why 4-20 mA is used in instrumentation?

4-20 mA standard is mainly to differentiate the conditions live zero and dead zero.

Let me tell an Example  ..Consider a level sensor fixed in a tank. Now the tank is empty .then the sensor will send the reading as 0mA this zero islive zero, if there is no standard as 4-20mA.In the same case if there is any problem in the sensor output then also the reading will be 0mA this zero is Dead zero. In order to differentiate the fault and the original process condition we have chose this range as the standard.

4-20 ma Signal in Industrial Instrumentation

Signal Flow from Instrument to Controller (PLC/DCS/Stand alone controller) -
A physical signal(Temperature / Pressure / Level / Flow etc) measured by a sensor is converted to an electrical signal by a transducer. These signals are non standard signals. These non standard signals are converted to a standard signal by a Transmitter. These standard signals are fed to processor (PLC / DCS / Stand alone controller ).

 

Different type of standard signals are used in Industrial Instrumentation

1. 4-20 ma

2. 0-5 VDC

3. 1-5 VDC. 

PLC/DCS use analog to digital converter to convert voltage signal to digital numbers which are understandably by CPU.
The measuring device has 250 ohm resistor. As for current signals,they are converted to voltage signals like this
4 ma * 250 ohm = 1 v
20ms * 250 ohm = 5v
The converted 1 -5 Vdc will go to analog digital converter.

Why 4 ma is used instead of 0 ma and why 20 ma is used?

If instrument is faulty,it can be easily identified. If the output is 0 ma it will be difficult to identify whether it is a faulty signal or a good signal.For error detection 4 ma is good.
20 ma is used as maximum because the human heart can withstand up to 30 ma of current only. For safety reason 20 ma chosen as maximum value.
Scaling of 4-20 ma is easy.
4 ma = 0 %
8 ma = 25%
12 ma = 50%
16 ma = 75%
20 ma = 100%

Advantages 4-20 ma signal  

1.Voltage Drop - Very long cables have resistance and that will cause a voltage drop. The voltage drop   will affect the reading. Current signal doesn’t have this problem. 

2.Electromagnetic interference - Current signals are immune to electromagnetic interference.

How to calculate percentage signal corresponding to ma signal ?
 

MV - Measured Value
LRV - Low range Value
Span = 20 ma - 4 ma = 16 ma.

Formula to calculate percentage signal

(MV - LRV) / Span * 100 %

 

Calculate percentage value corresponding to 14.98 ma.
(14.98 - 4 ) / 16 * 100 % = 68.6 %