why we use only 4-20 ma signal as AI and not 0-20 ma?
Answer Posted / 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
Is This Answer Correct ? | 11 Yes | 0 No |
Post New Answer View All Answers
Whatis the reset ofset and control system in instrumentation
Why the ammonia and Nitric Acid level indicators is with millimeter ? and What is the conversion form millimeters that level indicator indicate to weight ?
we are having yokogawa centum cs3000 dcs in our power plant. i need some of the operator guides NOT to appear in HIS0163 AND HIS 0162 but it shoul appear in HIS0164 HOW TO DO IT?
What are different types of valves?
sir, give me placement papers and interview quistion of birla group company of copper.
HOW TO SOLVE PROBLEM WITH ENRAF LEVEL TRANSMITTER 854 ATG THAT PASSWARD DO NOT Exist INTO TRANSMITTER TO ADJUST LEVEL WITH PORTABLE PET 847, WRITTEN WRONG PASSWORD , I AM SURE THAT DEFAULT PASSWARD (W2=ENRAF2) IS THIS TRANSMITTER NEED TO REPLACE SOFTWARE AND HOW?
what is performance test.need defination
how can we tune a PID. If it is by zeiger nichols method..please explain briefly with help of an example.
please send the drdo exam paper
How to calibrate smoke Dectector
What is furnace draft control?
why would you like to join our company
what are the use of belew gear in general ??
Any one can help me to identified, how can we interchange the device in FF
I need control valve ISA hand book. Is there any site to download that big book containing all the pages. Written by Paul Friedmann