Function of Sensors and Transmitters
Sensors are used in the field of instrumentation to measure various physical quantities of a process, including its flow, pressure, temperature, and so on. The output of the sensor comes in the form of voltage, and this voltage is transmitted to the transmitter that is coupled to the sensor. The transmitter turns the voltage signal into the current signal. Calibration of this transmitter determines the range of the current signal that it will create at the output. Calibration also determines the accuracy of the current signal. Generating a current signal in range of 0-20mA (or) 4-20mA is a function that almost all transmitters built to industrial process standards are capable of doing. The engineer who calibrates the transmitter to determine current signal over all ranges determines all.
Use of 4-20mA in Devices
The transmitters are typically devices that are powered by a loop, and they are typically provided by a 24VDC supply. The majority of industrial applications make use of two-wire loop powered transmitters, which include both the signal for the loop current & the power supply at the same point on the same electrical wires.
Assume that the pressure imposed by the fluid on the pressure sensor by the pipeline as it moves through the pipeline ranges from 0 to 10 bar, and that the transmitter is calibrated to provide a current signal in the range of 4 to 20 milliamperes, which is proportional to pressure that the fluid is exerting on the pressure sensor.
There will be no pressure exerted on the sensor by the pressure transmitter even when there is no liquid (fluid) flowing through the pipeline; yet, the output terminals will still receive a current signal of 4mA. In addition to this, the transmitter will provide a current signal of 20mA at the output terminals if the pressure that is being applied to the pressure sensor is at the maximum calibrated value of 10 bar. It is thus possible to derive the conclusion that an open circuit failure has taken place if there is no passage of current via the wire that connects the transmitter output terminals & the input card of the controller.
Use of 0-20mA in Devices
On the other end, the fluid moving through the pipeline creates a pressure in range of 0-10 bar, and that the transmitter is calibrated to provide a current signal in their range of 0-20 mA. In this scenario, the current signal would be generated by the transmitter. The transmitter will create 0 milliamperes of current at the output terminals whenever there is no fluid flowing through the pipeline. In this scenario, it would be very challenging to determine whether the 0 mA current is the result of an open circuit in the transmitter or a lack of pressure in the fluid. Both of these possibilities are equally plausible. Therefore, errors such as open circuits may be readily identified if the transmitter is calibrated to provide a current signal in range of 4-20mA.
Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?
If a current output of 4-20mA is supplied to the input card of any controller, then a resistor with a value of 250 Ohm must be placed in the route in order to transform this current signal into a voltage signal with a range of 1-5V. The ADC of controller will only handle voltage signals that fall within the range of 1-5V as a matter of routine procedure. Using a current signal with a range of 4-20mA is recommended for this reason, among others.
Conclusion
As a result, it is recommended that a current signal of 4-20mA be used rather than 0-20mA. Not only does this range assist us in the detection of open circuit failures, but it also makes it simple to convert this signal into a voltage signal in the range of 1-5VDC, which will be managed by the input card of the majority of controllers.