Inst & DCS

It could be any of a number of things. Eliminate the possibilities of loop voltage too low/loop resistance too high, and instrument ranging errors, and re-post afterwards with the findings. Do feel free to describe the instrument and the DCS in use as well. _<subscribes>

what is distance between Instrument and system, some time problems are very basics don't forget to check scaling in DCS for that instrument.

To what tolerance are you reading the outputs and comparing them?

We try very hard to not put in an instrument display if the system HMI is also to display the same reading. (never happens - customers always want an instrument readout) The conversion from 4-20ma (analog) to digital in the processor introduces a potential for a minor error. It is very common for a temperature transmitter to display, for example, 25.67 degrees C while the HMI on the DCS shows 25.64 degrees. Is this what the problem is? If so, little can be done, other than setting one display to round to the nearest whole number. At that tolerance they should match. If not, then you have more serious problems and we need more information to decipher.

How big is the difference? If it's small, it could be a calibration problem, as Phys says.

If it's big, it may be a scaling problem. Your instrument is, say, 0 to 100°C, and converts temperature to 4 to 20mA. DCS, however, is configured to interpret 4 to 20mA as 0 to 200°C. I've seen this happen a lot.

regards,

Vulcan