Using a wireless radio battery to power a pressure transmitter

In an ideal world, every place you need to install a transmitter would have easy access to electrical power. But we don’t live in the ideal world, and there are locations where power isn’t readily available, and getting electrical power to those areas can be quite costly.

This was the situation at a local plant, in their flammable fuel storage facility. The site is required to keep a record of the line pressure that supplies water in the event of a fire.

The Problem

The water pipe is located 6 to 7 feet (2 meters) below grade, and the engineers planned to build a vault where they could install and maintain the necessary submersible pressure transmitter.

The planned location is more than 500 yards from any available power source. The signal coming from the pressure transmitter would have to run that distance plus an additional 200 yards to reach the control room.

Not surprising, the trenching cost for an electrical installation was quoted at a five-figure minimum price.

Finding a solution

The project engineer started considering a wireless solution, and then asked, “Can a battery-powered industrial radio also power a two-wire loop-powered pressure transmitter?”

The answer requires consideration of a couple of factors:

  1. The obvious problem is that providing continuous power to a loop-powered transmitter would run the battery down in no time at all. But in this case, the monitoring equipment isn’t for continual updates. It only required two readings per day. But, the site wants readings every 10 minutes to prove that the system is up, running, and functional.
  2. The other factor is in learning how much power is required to initialize any specific transmitter. Luckily, the IP68 submersible pressure transmitter being installed in this application used a very modest amount of power. It quickly and easily powered up at just 15 VDC.

Banner Engineering’s DX-80 FlexPower wireless node radios are designed to provide power for periodic monitoring at extended intervals. The radio’s ‘Switched Power’ wiring terminal does exactly what the engineer needed — power the loop-powered transmitter at a periodic interval.

Configuring the power to the terminal takes just three settings:

  1. Deciding how often the ‘Switched Power’ is turned on
  2. Defining the duration of the Power-On event
  3. Determining the DC voltage level the battery power has to be boosted to power the transmitter.

In this situation, a boost to 15 VDC for five seconds would initialize the pressure transmitter and provide a stable reading. The engineer set the interval between Power-On events at 10 minutes, to show the inevitable minor pressure changes on the control room’s digital display.

A Banner gateway radio at the control room reproduces the pressure value as a 4-20 mA signal, which is easily handled by the control system. The field nodes and gateway made for a clean installation, getting this critical pressure signal back to the control room.

The best part?  Total cost for two submersible pressure transmitters, two battery-powered wireless field nodes and a gateway radio: an easily affordable $5,000.

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