Instrumentation and SCADA · Signals and Basic Control
What Is an Analog Signal?
An analog signal changes smoothly as the measured condition changes.
A common plant signal represents the low end and high end of an instrument range. The most widely used is 4 to 20 milliamps (mA): 4 mA is the bottom of the range (0 percent), 20 mA is the top (100 percent), and 12 mA is the middle (50 percent). The exact signal standard and scaling are set by the plant. Operators should use the loop information provided for their facility.
If tank level rises smoothly but the displayed value stays fixed, the loop may be frozen, disconnected, or held in a manual state.
An analog loop represents many values between its low and high ends. The receiving device must use the same scale as the transmitter. If one end is set for a different range, the display may be wrong even though the signal is healthy. Operators may notice this after a replacement or programming change. Compare known process points when possible. A reading that is always off by the same amount may suggest a different problem than one that is off by a percentage.
A scaling mistake can create a smooth, believable, but completely wrong value. Because the bottom of a 4 to 20 mA range is 4 mA, not zero, a signal near 0 mA points to a broken wire or lost loop power rather than an empty tank. The NAMUR NE 43 industry recommendation, which many transmitter makers follow, treats a signal below 3.6 mA or above 21 mA as a device failure.
Key point: If the receiving device uses a different scale than the transmitter, the display can be wrong even though the signal is healthy.
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Wastewater Instrumentation, SCADA, and Process Control Fundamentals
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