Why do we use 4-20mA instead of 0-20mA current signal in industrial Automation?

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In process instrumentation, sensors are used to measure physical quantities such as pressure, temperature, flow etc. of the process. The sensor generates output in the form of voltage which is sent to the transmitter attached with the sensor. The transmitter converts voltage signal into current signal. The range of current signal that the transmitter generates at the output is decided through the calibration of the transmitter. Almost all transmitters that are manufactured on industrial process standards can produce current signal in the range of 0-20mA or in the range of 4-20mA. It all depends on the choice of an Engineer who calibrates the transmitter to set current signal in any of the range.
The transmitters are generally loop powered devices and usually supplied through 24VDC supply. Most industrial applications use 2-wire loop powered transmitters that contain loop current signal and the power supply at the same electrical wires. The following figure shows how power supply is connected in loop with current signal generated by the pressure transmitter and later on, how current signal is fed to the input card of the controller.
Suppose that the fluid passing through pipeline exerts pressure in the range of 0-10 bar and the transmitter is calibrated to generate current signal in the range of 4-20mA; which is proportional to the pressure being exerted by the fluid on the pressure sensor.

When there is no fluid flowing through pipeline, no pressure exerts on sensor but pressure transmitter still generates 4mA current signal at the output terminals. And if the pressure exerted on the pressure sensor is at its maximum calibrated value of 10 bar, the transmitter generates 20mA current signal at the output terminals. So, if no current flows through the wire between transmitter output terminals and input card of the controller, it can be concluded that the open circuit fault has occurred.
On the other side, suppose that the fluid passing through pipeline exerts pressure in the range of 0-10 bar and the transmitter is calibrated to generate current signal in the range of 0-20mA. When there is no fluid flowing through the pipeline, the transmitter generates 0mA current at the output terminals. In this case, it would be extremely difficult to identify that either 0mA current is due to open circuit of the transmitter or it is due to no pressure of the fluid. Hence, if the transmitter is calibrated to generate current signal in the range of 4-20mA, the faults like open circuits can easily be detected.
It can also be seen from the above graphical illustration that if 4-20mA current output is fed to the input card of any controller, we use 250 Ohm resistor in path to convert this current signal into voltage signal of range 1-5V. As a standard, the ADC of the controller only process voltage signals that are in the range of 1-5V. It is another reason to use current signal in the range of 4-20mA.
Hence, it is best practice to use 4-20mA current signal instead of 0-20mA. Not only, this range help us in detecting open circuit faults, but also, it becomes easy to covert this signal into 1-5VDC voltage signal which will be processed by the input card of mostly controllers.
 
Actually the elevated "live zero" that you describe is convenient for detecting broken wire open circuits, but it is a secondary reason for the widespread use of 4-20mA signals. The primary reason that 4-20mA is used instead of 0-20mA is because an elevated 'zero' allows for powering the field device on the same two wires that the signal uses.

Passive, loop powered field transmitters use the DC electrical currrent below 3.5mA to power the transmitter to make the measurement. The voltage is provided by the loop power supply and the loop current is regulated by the field transmitter which acts as a transmitter, regulating the loop current in proportion to the measurement. The aspect of power-over the-wires is what drives the market for passive, 2 wire, loop-powered field instruments because otherwise they would have to be 3 or 4 wire units, with the increased installation cost.

Most all field transmitters will drive slightly below 4.00mA and above 20.00mA for signal outside the calibrated range. And there's a fail-low setting at about 3.6mA (varies depending on vendor) to signal the loss of a true measurement. But the current below 3.5mA is 'reserved' for powering the transmitter.

Active 4-20mA outputs, like a controller's output, are powered by the device's power supply and can output 0-20mA because the power supply to drive the loop current is local, and powers other functions.
 
Thanks Mr David . I really missed to mention this point which is applicable for loop powered Transmitters.

I take this opportunity to mention why we choose current signal transmission instead of voltage.

  1. Current signals are often used in sensors because the current response is usually more linear than the voltage response.
  2. Current signals also typically provide a low impedance for sensors, with the benefit of better immunity to noise
  3. Current signals can extend much further than voltage signals, allowing robust signal wire lengths of up to 1,000 meters.
  4. 4-20mA current loops is safe enough. Even if the power supply is 24 volts, shock isn’t a danger .
  5. 4-20mA current loop is intrinsically safe for hazardous areas .
 
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