Why a 4-20 mA signal is used, not 0-20 mA or 0- 30 mA signal in Instrumentation ?

Any process parameter such as Temperature, Pressure, Flow, Density is sensed, measured by a sensor and is converted to an electrical signal in the form of voltage. The voltage from the sensor is sent to the transmitter to convert in a standard form of the current output signal.

Most of the applications use 4-20 mA signal instead of 0-20 mA or 0-5 V, or 1-5 V.

1 (a). Live Zero i.e 4 mA. It is easy to detect that the signal wire is not broken. If “0” mA, unable to know that it is a signal or wire broken.

(b). The 4 mA is used as “range zero”. If the range zero were truly zero mA, then there would be no current, and therefore no power to drive the field instrument. Field instrument would require 3 or 4 wire devices. Additional wiring costs come into account.

(c). Another reason for choosing 4 mA is when the transmitter was first introduced, it was working on pneumatic pressure signal for transmission. Later, electronics chips were introduced those require a minimum of 3 mA of current to function. So, a margin of 4 mA is taken as reference.

2. Scale and Calculate: with 4-20 mA signal it is easy to scale and calculate the process parameter.

(a). Scale: 4-20 mA signal

mA        Percentage     

4 mA           0                     

8 mA         25                   

12 mA        50                    

16 mA        75                    

20 mA       100   

(b). Calculate percentage of signal corresponding to mA signal

(Measured Value -. Lower Range value)/Span * 100 %  

For Example: 15 mA signal is (15-4)/16*100% =    68.75 %        

3. Analog to Digital conversion: CPU in PLC/DCS/ Standalone controller need digital signal 1-5 V. PLC/DCS converts analog to digital signal so that the CPU can read it. For conversion of current from 4-20 mA to 1-5 V needs a 250 ohms resistance.

As we know V= I R

V= 4 mA X 250 Ω = 1 V

8 mA X 250 Ω = 2 V

12 mA X 250 Ω = 3 V

16 mA X 250 Ω = 4 V

20 mA X 250 Ω = 5 V

4. Reason for 20 mA: 20 mA is used as maximum because the human Heart can withstand up to 30 mA of current only. So, as a safety point of view, 20 mA is chosen.

Advantages :

It is more robust against electromagnetic interference.

Very long cables have resistance, that causes a voltage drop. The drop in voltage affects the reading. The advantage of using a 4-20 mA signal is that Current signals do not have this problem.

Disadvantages:

The current can induce a magnetic field and cross talk to parallel lines. One should use twisted wire and keep it distant from other communication lines.