Generally in which location ( JB or IO card or DCS) the
thermocouple compensation is done.Also where they measure
the atmosphere temp ( for compensation) in JB OR DCS panel?
How they compensating that atmosphere temp? is any circuit
for that ( like in rtd wheatstone bridge)?
Answer Posted / carl_ellis
Cold junction compensation (CJC) is done at the point where
the thermocouple wire or thermocouple extension wire
connects to the thermocouple input, either an I/O card, a
temperature transmitter, or a thermocouple input on a
recorder/controller/indicator/alarm. RTD's do not need
CJC. Thermistors do not need CJC.
The purpose of CJC is to measure the temperature of the
termainal connection block with respect to the freezing
point of water (because that is the 'cold junction' and the
national standard thermocouple tables are referenced to) a
cold junction at the freezing point of water) and add that
value to the temperature value determined by the EMF/mV
output of the thermocouple.
The temperature of the terminal connection block is affected
by ambient, room, or atmospheric temperature but is also
affected by the heat generated by the device's electronics,
other electronics in close proximity, and whatever cooling
the installation provides. Because all of those factors can
vary in temperature, the CJC involves measuring the
tempeature of the (terminal) connections.
Temperature-sensing ICs, thermistors, and RTDs are used for
CJC measurements. Thermocouples cannot be used for CJ
measurement.
So the answer is that the CJ measurement is made at the
thermocouple input, presumably the I/O card or temperature
transmitter, never at a junction box.
The actual temperature calculation, measured T/C EMF plus
CJC, can be done on the I/O card or in the 'system', a PLC,
temperature transmitter, controller, DCS, whatever,
depending on design.
CJ should not measure room temperature or atmospheric
temperature (although some poorly designed thermocouple
devices do so); CJ should measure the terminal temperature
where the thermocouple wires connect to the I/O. A
temperature transmitter measures the CJC in order to do the
calculation for temperature.
There are special techniques for CJC for high accuracy
calibration involving an ice bath, but industrial devices
use integrated CJC.
Tell 'em Carl sent you.
Is This Answer Correct ? | 3 Yes | 0 No |
Post New Answer View All Answers
Hi Friends i am going to write BARC written test by this month 20th (20-5-2008). So please if any body knows the details about this or having previous question papers pls mail me to kkumarei@gmail.com. For Suggestions give one message to 9962122264
Why do we have earthing cable in branch cable and shield in the same cable?
How an rtd work, and what volts/current goes to a rtd to measure the resistance?
how can be calibrate flow indication transmitter (rosemount) it shows excess milli amps nearly about 24 ma on full scale, but its scale was set on 20ma on full scale..???how can be simiulate..???
Sir,I am going to sit for IOCL written test exam. recently.Can anyone tell me the name of 'Objective type Question Book on Instrumentation Engineering ' which will help me. Pl. mail to bidyut_71@rediffmail.com / bidyut.saha2008@indiatimes.com / bidyut.saha09@yahoo.in
Please asked me How calculate DCS controller memory.
I need control valve ISA hand book. Is there any site to download that big book containing all the pages. Written by Paul Friedmann
What is exact difference between the panel earthing and system earthing in DCS system. What is the value of resistence.
What is the working principle of the magnetic meter?
what is your eligibility criteria for freashers?
what is a half wave, full wave, and bridge rectifer.
Exothermic reaction./ endothermic reaction
List three types Level Switches with their working principles
what is KV and XV valve
why is not useing in boiler skin thermocople