E-Zine May 2011
Laser level measurement sensors emit a laser beam towards the material and measure the remnants of the beam that are reflected from the material. These systems determine the location of the material using the time-of-flight that the laser beam takes to travel to and return from the material. The distance between the sensor and the material can be calculated as one-half of the measured time-of-flight times the speed of the laser beam. Mechanical dimensions can then be used to determine the level in the vessel.
Noting that the laser beam travels to and returns from the material, degradation of the beam strength between the sensors and the material can cause laser level measurement systems to fail. Degradation can occur at the sensor, in transit to/from the material, and at the surface of the material.
Dirt or other coatings on laser transmitter/receiver can cause the received laser signal to be weak. When accumulations over time are normal for the process, routine maintenance may be required to keep the transmitter/receiver operating. In many applications, the sensor may be shielded in a tube and/or continuously purged with gas to keep it in operation. Similar concerns exist when the laser beam travels through a sight glass that can become dirty and cause attenuation of the beam.
Excerpted from The Consumer Guide to Non-Contact Level Gauges