Explain why 4-20ma is used for instrumentation
Transmitter is a device that measure process variable and transmit analog signal mostly
Why 4 mA?
Electronic chips need minimum 3 mA to act
And eddy current has a bad effect on the signal less than 4 mA
So they choose 4 mA as a minimum signal
Why 20 mA ?
Because human heart effected with signal begin from 30 mA
So they made the maximum signal less than 30 mA
So transmitter send 4:20 mA then converted to signal 1:5 V by 250 ohm resistance to operate ADC Analog to Digital converter that convert the incoming signal to binary signal.
4:20 mA is better than 0:20nA
4 20 ma electromagnetic interference immunity is better than 0-10v and can monitor sensor break / fault if signal is 0 ma.
4:20 mA is better than 0:10v because the voltage signal has voltage drop across distance
V= IR 4250/1000=1v 20*250/1000=5v
Input Signal 4mA to 20 mA output good signal Linear pressure 3psi to 15 psi and start from 4mA because check to feed back signal for DCS