High Temp thermocouple readings are off.

[BadgerNuke]

Hello,

I am using several type-K thermocouples which are attahced to a 1303 SXCI board, plugged into an 1102 SCXI, which itself is in a SCXI 1001 chasis.  I am using a PCI 6289 to read this data.  Furthermore, I have the 1303 set up with 10 megaohm resisters in both the bias and pull-out resister locations, so the 1303 should be able to read both floating and ground referenced signals.  My readings are fine until I get up to about 600C - 700C.  At these temperatures the readings start to vary wildly, then fall precipitously.  If I use a handheld thermocouple reader to double check the readings, I get the correct reading.  The type K thermocouples themselves are rated for at least 900 C and can proably reach 1250C.  Any idea what could be causing this problem and how to fix it?  I have attached my program for what its worth. 

[Hal_L]

Hi BadgerNuke,

This is some strange behavior that you are witnessing. At this point, I am not sure what is causing the discrepancy in your data. There are a couple of things that I want you to try that will help us figure out what is causing this problem:

- read the thermocouples as voltages and as temperates in Measurement and Automation Explorer (MAX) to see if this behavior is repeated

- use a shipping example to acquire the data such as the Acquire Thermocouple Samples which is located in the Example Finder in LabVIEW.

Also, would it be possible for you to post some of your data, preferably some of the accurate readings as well as the high temperature readings that are causing the problem? Finally, I am including the link to a tutorial with more information about taking thermocouple readings with the SCXI 1102: How to Use Thermocouples with an SCXI-1102 Module.

Regards,
Hal L.

[Logan_K]

I looked at your code briefly, and noticed that you are using a "Constant" CJC source for all of your thermocouple measurements. This will result in undefined, as possibly very large, measurement errors - I would strongly recommend using the "Built-In" CJC source.

Also, when you say that your measurements vary wildly and then suddenly fall - this sounds like behaviour that could be attributed to a PID control loop that isn't tuned properly, or an input to it that isn't measuring properly. Since you are using a constant CJC source, that could be the source of a large measurement error that could be causing the PID loop to fail.

Regards,
Logan K

[BadgerNuke]

Hello again,

Here's my data, its in an Excel work book format with the graphs highlighting the behavior I'm talking about.  I ran this test using both a type B and a type K thermocouple for comparison.  I experienced the same behaviour with the type B thermocouple at ~980C.  I have looked at the temps and voltages in Measurement and Automation, and get the same erroneous temperatures (the voltages varied between 0.0343 and 0.034 (not sure what this meant) and the temp was 631C when the temperature should have been around 800-900C).  I have since changed the CJC to the built in and it did not change the behavior.  I'm pretty sure the PID settings are set correctly, I'm able to hold a constant temperature within ~1C using the current setting ( although obviously not at the temperatures I need).

[BadgerNuke]

I forgot to mention; once the k-type thermocouple started to read incorrectly I removed it for a short amount of time before reinserting it, removing it again, then reinserting it and leaving it in.  I also killed the immediately heater once I lost both themocouples. 

[Hal_L]

Hi Badgernuke,

There is one more thing that I want you to try. Could you connect the thermocouple to two channels? From there, acquire one channel as a temperature and the other as a voltage and write the data to a file? Could you pinpoint exactly where you start to experience problems in this data? I would like to see how the voltage data varies with the temperature data at extremely high temperatures.

Regards,

Hal L.

[BadgerNuke]

So I did what you suggested and I have attached the file.  To save space, I included some graphs pinpointing the first excursions as pictures.

[Hal_L]

Hi BadgerNuke,

I took a look at your data and your description in your first post was extremely accurate. It looks like some overvoltage protection is being applied when the temperature readings reach around 600ºC.

What is the tip of your thermocouple connected to? It is possible that you could be experiencing a very high common mode voltage. Would it be possible for you to isolate the thermocouples?

Regards,
Hal L.

[BadgerNuke]

Problem solved. 

I tried the resistor combinations again.  I'm pretty sure that this isn't supoosed to be wired up this way, but it works for grounded, ungrounded and exposed thermocouples.  The resistor configuration is refered to as the factory-shipping configuration and should be the default value.  It uses 10 ohm resisters in the Bias slots, and 10 megaohm in the pullup resistor slots.

Thanks for the help.