Answer / carl_ellis

Historically, the 4-20 mA electronic control signal replaced the 3-15 psi, 20-100 kPa or 0.2-1.0 bar pneumatic control signals and its competitor 10-50 mA.

The ratio of contorl signal span to its 'live zero' is the same in all cases: 4:1

The ratio of span to the 'live zero' for 3-15 psi is:
span = 12 psi (15-3)
live zero = 3
ratio span:live zero = 12:3 = 4:1

The ratio of span to the 'live zero' for 20-100 kPa is:
span = 80 psi (100-20)
live zero = 20
ratio span:live zero = 80:20 = 4:1

The ratio of span to the 'live zero' for 0.2-1.0 bar is:
span = 0.8 bar (1.0-0.2)
live zero = 0.2
ratio span:live zero = 0.8:0.2 = 4:1

The ratio of span to the 'live zero' for 10-50 mA is:
span = 40 mA (50-10)
live zero = 10 mA
ratio span:live zero = 40:10 = 4:1

The ratio of the span of the active control signal to its live zero is identical in all cases.

Carl Ellis
Measure First

Answer / satish singh

because in this range the current value is continuity /or we can say that in thios range the value is linear

Answer / sanju6192@gmail.com

if zero signal is send in process then there can be cable cut then not recognize that this is signal transmit or cutting signal

Answer / marmikjani

Why we use 4-20mA signal?? Not start from 0-03mA? & 20mA??





· We can not use 0-20 mA signal cos then it becomes very difficult to distinguish between "zero" signal and "instrument fail" signal.



· Where as if we use 4-20 mA. its very easy like when we get 4mA means the signal is at low level and if it is below 4mA it shows us that the instrument is not working or it is failed




•That may have been one of those arbitrary decisions that is sometimes made when a standard is developed. In addition to providing a non-zero minimum to allow detecting signal loss, the minimum signal voltage provides for a margin between the minimum signal and any noise that might be present. Perhaps someone determined that 4ma was large enough to help with noise and thought that the numbers worked out conveniently. A 16 volt difference between min and max, difference = 4 X min, range of 20% to 100%, perhaps seemed easier to work with than 17 volts, 5.67 X min and 15% to 100% or 18V, 9 X & 10% to 100%


•This 20 mA signal is not enough to trigger a spark and therefore we can limit fire hazards in plant area


•As there is a leakage current so 0 to 3 MA, so we consider as 4-20MA. But in some cases we will find some transmitters 0-20MA which are more costly than 4-20mA

Answer / carl_ellis

@Marmkijani
The main reason for 4-20mA is that 2-wire field instruments need to use 3.5mA to power their own electronics in order to make a measurement and to control the regulated output.

4-20mA is also, as you pointed out, a range in which the energy in the circuit is low enough to qualify for Intrinsic Safety approval (when used installed properly with an approved DC power supply and I/S barrier).

The "live zero" is nice and convenient as a diagnostic for an open circuit fault, but it was not the main reason for 4-20mA; powering field instruments was the main reason.

what should he know if he is an assitent engineer instrumentation in cement industries of experience of 10 - 15 year.

0 Answers  

What is the Temperature Co-officient of Resistance of PT- 1000 and PT-2000 ?

2 Answers   Adani, Birla, MEIL,

how u meassure room temp?is it possible 2 meassure room temp by rtd or thermocouple?between rtd n thrmocouple which is more regard?3 meassure diff. between rtd n thermocouple?

3 Answers  

What are the logic behind 2x2 and 2x3 logic and more when ask in interviews

1 Answers   Reliance,

In Safe Guarding test what means by "Failed to Danger" ?

0 Answers   Optimal,

what is the working principle of ABB gas analyser of kiln in cement industries

4 Answers  

transmitter in 3 connection +,-,And test mode so how Use To Test Mode

1 Answers   Essar,

what is the different between plc and dcs ? I know full name of this two but I want to know what structure and working style and speed I/O capacity and controller difference?

2 Answers   L&T,

If we interchange HP and LP tapping in drum level measurement and reverse LRV & URV correspondingly.What is the difference between the normal tapping(HP,LP) and reversed tapping(LP,HP)?

2 Answers  

What s PDS & PDMS tools ?its role in instrumentation?

1 Answers  

My flow transmitter range is 0-1000mmwc.DCS range is 0- 100m3/hr.What is correspond out put in terms of milliamps if my flow is 65m3/hr

4 Answers  

There are many type of control cable like pair,core,tried, What are defination each one

0 Answers   Birla,