an i/p converter converts current to pressure. To calibrate
we first need to determin the range of the input and the
output. The current input is usually 4-20mA and the output
is usually 3-15psi (usual industry standards).
Calibration assuming the above input/output:
First connect a milliamp source to the input and a pressure
indicator (pressure gauge, druck etc.) to the output.
Inject a 4mA signal (0%) and check the output is reading
3psi, if not adjust the zero until the output reads 3psi.
Next inject a 20mA signal (100%) and check the output is
reading 15psi, if not adjust the span until the output
reads 15psi. Again inject a 4mA signal and check the 3psi
output as adjusting the span can alter the zero and if it
has changed and adjust as required. Check the span again at
20mA and repeat until the converter reads 3psi at 4mA and
15psi at 20mA repeatedly. Next check the linearity by
injecting signals for 0, 25, 50, 75 & 100% the output
should read as follows;
0% (4mA) 3psi
25% (8mA) 6psi
50% (12mA) 9psi
75% (16mA) 12psi
100% (20mA) 15psi
I/P converter - all the instrument calibration principal
will be same. In beanch calibration we have to give
pnumatic input of instrument air/ nitrogen with pressure of
1.4 bar or kg/cm2 or 21 psi and we have to fix a test gauge
of 0 to 1.5 bar(kg/cm2)/ 0 to 20psi at pnumatic output. now
we have to connect mA source to I/P converter. we have vary
the mA 4 to 20. the result will be like this.
input 4mA output will be 0.2 bar(kg/cm2) / 3 psi
input 8mA output will be 0.4 bar(kg/cm2) / 6 psi
input 12mA output will be 0.6 bar(kg/cm2) / 9 psi
input 16mA output will be 0.8 bar(kg/cm2) / 12 psi
input 20mA output will be 1 bar(kg/cm2) / 15 psi
if output not match with input we have to set things by
adjesting Zero and Span adjesting point.
first give the 20 psi working pressure on inlet port . then give the input of 4 to 20 mA . first give 4 ma then check the out put in out let if nt get the 3 psi adjust the zero adjustement, then give 8 ,12 , 16,and 20MA,THEN WE GET Correspondent outm put of 6 , 9 , 12,15 psi,if not get the 15 psi adjust the span adjustment.(test equipment: pressure generating instrument,mA Source,regulator, pressure indicator)
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Other Instrumentation Interview Questions
What is the Relation Between flow and pressure.
How both terms related with each other.
You are looking at the RS-485-side of an RS-232/RS-485
converter and the power supply voltage for the converter is
+5V. Under idle conditions (no transmission), and assuming
you have enabled the RS-485 output: what would you expect
(a) Output A measured against output B
(b) Output A measured against ground
(c) Output B measured against ground