Ultrasonic level switches use high-frequency sound waves that are easily transmitted across a transducer gap in the presence of a liquid media, but are attenuated when the gap is dry. The Magnetrol® ultrasonic portal, echotel.magnetrol.com, provides information about the Echotel® series of ultrasonic switches and how they can improve the safety and efficiency of many processes. Periodically, MAGNETROL product manager Tom Kemme answers questions about ultrasonic level switches in the portal’s “Ask Tom” column. This week’s blog post shares some recent Q&As.
Question: Can ECHOTEL be used in applications that have entrained air?
Answer: Yes, ECHOTEL ultrasonic switches can be used in applications that have entrained air. All ECHOTEL gap switches feature a tip-sensitive transducer that provides superior performance over side gap transducers that are offered by other companies. Side gap transducers allow gas bubbles to adhere to the upper surface of the gap, which cause false dry gap alarms. Tip sensitive transducers allow these bubbles to pass through the gap. Applications with severe turbulence or entrained air should use the Model 961 switch, which offers a time delay adjustment. Up to 10 seconds of delay can be used to disregard entrained air and reliably detect the true liquid level.
Question: We are considering adding level alarm switches to our process to provide high-high level indication in several tanks. Instead of switches with relay outputs, we are considering the current shift output. What are the advantages of a current shift output?
Answer: Current shift electronics simply shift the current output from 8 mA when the level switch is in the normal operation, to 16 mA to indicate a level alarm. ECHOTEL Model 961 also has a user selectable fault signal of 3.6 or 22 mA. Current shift switches are 2-wire loop powered, which allows them to be offered with intrinsically safe approvals. This allows these switches to be put into hazardous area locations at a lower cost since rigid conduit is not necessary. Since current shift switches provide constant indication of either a normal (8 mA), alarm (16 mA), or a fault (3.6 or 22 mA) condition, they are sometimes referred to as a transmitter for the price of a switch.