4 to 20 ma conversion
The 4 to 20 milliamp DC standard is the most widely used form of signal transmission in modern industrial instrumentation systems. It is an analog signal standard, which means that electric current is used to accurately represent measurements or command signals.
In this standard, a current value of 4 milliamps represents 0% of the scale, while a current value of 20 milliamps represents 100% of the scale. Any current value between 4 and 20 milliamps corresponds to a proportional percentage between 0% and 100%.
To illustrate this, let's consider calibrating a 4-20 mA temperature transmitter for a measurement range of 50 to 250 degrees Celsius. We can establish a relationship between the current and the measured temperature values on a graph.
This concept is similar to the 3-15 pounds per square inch (PSI) pneumatic signal standard, where varying air pressure represents a process variable proportionately. Both the 3-15 PSI and 4-20 mA signal standards are known as "live zero" because their ranges start with a non-zero value. This "live" zero allows for easy differentiation between a legitimate 0% signal value and a failed signal, such as a leaking tube or damaged cable.
It is crucial to ensure compatibility in the range of instruments sending and receiving analog signals to accurately represent the desired variable. For instance, let's consider a temperature measurement system consisting of a thermocouple, a temperature transmitter, a 250 ohm resistor (to convert the 4-20 mA analog signal into a 1-5 volt analog signal), and a specialized voltmeter functioning as a temperature indicator.
It is important to note how the output range of each sending device matches the input range of its corresponding receiving device. If we visualize this system as a path for information flow from the thermocouple's tip to the transmitter, then to the resistor, and finally to the voltmeter/indicator, we can observe that the analog output range of each device must correspond accordingly.
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