William_H_Conley_IIIThe ability to monitor product inventory has always been a primary concern for companies. When the product is not a fixed, physical unit of measure, but a volume of liquid; the challenge increases. The evolvement of wireless technology presents an exciting opportunity for remote monitoring. The ability to wirelessly monitor a system can dramatically decrease the costs of typical wiring infra-structure and reduce maintenance expense – all with a fundamentally simple installation.

Initial concerns about wireless security and dependability are rapidly giving way as industrial strength products have been developed. Wireless has really come a long way. Systems are secure, more reliable and they can support several protocols such as Modbus, and Profibus. Although the project discussed below concerns water levels; any remote monitoring application should be considered as a good candidate for a wireless solution. Carefully understand the distance restrictions of any equipment you consider and talk to the product’s design engineers or technical support staff during your selection process.

This project will discuss the use of wireless in monitoring water inventory levels, pump status, and delivery pressure. In addition, wireless was used to monitor secondary system parameters such as power supply, backup battery, and radio signal strength.

The remote nature of this application makes it an ideal candidate for wireless connectivity. Increasing demand for water, aging components and desert temperatures exceeding 100+ degrees in the shade put a severe strain on motors, causing fuses to blow or breakers to open. While a large holding tank provides reserve capacity, it can also mask problems. It can take as long as two days for the tank to drain and the water to stop flowing. What is needed is a reliable way to monitor all aspects of the well system and warn of any trouble before it becomes catastrophic. In most cases, the well site is located far from homes or the businesses it services. Effective wireless monitoring is a natural and cost effective solution.

AZwaterwell_1settingWell System Overview
This tank farm site is a typical well system in southern Arizona. It feeds 12 rural homes situated on a 100 hundred-acre development. It consists of a well pump located at a depth of 700 feet and submerged in 20 feet of water; a 15,000-gallon holding tank; a pressure pump; and a 1,000-gallon pressure tank used to provide water pressure. A control panel distributes power to the pumps and houses a very simple control system. The controls turn the Well and Pressure Pumps on and off as necessary to fill the Holding Tank and pressurize the Pressure Tank.

The control system is nothing more than several float switches. The float switch follows the level in the holding tank. When the level drops to 3000 gallons; the float switch changes angle to close an internal contact. This energizes the well pump, which refills the holding tank.

A pressure switch monitors the pressure tank. The pressure pump forces water from the holding tank into the pressure tank. The pressure in this tank is a relative indication of how much water is pressurized, and, of course; how much water pressure is available to a respective home. (A PLC could have easily been added to replace the control system but it was unnecessary for this project.)

The Remote Monitoring System
The basic system required the ability to monitor well system performance, water inventory, pump status, and the water delivery pressure. The tank farm was located approximately ¼ mile from the homes it services; making wireless an attractive option. A Modbus-ready HMI was used to display the data, giving a user the ability to glance at a monitoring panel in the comfort of a home or office to check the holding tank level, see if the pumps are on or off, how much current was being drawn, and look at how much pressure was in the pressure tank.

The monitoring system was developed around B&B Electronics’ Zlinx™ wireless I/O product line, specifically the ZZ9D-NA-LR. The biggest contributing factors for selecting the ZZ9D-NA-LR were: security, plenty of onboard I/O, long range, and it communicated native Modbus RTU.

The goal was to monitor the basics. This included water level in the holding tank, AC current drawn from the well and pressure pump and the pressure in the pressure tank. Wherever practical, sensors with a conditioned current output (4-20 mA) were used. A current type sensor eliminates most induced noise and can easily support longer connection lengths.

Also, keep in mind that you should use a sensor/transmitter with a full scale measurement greater than what is being monitored. If you plan on measuring a motor with a maximum current draw of 20 A, select a current sensor with a full scale of 50 A. While some resolution may be sacrificed, it can provide the opportunity to catch potential failure modes that would otherwise be missed if the sensor maxed out at rated load.

AZwaterwell_2panelI recommend you use 50 A current sensors, one on each leg of the incoming 220 VAC power cable. A 50 A current sensor is used to detect any problems with the motor in case it starts to go bad. Large current draw is an excellent indicator of Start or Run capacitors going bad.

Make sure you are using a level ‘transmitter’, not a level switch or other device, it’s sometimes confusing. Flowline’s LU12-5001 level transmitter was selected because of the low cost and it gave a 4-20 mA output signal with respect to a 4” to 40’ depth detection.

Mounting the level sensor was one of the most difficult parts of the project. It required mounting the sensor as well as running conduit to install the wiring.

The pressure sensor outputs a 4-20 mA current signal with respect to its full scale measurement of 0-100 psi. It simply took the place of the old pressure gauge.

Other Suggested Applications
The fundamentals of this application can be easily modified to fit other liquid or gas inventory management systems. Natural gas or fuel tank farms are a logical extension. In addition to inventory management; system monitoring of motors, pressure tanks, temperature, flow, and other mission critical parameters are recommended.

Wireless Modbus RTU nodes can also be used to extend the I/O count on an existing PLC with limited I/O count - effectively expanding a system. Wireless continues to be one of the most exciting developments in industrial remote monitoring and management of critical systems.