Tom Jenkins
Lifetime Supporting Member
I'm pretty sure 4-20 mA came first, and then loop powered devices followed as electronics improved. Loop powered devices, indicators or transmitters, run the current through a diode to create a voltage drop (or I suppose sometimes a resistor, but usually a diode so you have a constant voltage). This voltage is used to power the electronics.
It isn't the first or the last mA, just all of the mA. Obviously, if you had 0-20 mA you couldn't have loop powered devices. However, I believe the loop power took advantage of the 20% zero offset, the offset wasn't created to allow loop power.
As I hinted above, the old pneumatic controls had a 20% zero offse as well, 3-15 psig. This may have had a benefit in providing energy for moving bellows etc. at a low percentage of full scale signal, but I don't think that is why they went 3-15 psi and I doubt that is why 4-20 mA emulated it.
The big reason, from what I have ben told and read, is that the 4 mA gives you a way to distinguish between a true zero process variable and an open circuit. Ganutenator was right.
It isn't the first or the last mA, just all of the mA. Obviously, if you had 0-20 mA you couldn't have loop powered devices. However, I believe the loop power took advantage of the 20% zero offset, the offset wasn't created to allow loop power.
As I hinted above, the old pneumatic controls had a 20% zero offse as well, 3-15 psig. This may have had a benefit in providing energy for moving bellows etc. at a low percentage of full scale signal, but I don't think that is why they went 3-15 psi and I doubt that is why 4-20 mA emulated it.
The big reason, from what I have ben told and read, is that the 4 mA gives you a way to distinguish between a true zero process variable and an open circuit. Ganutenator was right.