RTDs with SC2345 and SCC-RTD01

[RMaso]

Hello,

I'm having trouble with some RTD measurements using the PXI-6289, SC-2345, and SCC-RTD01 setup. I've set up a DAQmx task in the project, using the following scheme:

Name: RTD_Temperatures

AI Channels 16, 17, 19, 21, 23

Ro = 100

PT3851

0 to 250 deg F

1mA external excitation

Acquisition Mode: 1 Sample (On Demand)

I'm using a 4 wire PR-12 RTD from Omega. I've also attached the code I'm inquiring about.

My issue is that I'm not getting the right value from the PXI for the RTD temp. I've measure the resistance at of the probe (109.3 ohms) with a voltage of 0.109V. Using those values, I verified the expected 1mA excitation source. Looking up the resistance of the probe and comparing to a R/T table, the temperature should have been ~75 degrees (which felt about right). However the "temperature" I read from the PXI was 562. I have 5 different channels, and each channel read the same when the probe was plugged into it. Without a connection to a probe, it was -400 or so. I figure it was just due to no connection. I'm concerned about the high reading.

I modeled the code after a thermocouple VI that we already use, and is proven. The Thermocouples connect to the other connector of the 6289 thru a SCC68. I figured changing the task to an RTD DAQmx task would be sufficient. The filter is just a smoothing filter. Any tips or thoughts would be greatly appreciated.

Thanks!

[RMaso]

Also,

When I try to use the Test Panels in MAX, or create a task, the temperatures are exactly what I read in my LabVIEW code (still wrong however). I created a task to read Voltage and another for Resistance on the AI channel, and both came up with weird values; they were drastically different from what my Fluke DMM read. The voltage MAX recorded was 2.7V and the Fluke read 0.109V. Resistance should have been 109 ohms, but this brought about another weird issue: The value of the resistance seemed to vary depending on the maximum range value. If it was set to 500, resistance was ~540; if the max was set to 1k, the resistance read 1.4k ohms.

This seems even more confusing than just a wrong value, but I imagine solving that would solve the main issue. Any help is always appreciated.

Thanks!

[PeterGC]

RMaso,

When you are taking the measurements with the Fluke, is the RTD still attached to the SCC module?  

I know you are reading incorrect voltages/resistance, but have you tried creating an RTD task in MAX and checking the results? 

Sincerely,

[RMaso]

Peter,

Thanks for your reply.

When I measured the resistance across the RTD probe, that was without the connection to the SCC module. The voltage was measure with the connection, of course. I also tried to measure the current out of the module by connecting all wires to the probe/SCC but running the power through the multimeter to the probe. However, I didn't ever get a value for the current. But taking the voltage and resistance I measured, the calculated current was 1 mA, which is what is expected.

I did try a simple task in MAX (3 actually: voltage, resistance, and RTD temp). However every value didn't match the expected. The MAX Temp task also gave exactly what the LabVIEW project produced. Here are the results from the tasks in MAX:

RTD Task:

max temp = 100; Temperature = 562

max temp = 250; Temperature = 562

max temp = 1000; Temperature = 2.489k

Resistance Task:

max ohm = 100; R = 104.9

max ohm = 250; R = 525.9

max ohm = 1000; R = 1.05k

Voltage Task:

max volt = 10; V = 2.73

max volt = 1; V = 1.05

It seems strange to me that the measure values vary depending on the maximum. Is this due to internal (to the software) scaling of units? That seems plausible until the resistance task; resistance shouldn't change based on the maximum. I mean, it is what it is. I feel that this shouldn't be this convoluted; but maybe there's a step I'm missing somewhere along the way. It concerns me that I may be working the signal over twice, with having the SCC-RTD module AND RTD task?

I did play with the internal vs external current sources, and internal just gave me an error saying the device couldn't produce that. So I stuck with external (which I assume means the SCC is supplying) at 1mA. I also adjusted the sample rate from my code, but all that seemed to do was provide sporadic readings on the indicator.

I do appreciate your help, but do you have any other thoughts?

Thanks,

Rex

[PeterGC]

RMaso,

I agree, it is odd.  You have done a lot of the troubleshooting that I normally do with these devices. 

One thing to confirm.   I'm assuming you are supplying power to the 2345 from your DAQ card, which is fine, but I would measure the current from the RTD01 and make sure it is ~1mA.   

You are correct about the scaling, but I completely agree, the resistance should not change.  Very strange...

You have plugged the RTD01 into one of the analog input sockets in the SC-2345 carrier (sockets J1 - J8), correct?  Also, you mentioned a thermocouple setup on a SCC68.  Can you borrow that terminal block and see if you can get reasonable readings with that setup?

Glad to help, if I think of anything else I'll let you know.

Best,

[RMaso]

Peter,

I finally had some more time to do a little troubleshooting on this project. I re-measured the current and voltage coming out of the RTD modules. I was very surprised at what I found:

V source to SC2345 (with POW3) : 24VDC (well within range of 7-42)

V out of RTD: 28V

I out of RTD: 1 AMP

I thought the RTD was supposed to keep a current of 1mA constant? The 1A seems very wrong. Every module is putting out 1A and the pot will only adjust down a few hundreths of an amp.

Any thoughts?

Thanks,

Rex

[RMaso · Accepted solution]

BLUF: When using the SC2345 and SCC-RTD01, create a new device in MAX under your PXI for the SC2345 and configure the modules you have in each slot of the SC2345. Then create a task in MAX (or project) and select the SC2345/MOD#:ai# (or something similar to that) as the physical channel to read.

I finally figured it out last minute yesterday! I was setting up the tasks in MAX under the DAQmx card/slot that had the SC2345 connected. This seemed logical to me; but my assumption was that the RTD module conditioned the signal and output a value (voltage or what not) to the card and the user just reads what is on the card. But in order to make it work, I had to setup up the SC2345 as a new device in MAX under the PXI devices. This brought up a dialog box to configure the SC2345 with the locations of the RTD modules. After doing this, when I created a new task, new channel options appeared for each module in the SC2345. Then you could pick what input channel on the module (ai0 or ai1) to read from. Setting up the task to read from the SC2345/MOD#:ai0 instead of say, PXISlot6/ai0 (where the SC2345 was actually plugged into the PXI), gave the correct results. I'm not sure what this means though, as far as what is the signal actually doing between the module and the PXI, but whatever it does, setting up to read from the module worked.

[PeterGC]

RMaso,

 I had a feeling it was a configuration issue.  If the SC terminal block/modules are not setup correctly the drivers may not interface with the hardware correctly.   I'm glad you were able to figure it out!

Let me know if you run into any other issues.