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

i am confusing about differential pressure transmitter as i seen so many DPTs in different ranges how can we calculate level and flow with mmh20 plz tel me

1 Answers  

---

what is the principal of operatio of dc motor

1 Answers  

---

what is transducer?

13 Answers   Sofcon,

---

Explain about the leakage class of the control valves

0 Answers   CCIE,

---

Why we. Don't measure current in parallel? Why we measure current in series and why we measure voltage in parallel only?

2 Answers   Almeer Technical Services,

---

---

why we take 4 to 20 mA as out put of every instrument like pressure transmittre ?why we cant take 0 to 20mA as output?

6 Answers   OGDCL,

---

what is absolute pressure and where use in process?

3 Answers  

---

why to use 3 wire at RTD ? what's the reason for common wire ?

4 Answers  

---

what is the electrical operation procedure for switching on 27.6MVA electric furnace.

0 Answers   Jindal,

---

how to calculate MMWC values in to Tonns(MMWC values showing in DP transmitter and Tonns value showing in Totalizer). please tell me details?

2 Answers   Chemin C I, MSP,

---

what is mean of 3 by 2 automation ?

5 Answers  

---

If fire would be inside the plant which fire detector is to detected the fire.

0 Answers   KOC,

---