Current-Based Signals

<4mA or >20mA indicates instrument range error or circuit fault.

Current-based signals are practically immune to electromagnetic interference from other nearby circuits.

4-20 mA DC current is used to indicate sensor output or controller output, and the DC is not subject to externally induced currents by AC sources, since AC is required to induce current.

The reason 4 mA is used as the low end (rather than 0 mA) is typically used for ensuring that a process set off an alarm or shut down when no current is detected, since an open circuit between sensor and controller will also produce 0 mA current.

Some other common output/input current are 10-50 mA, 0-10mA, 0-50mA, but 4-20mA is the most common.

I think the best way to think of it is to compare it to the other common alternative, 0-10VDC (or 0-5VDC). Using a DC voltage analog output signal has 2 main problems:

1) It is much much more susceptable to errors cause by noise, i.e. induced voltages from nearby AC circuits or RF interference.

2) It is more susceptable to voltage drop over long distances and drift due to temperature variations.

Using a current signal significantly reduces those issues to the point where they can be dealt with by monor adjustments in initial setup calibration. Then if you have decided on a current signal, the other issues posted above relate to why you want to use 4-20ma instead of 0-anything.

Why Not!?!

Ya gotta probem with that.

4-20 ma output is much more reliable than 0-20mA because of noise,vibration if nearby another machine works etc.

For example turbine flow meter its analog value set up is 200 cubicmeter/hour and counter will start to count ,If flow of oil in the pipe is 100% then output of sensor will read 20mA(200 cubicmeter/hour),If 75%(150 cubicm/hour),50%=100cubicm/H,25%=50 cubicm/H,

(If output of sensor is less than 25% then counter will not count,when exactly some flow in the pipe it will always more than 4mA.)

If it would start from zero ,then counter for Sump(Sump=total how many cubicmeter for 1dayor 2 day or 1 year) will count always even there is no flow of oil in the pipe ,because sensor output is always exist due external noise(nearby some motor can run),and its value can be 0.5mA or 1mA

Since our counter will work only if output of sensor more than 4mA.

So This type of meter (4-20mA) can reduce the error than 0-20mA