Temperature transmitter to SLC

[Dave Soper]

I have a Temperature transmitter connected to a type "J" thermocouple. The output is scaled 0 to 400 deg. F at 4 - 20ma.
This output is going to an A-B NI16NI analog input module. In the program I am using an SCP for this input. The raw data is set for 3277 for the 4ma input and the max raw data is set for 16383 for the max raw data. the output from this is 0 to 400 deg F. I began troubleshooting this yesterday. I have been told that it has been working ok during the past few years, until a few months ago when it started indicating the wrong temperature. The transmitter is a Rosemount Hart compatible, I think it is a 3144, No name plate left. Using the Hart communicator, it shows the correct temperature and the correct output. at 204 deg. F it showed 12.12 ma. The plc program online indicated 11392 for the raw data. It should be about 9830 or so. Connecting a ma meter in series indicates the 12.06ma.
Any ideas what would cause the raw data to be so far off. I even tried it on 3 or 4 different inputs, with the identical results. so I think that eliminates the plc as being the problem, but right now I am lost! HELP!

 

[keithkyll]

Use voltage to check card, not current. 4-20 mA will be across a 250 or 500 ohm resistor. Reading will be 1-5 volts or 2-10 volts. You can measure this at the input points on your card.
Voltage is what the input card is actually converting. If the voltage looks correct, replace the analog input card.

 

[Dave Soper]

The Analog input module that this inputs to is a ma type module, not the voltage module. it has 24vdc supply. there is not a resistor in the circuits, other than what may be present internall on the module. I did read the voltage present, and it was 21.17 volts dc.

 

[keithkyll]

There's nothing magic about an Analog Current module.
The difference between a voltage module and a current module is 1 resistor. The resistor is internal, but connected across the input pins. You can convert a voltage module to a current module by adding a 250 ohm precision resistor across the input. Now the 1-5 volt range will be 4-20mA. It's as simple as that. The resistor is a "current to voltage converter". After the resistor, the rest of the module is the same as a voltage module. The PLC is still reading voltage, not current. With this in mind, you can use many of the same tests you would with a voltage module.

You should be able to read the voltage directly across the input. Negative lead to "-" input, not ground.

 

[DenverGreg]

ABs 1746-NI16Is

Dave,
Is the TT the only input that is being used on the card? Make sure that ALL unused inputs are 'shorted' like AB shows in the manual. I've been having severe probs. with the 1746-NI16I cards for three months now. I have had AB send three replacements. It is almost like the cards are losing their isolation, the internal circuitry is not letting the loops stay isolated from each other.
Keith's idea of checking voltage, not current, is a good one. You are testing the entire circuit mA-to-Volts By applying your 'known' field mA input and checking the voltage at the card. The card is apply a resistor that knocks the mA to volts and then uses the voltage (1-5) internally.
We have had some killer lightning storms since April, and my weird NI16I readings closely coincide with these. THAT is why I feel that the module is losing its isolation. Some of the inputs reflect a raw value of 21000, while others show 3000, its all very strange. I am now installing some lightning/surge protevction on EVERY input, installing new modules, and crossing my fingers.
Is your TT located outside? And is there a chance that it had some stray voltage induced on it also?

dG

 

[DJM]

I'm not sure what the issue is about voltage or current as a form of measurement. While the statement is true that current cards use a resistor to derive a voltage drop (usually 1 to 5 volts using a 250 ohm resistor for 4-20mA)this isn't necessary here. Dave has already read the current and his reading (12mA) was consistent with what he expected to read at 50% of his range. No need for a voltage reading. The problem is with the analog input or the configured scaling (engineering units) of the card. Since I know nothing of how the scaling is done on AB, I can offer nothing further.

 

[keithkyll]

Agreed. Dave's first post shows he checked everything. I still suspect the Analog card, as does dG. The reason I stress voltage is for further testing.
To test the PLC, you don't need a calibrated current source.
Connect a voltage source, such as a 1.5 volt battery directly to the input. Understanding that 4-20mA scaling is the same as 1-5 volt scaling, check the reading.
I thought his scale was at 0-20 mA, but the numbers are still too far off, and programs don't usually change themselves. Therefore, it must be hardware. The battery test eliminates everything but the PLC.

 

[DJM]

That's a good strategy Keith, mind if I use that?

 

[keithkyll]

Please do. Get a battery holder from somewhere like Radio Shack. Connect the wires from the holder directly to the module, then put your Fluke on the holder to measure actual voltage. If the Fluke is calibrated, you now have a calibrated current source. Okay, it's not NIST, but still accurate.
Ohm the input first. Some modules (AD, I think) use 500 ohms and the 10 volt scale.

 

[DenverGreg]

Calibrate the Module

Dave,
Have you tried calibrating the module? What happens when you apply 4, 12, and 20mA from your calibrator? I have been able to calibrate out the drift that I have been noticing in our NI16Is and have begun tracking the errors. Like I said prev., I've now sent back three modules to AB, I really hope that I can track the modules thru their system so that I may be able to see what is going wrong.
Let me know what happens, this appears to be almost the same prob. that we're having.

dG

 

[WireGuy1950]

Another thought, Is the module calibration data still correct and not corrupted. There should be some section in your program that addresses the Analog Input card and gives it the proper information as to how the channels are to be configured, if the data has changed in the configuration words then that could change your calibration.