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Field Instrumentation Basics
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0:00
If you see here, this is a level
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transmitter. This instrument measures
0:06
this tank level and generates an
0:09
equivalent 4 to 20 milliamps signal and
0:13
then it sends the signal to the DCS
0:17
system. I will show you one more
0:19
instrument. It is control valve. Our DCS
0:23
system sends analog output which is 4 to
0:27
20 milliamps signal to this control
0:30
valve. If you see here, this is the
0:32
valve positioner. This valve positioner
0:35
receives this 4 to 20 milliamps signal
0:38
from the DCS system and then accordingly
0:42
regulates the instrument air flow to the
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actuator. Based on the instrument air
0:49
flow, the valve will open or close.
0:53
Let's say DCS given 50% signal to the
0:57
valve. What is the 50% equivalent of the
1:00
current signal? It is 12 milliamps
1:03
signal. So, when the DCS sends 12
1:06
milliamps signal to the valve
1:08
positioner, it regulates the instrument
1:11
air flow such that the valve will open
1:15
50%. This is analog output. If we are
1:18
also using a valve feedback, then we
1:22
will have one analog input to the DCS
1:25
system for measuring the actual valve
1:29
feedback.