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Remote level monitoring

   

Remote level monitoring

A Moore Industries customer wanted to monitor levels in new storage tanks that are remotely located on their tank farm and are not connected to their existing Modbus network. Twenty existing tanks are monitored using Modbus continuous level sensors with data transmitted over a RS-485 twisted pair network.

Five new remote tanks were added and needed to be monitored, but were built with a level sensing device that only provided a dry contact output from a float cage to indicate level, no Modbus output available. The customer needed to convert the dry-contact output from the new remote tank level devices to a Modbus signal, and then transmit the data for monitoring at the Distributed Control System (DCS). The new remote tanks could not be connected to the existing Modbus network using twisted pair wire, because they are located across a river from the existing tanks and the DCS.

The first step involved using a pair of resistors and a bulk DC power supply to convert the dry-contact relay output from each tank’s float cage to a 4-20mA signal. Next, a Moore Industries TMZ PC-Programmable Modbus Transmitter (Signal Converter) was used to read and convert the 4-20mA signal. And finally the new Modbus signals had to be connected to the existing network across the river. This involved installing a pair of Moore Industries WNMs (Wireless Network Modules) that utilized a point-to-point wireless network link across the river.

The Moore Industries WNM Wireless Network Module is a bi-directional, point-to-point or point-to-multipoint, 900 Mhz frequency hopping spread spectrum radio modem that includes a RS-485 port. In order to connect the two Modbus networks together across the river, a set of WNMs was used in point-to-point configuration. Each WNM can also be simultaneously configured as a tank site source and a wireless repeater, allowing for virtually unlimited transmission range.

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