we have yokogawa centum cs3000 dcs. the problem is a thermocouple inserted in bucket elevator carrying CARBON pellets is showing a large flucuation in temperature of range about 10 to 15 degree celsius (eg.jump from 164C to 158C direct). we have checked milivolts at thermocouple,JBs terminals & at dcs card channnel BUT there is no Fluctuation in millivolts.

pls give me some suggestions to remove this problem.
thanks in advance!!!

 

Hi,

First of all since TC gives mV output, its prone to EMI disturbances and noise. So,make sure wires between sensor and transmitter is not long & possibly if you can use head mounted transmitter, better it is.

If still problem persists then please check the grounding voltage which should be not more than 2 V.

At last you can use signal isolator if your transmitters is not isolating its input & output.

Regards
Sushant Chanana
[email protected]

 

Suggestions:

1. Disconnect TC from DCS card and use a TC calibrator to check the DCS card. If things work okay, then it sounds like you have a wiring/installation issue. If you still get error's in your TC readings, you might want to check the following:

a. Short out unused channels on the TC card. You might be experiencing multiplexer "switching noise" on the TC card due to open and unused channels.

b. Check to see if your TC card is actually "linearizing" the TC. It should be, however, a 10-15C error may be due to the thermocouple sensor non-linearity. You may need to add a linearization routine to the measurement.

2. If the DCS card works okay with a calibrator, then you may have a wiring/installation problem. How long is your TC wire run? If very long, you may have a ground loop issue, noise pickup, marginal/broken wiring, or external sources of heat near the wires causing errors.

a. With a TC calibrator at the sensor end and a TC meter near the DCS, you could check out the TC wire run to see what an independent and isolated source/meter reading produces. If this reads okay, then you may have ground loop issue. In this case, in isolated tc sensor or moving to an isolated field transmitter may be the best solution. Otherwise, if the reading is still moving around, you may have other sources of error as mentioned above.

If you have any questions or we can assist, please let me know.

Kind Regards,
Donald Lupo (dlupo [at] acromag.com)

 

The thermocouple output signal is mV. If your mV output is stable but your deg C indication is not, then the problem is not in your TC but in your instrument doing the converting to deg C.

The mV signal sent by a type K is fairly low. A change of .5 mV could cause the degree of temperature change that you are observing.

If you find that your mV signal is not stable then your problem could be insufficient insulation or electrical noise interfering with the type K mV signal.

Electrical noise problems in thermocouples can often be surmounted simply by using twisted shielded extension wire or specifying an RTD instead of a thermocouple. As another poster mentioned an in head transmitter would also solve your problem if it is being cause by electrical interference.