Answer / ahmed eid

Ohm's law voltage = current * resistance
of the standerd voltage in the dcs from 1 to 5 voltge
and the standerd resistance of loop 250 oums
so
V=I*R
the first in the low signal 1 volt
1(v)=I*250(r) =.004 A
.004*1000 = 4 MA
the sacand in the high signal 5 volt
5(v)=I*250(r) = 020 A
.020*250 = 20 MA

Answer / d tilston

Two wire transmitters require some power to work. 0-20 mA
would not provide any power at 0 mA.

Power = current x volts, (usually 24 VDC)
Power = 0.004A x 24VDC
Power = 0.096W

Not much power.

Answer / carl_ellis

Question:
why do we use 4-20mA as o/p and why don't use 0-20mA or 5- 21mA?

There are systems that use 0-20mA as an output from a
controller. Those are active', powered outputs, not
signals from 2 wire, loop powered field transmitters. 2
wire, loop powered instruments use about 3.5mA to power the
transmitter's electronics, so 2 wire, loop powered
instruments can never go below 3.5mA without losing the
current needed to power the instrument.

An active analog output from a controller that uses the
controller's power from the controller to drive the current
signal can be a 4-20mA signal or a 0-20mA signal.

People in the process industries use the 4-20mA output
because it is a 'live-zero' that immediately indicates a
broken wire or an open circuit if the loop current falls to
0.0mA. Those using 0-20mA probably have a 0-10Vdc or 0-5Vdc
analog input and want to use the entire span with a 500 ohm
or 250 ohm resistor.

As to why 5-21mA is not used, there are conventions used
that make systems maintainable over time. If every analog
output used oddball ranges or spans, it would be near
impossible to maintain the system over time. Given that
many control systems operate in excess of 10 years, it is
important to use standard analog signals in order to keep
the system maintainable.

The convention is 4-20mA, established by the ISA SP-50
standard in 1967. Deviations are punishable. Be warned.

Carl Ellis
Measure First

What is feedback system.

1 Answers   CCIE,

---

what is fire and gas system?

2 Answers   Andhra Petrochemicals Limited,

---

why direct mounted pressure transmitters is always installed with DBB ( double bleed block) valve in process line or top of the tanks ?

0 Answers  

---

what is the difference between transmitter and smart transmitter ?

4 Answers   Aditya Birla,

---

Make Heater Process Logic.

0 Answers  

---

 why Fahrenheit unit of moistur analyzer transmitter

0 Answers   Infosys, Samsung,

---

how we use the positioner feed back in split range control valves?

1 Answers   Tata Power,

---

The performance of a pump is monitored by measuring the ________ and _________ pressures

1 Answers  

---

what is the working principle of hydra step(electronic level indicator)?

2 Answers   Bengal Energy, Lanco,

---

What is the difference between PLC and DCS?

3 Answers  

---

Hi i m B.E. In elect. & comm. I m workin in bio mass plant from 6 mnths as inst. Engg. but now i m in dept. Of operation as shift engg...which is best operation or inst. 4 me? M confusd..plz help and reply me..

2 Answers  

---

what is the difference between 3,4 wire RTD sensor?

6 Answers   HTA Instrumentation,

---