Wireless Question – How to Connect Level Switch to Light or Horn

As wireless instrumentation based on the WirelessHART standard continues to move into the mainstream, I get great questions about it by email from time to time. Here’s one that came in this week that I thought I’d share:

Do you guys know of a wireless alarm system that we could purchase that would accept an input from a level switch and output a wireless signal to a receiver which is 1500ft away to activate a light/horn. The level switch is in a Class 1, div 2 area. We are just looking a for a stand alone system.

One on the common plant situations that leads automation engineers to think about wireless is distance. Many times, infrastructure like cable tray or conduit paths are not in place. Or as in this case, the annunciator (light/horn) is a great distance from where the measurement takes place and where the automation system is located. In most cases like this one with long distances involved, it’s not economically feasible to add the cable infrastructure to solve the problem at hand.

I’m not sure about a standalone solution, but know it could be coupled with a small PLC or to an existing automation system. I checked with Emerson’s Wireless Manager, Dan Carlson, about how Emerson might address this application. Dan responded:

The end-user could implement a network of 702 discrete transmitters made by Rosemount that take discrete inputs from any type of switch, including level switches. The signals can then be sent to a Smart Wireless Gateway with either Modbus or OPC outputs routed into a PLC-type device [or natively into an automation system like the DeltaV system] to provide signal response.

Once the wireless gateway is in place and communicating with the automation system or PLC, you can add new wireless measurements or final control elements to the wireless network to provide an answer to wide range of applications.

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