Monitor Flow and Level Wirelessly at Remote Sewage Lift Stations

Monitor Flow and Level Wirelessly at Remote Sewage Lift Stations

Back in the late 1980’s a local city municipality had six lift-stations wired underground directly to their Wastewater Plant located a few miles north of Daytona International Speedway. However, in 1995 a forest fire burned the entire surrounding fields which destroyed the Level and Flow sensors and the underground wires going to the wastewater plant were melted and rendered useless.

Cost issues have been a concern for this more rural municipality. They were searching for a more economical and cost effective way to retrofit the three lift stations than reinstalling all the old hardwired systems.

Over the years, all six of the lift-stations have been repaired and upgraded. However, they have only hardwired three of the main lift-stations back to the Wastewater Plant using a data acquisition system.

The city would like to tie the remaining three lift-stations into their data acquisition system by using Wireless Telemetry. By doing this they will be able to monitor two 4/20 mADC signals coming from a Flow and Level sensor at each of the three remaining lift-stations.

The decision to use Wireless was made to help alleviate the destructive issues from the aforementioned forest fire as well as help maintain the integrity of the system given Florida’s high winds (upturned tree roots), excessive high level ground water, and encroaching metropolitan development.

All three lift stations are located about 1,000 feet apart from each other deep into the forest. However, there is a 40 foot wide clearing next to the lift-stations which the city keeps mowed as an access road to each lift-station.

DR9011 With 3 Element Yagi Antenna

DR9011 With 3 Element Yagi Antenna

In order to test the effectiveness and durability for the new proposed wireless system one of the remote non-wired stations was configured in the following manner:

·        3-Element, 8 dBi Gain Yagi Antenna was installed on a small 10 foot tower at one lift-station

·        A DR9011 wireless transmitter was installed in the existing enclosure along with the Flow and Level sensors producing the 4/20 mADC signals.

·       A DR9021 Wireless Receiver was mounted approximately 3000 ft away at one of the main lift-stations.

DR9021 Wireless Receiver

DR9021 Wireless Receiver

·        The 4/20 mA signals out of the DR9021 Receiver were wired directly into the existing data acquisition system to be sent back to the Wastewater Plant for monitoring.

The system has been running trouble free for over two years (2012) and the two other wireless systems are due for installation when time and funds are available.

Note: The lift station supervisor said that by using the Wilkerson Instrument Company wireless system they will be saving the City over $25,000 for the 3 installations.

Ground Loop Primer

Ground Loop Primer                                                       Printer Friendly PDF

One of the most frustrating problems of the measurement and control industry is that of the ground loop. Its effects can appear and disappear with no apparent reason and can range from mere annoyance to downright destructive. It seems that ground loops carry some mystical connotation to the point that our industry has made it a “catch all” culprit for anything that cannot be explained.

While ground loops can be complex problems and may not always be predictable they can be understood and dealt with if we have a little insight into just what they are and how they can affect a transmitted signal. Let us first refer to Figure 1.

This Figure depicts what we might consider to be a typical measurement loop. It has a transmitter sending a signal to a receiver, some finite distance away, over a pair of wires. One side of the signal current has become grounded via internal circuitry and ultimately is tied to earth ground usually via the instrument case.

As depicted in Figure 1, this measurement loop would probably work fine and not have any influence from ground loop currents. However, reality sets in and we have to abide by plant safety procedures, the National Electric Code, etc. Safety procedures almost always will mandate that each piece of equipment be grounded to earth at its respective installed location. This is where the trouble starts.

Once we ground two pieces of equipment at two different locations we have set the stage for ground loop problems. If we could take a volt meter (Figure 2) with very long leads and measure the voltage between the ground points of the transmitter and the receiver we would measure some voltage.

It may measure in millivolts or it could be many volts. Either way, if there is a potential difference, then current will flow between these two points. Since the earth presents itself as a resistor between these two ground points the amount of current that flows between the points will be directly proportional to the voltage difference and inversely proportional to the resistance.

For those who are fans of Ohms Law you will recognize this equated as I=E/R. I being the ground current; E being the voltage between the ground points; and R being the resistance of the earth between the two ground points.

Ground Loop Figure 2

Figure 3 shows that we now have two currents that can flow through the wiring between the transmitter and receiver. If it all stopped here we could just calibrate the measurement loop to nullify the effects of the ground current and go on about our business.

Many times this is exactly what happens. A technician calibrates the loop and comes back a few days later to find that his calibration is no longer accurate. What has happened? It could be a lot of things. Maybe it rained and the resistance of the earth changed. Suffice it to say there are many phenomenons, either natural or man-made, that can change the resistance or the voltage between the two ground points thus effecting the calibration of the loop.

Ground Loop Figure 3

It is apparent that we are “fighting a losing battle” thinking we can anticipate the interactive affect of ground loop currents on our measurement loop. What can we do to get around this problem? The answer is to provide DC isolation between each component in our loop. (Figure 4) DC isolation can be accomplished either in the transmitter, the receiver, or with a third component as shown in Figure 4.

Ground Loop Figure 4

Figure 4 shows DC isolation being accomplished by using a transformer. An isolator module, of course, is much more than just a transformer, but it is the transformer component in a signal isolator that, in fact, provides the isolation since DC cannot pass through a transformer. Now that we have inserted this “transformer” into the circuit, the ground loop between the transmitter and the receiver no longer exists, thus eliminating its effects on the signal current.

The Wilkerson product line provides several options for implementing DC isolation depending on how the isolator is powered. There are three basic ways of powering an isolator.

These are listed below with the respective modules:

1. Input Powered or Loop Powered

DM4391-1 DM4391-2

2. Output Powered or Two Wire

TW810X

3. External Powered

MM4300 Series MM4380A

DM4300 Series DM4380A

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