reflected signal takes more time to return to the transmitter when the probe is less covered. The covering of the probe is determined by measuring the time it takes the reflected signal to return to the transmitter. The strength of the reflection (and the ability to utilize this technology) is determined by the dielectric constant of the material.
Submerged ultrasonic technology emits ultrasonic energy upwards from a sensor located the bottom of a vessel. The ultrasonic energy is reflected back to the transmitter by the liquid/gas interface. The level measurement can be derived from the amount of time measured for the ultrasonic energy to travel to and return from the interface. The ability to use this technology is dependent upon the ability of the interface to reflect ultrasonic energy back to the sensor.
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Excerpted from Excerpted from The Consumer Guide to Non-Contact Level Gauges