Why 4-20 ma signal is used in Industrial instrumentation?



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 . 4-20 ma,0-5 VDC, 1-5 VDC.

4-20 ma

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%

What are the advantages 4-20 ma signal comparing to voltage signals (0-5v ,1-5v) ?

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 %

4-20mA transmitter - basic
4-20 mA Current loop - Calculation of process variable

Why 4 ma is used instead of 0 ma?

The primary reason for 4mA as a ‘range zero’ is not its live-zero error detection functionality; the main reason is that a 4.0mA ‘range zero’ allows about 3.5 to 3.6mA of electrical power to be used to “loop power” field instruments using the same 2 wires for power as are used for the signal. 2 wire loop power functionality was the driving factor for a 4mA range zero.

4 wire 4-20mA signals, like a controller output that drives a valve positioner is not loop powered but the device it drives, the positioner uses 3.5 to 3.6mA to drive its internals.

If the range zero were truly zero mA, then there would be no current, and therefore no power to drive a field instrument and field instruments would have to 3 or 4 wire devices, with all the attendant cost of additional wiring.