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Earlier, instead of 4-20 mA, 0-100 mA was used. But the usage of 0-100 mA led to sufficient power dissipation. Hence the upper range was reduced to 20 mA to optimize the power dissipation (this was found by experiments). In this way the upper limit was set. But this 20 mA is limited to cable length of 50 meters.

For the lower limit i.e. 4mA . Zero was not used as the lower limit because it give no intimation about actual zero value or zero transmission due to some breaks in the circuitry. Therefore 4mA was set as the lower limit as it is also above the maximum leakage current in electronic circuits. In some transmitters, some times the transmitter shows 2 mA. This is used to indicate that there is some fault in the sensor. Actual zero is indicated by 4 mA.

The scale is linear. For example, in a pressure transmitter of range 400bar.

0 bar - 4 mA

100 bar - 8 mA

200 bar - 12 mA

300 bar - 16 mA

400 bar - 20 mA.

The reason for not using voltage is that voltage continuously drops over the length of the wire whereas current can be transmitted over a given length without any drop.

The main purpose of using electrical quantities is transmission to remote locations.

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