E-Zine December 2008
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Level measurement systems that have non-mechanical sensors that do not contact the material include laser, non-contact radar, and ultrasonic level measurement technologies. In general, these level measurement systems use level sensors that are located above the material and are not directly exposed to the material. These sensors are exposed to the environment of the material in normal operation and can come in contact with the material when the level of the material is excessively high.
Laser sensors emit a laser beam that is reflected back to the transmitter by the material. The level measurement is derived from the amount of time measured for the laser beam to travel to and return from the material. The ability to use this technology is dependent upon the ability of the material to reflect part of the laser beam back to the transmitter.
Non-contact radar sensors emit radar energy that is reflected back to the transmitter by the material. The level measurement can be derived from the amount of time measured for the radar energy to travel to and return from the material. However, some designs use modified radar energy waveforms to improve measurement. The strength of the reflection (and the ability to utilize this technology) is determined by the dielectric constant of the material.
Ultrasonic sensors emit ultrasonic energy that is reflected back to the transmitter by the material. The level measurement can be derived from the amount of time measured for the ultrasonic energy to travel to and return from the material. The ability to use this technology is dependent upon the ability of the material to reflect ultrasonic energy back to the sensor.
Level measurement systems that have sensors located externally to a vessel and not in contact with the material include nuclear level and weight measurement technologies. In addition, non-contact radar level sensors with isolators, "bottom-up" ultrasonic transmitters, and laser level sensors that measure through sight glasses can effectively exhibit these attributes in some applications.
Nuclear level measurement systems typically consist of a nuclear radiation source that is directed into the vessel so that it passes through the vessel walls and the material. Sensor(s) on the opposite side of the vessel measure the amount of radiation remaining. As level increases, the material blocks more radiation so less radiation is received at the sensor. The level measurement is derived from the amount of radiation received by the sensor.
Weight measurement devices such as load cells can be used to measure the total weight of a vessel. Knowledge of the density of the material in the vessel and weight of the empty vessel can be used to infer the level of material in the vessel. Note that the load cells are typically part of the vessel support system, so careful mechanical design of the entire system is advised.
Excerpted from Excerpted from The Consumer Guide to Non-Contact Level Gauges