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E-Zine November 2011Many ultrasonic sensors have integral temperature sensors to compensate for the effects of vapor temperature on the measurement. Recognizing that the sensor temperature may not be representative of the vapor temperature in the vessel, some ultrasonic level gauges can use external vapor-space temperature measurement(s) for compensation. Noting that the ultrasonic energy travels to and returns from the material, attenuation between the sensors and the material can cause ultrasonic level measurement systems to fail. Degradation can occur at the sensors, in transit to/from the material, and at the surface of the material. The design of the ultrasonic sensors is important. In particular, dirt or other coatings on ultrasonic sensing system can cause the emitted and received ultrasonic signals to be weak. As the signals become weaker, the measurement system can become erratic and fail to operate. When accumulations over time are normal for the process, routine maintenance may be required to keep the sensors operating. Careful analysis of the sensor design should be performed. Some designs inherently tend to maintain the sensor clean while others tend to inherently accumulate material. For example, the sensor system on the right in the figure emits ultrasonic energy upward to a reflector that redirects the energy down to the material. In this design, material can accumulate on the sensors and stick to the reflector. Both of these phenomena can attenuate ultrasonic energy and cause the level measurement system to fail to operate.
Excerpted from The Consumer Guide to Non-Contact Level Gauges ISSN 1538-5280 |
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