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13.1: SCADA?

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    SCADA is the acronym for Supervisory Control and Data Acquisition. It is a computerized system allowing a water system to operate automatically. A SCADA system usually consists of three (3) basic components: field instrumentation, communications (telemetry), and some type of central control equipment. The field instrumentation will measure various parameters such as, flow, chemical feed rates, chemical dosage levels, tank levels, etc. These instruments will then gather a series of signals and transmit them through some type of communication device(s) known as telemetry. The telemetry communication can be radio signals, telephone lines, fiber optics, etc. This information is sent to a central control computer typically located at an office or operations control center. This computer will have software interpreting the signals and displaying the actual values of the parameters being measured. Below is an example of a typical SCADA computer screen.

    Figure \(\PageIndex{1}\)

    A common measurement used to analyze the various field parameters of a water system is the 4-20 milliamp (mA). A 4-20 mA signal is a point-to-point circuit and used to transmit signals from instruments and sensors in the field to a controller. The 4 to 20 mA analog signal represents 0 to 100% of some process variable. For example, this 0 to 100% process variable can be a chlorine residual from 0.2 to 4.0 mg/L or a tank level of 0 to 40 feet. The 0% would represent the lowest allowed value of the process and 100% the highest. These mA signals are then sent through the SCADA system and processed into understandable values such as mg/L or feet, depending on the parameter being measured.

    This first example is using the 4-20 mA signal to measure the level of water in a storage tank. The tank is 40 ft tall and has a diameter of 30 ft (not to scale).
    Figure \(\PageIndex{2}\)

    There are a couple of things to point out with storage tanks. First, although the height of the tank is 40 ft, the water is never filled to that height. Why? Because the inside roof of the tank would be damaged. Therefore, all storage tanks have an “overflow” connected at the top of the tank off to the side. The second thing to point out is that the “bottom” or zero level of the tank is never at the actual bottom of the tank. Why? Because you never want to run a tank empty. There is always a several foot distance from the actual bottom to what is referred to as the “zero” level. In many questions, the “overflow” (actual top-level) and the “bottom” (actual location of the zero level) will be mentioned.

    Therefore, in this example above since there is no reference to an overflow or where the zero level is located, the 4 mA signal would represent 0 ft and the 20 mA signal 40 ft. What this is saying is if your meter sends out a signal of 20 mA, then the corresponding level in feet would be 40. Likewise, if the signal was 4 mA the corresponding level would be 0 ft.

    What do you expect the mA reading to be if the tank was half full (20 ft)?

    If you initially thought 10 mA that would be a logical guess. However, let’s think about this for a minute. Since the bottom or 0 ft is at 4 mA and the top or 40 ft is at 20 mA, the span, or difference between 4 and 20, is only 16, not 20. This “span” is an important number when solving these problems.

    Now, if your second guess was 8 mA that would be a logical answer too, but it is also an incorrect response. Yes, 8 is half of 16, but we are not dealing with a span of 0 - 16, we are dealing with a span of 4 - 20. Therefore, half of 16 is 8, but the halfway distance between 4 and 20 is 12! Anyone who guessed 12 mA, give yourself a hand. Whatever read you have on your meter, you must subtract out the 4 mA offset.

    Once you understand this the equation is quite simple. The meter read minus the offset divided by the span equals the percent of the value being measured.

    • (mA (reading) - 4mA (offset))/ 16 mA (span) (20 - 4) = percent of the parameter being measured

    Let’s use the 40 ft tank example to illustrate the solution.

    In a 40 ft tall tank, a 10 mA reading was collected for the height of the water level in the tank.

    • (10mA (reading) - 4mA (offset))/16 mA (span) = 6 mA/16 mA = 0.375 or 37.5% full

    If the tank is 37.5% full then multiply this percentage by the total height.

    • 0.375 x 40 ft = 15 ft


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