Incorrect thermocouple reading

I've got a NI PXI-1033 with a NI TB-2709 block and I'm running Labview 9. I'm simply trying to read the ambient temperature with a thermocouple from any of the eight channels. Unfortunately, when I look at the reading in the Measurement and Automation Explorer task menu the value oscillates around with the difference from the peak to valley of about 50 deg C. I've tested the thermocouple on a multimeter and it gives a good reading. What's wrong. Please help.
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It looks like you might have a ground reference issue. Check whether you have a differential signal or a ground referenced signal and check that the setup in MAX reflects this. Otherwise, you are measuring a voltage with reference to another voltage that is not attached to anything. You measure a strange signal, because there is always loose cross-talk and this unattached reference is loosely tied to something, but as charge moves around, this reference shifts. There are some good knowledgebase articles about single-ended (referenced and unreferenced) and differential measurements on the ni.com site.

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Tracing OSA data automatically when thermocouple reading stabilized

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   By saying user input, I mean specifying the location of file. In the code 3650.VI, when temperature condition is met, a dialog appears and asks me whether or not to trace the data in a trace.csv file and asks for a file location. I want a stationary file and write all the OSA data,only power column, to be written into that file.Every time to a different column,of course.
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Thermocouple reading

Hi,
I am working with a type K thermocouple and I use a LM 35 temperature sensor to get an accurate CJC. I have got an external serial device were I do the voltage measurement of both (K and LM35), that means I get the voltage value as string using VISA in Labview.
I would like to use the thermocouple reading vi for the linearization. My questions are:
Which option should I choose on the CJC sensor input of the thermocouple reading vi?
Can I connect the value that I get from the lm 35 to the CJC voltage input of the thermocouple vi?
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Hi Navarro,
thank you for your answer.
The first thing you have to define, is the Temperature Unit you want to get as the output of the Converter VI. Please refer to the LabVIEW help for the options:
Convert Thermocouple Reading VI - LabVIEW 2014 Help - National Instruments
http://zone.ni.com/reference/en-XX/help/371361L-01/lvinstio/conv_thermcoup_read/
As you described, your serial device returns scalar singular values to you, choose the scalar option of the converter VI.
The CJC options depend on your reference sensor. You can use a voltage or current reference, depending on what signal comes from the reference. Please refer to the help and the following link for more information:
What is CJC and Why Does My Data Acquisition Board or Signal Conditioning Unit Include a CJC Sensor? - National Instruments
http://digital.ni.com/public.nsf/allkb/E332E1D829059FB5862570AE005066B3
I hope, this helps.
Best regards,
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Thermocouple reading stop updating

Hi,
I am having a problem with a cRIO application and a NI 9213 module. I have a heater application and I am using the NI 9213 to read the temperature values of the heaters.
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Hi Sebastian,
To get the 9213 working again I have to reboot the controller or make any change in the module properties like change from "hi rtesolution" to "hight speed" or something like that.
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Reading AI and thermocouple reading using virtual channels

Hi All
This is the configuration of my apparatus
SCXI 1102- thermocouple readings
PCI-MIO-16E-4 card
i want to read both analog inputs (from a front feedthrough panel) and a thermocouple reading simulataneously. I tried defining the channels as DAQmx global channels and tried to use the specific DAQmx polmorphic VI to read both the AI and the thermocouple reading simultaneously. But it looks like the AI affects the thermocouple reading. When i change the AI, the thermocouple reading also gets erratic(changes to lower values, sometimes even negative). Can anyone help me on this. Im absolutely new to this. Excuse me if the question was pretty trivial. Thanks a million
sambar

Sambar,
To which pins on the feedthrough panel are you connecting your signal? Make sure you are not connecting it to channel 0. All of your SCXI channels are multiplexed to channel 0 on your PCI-MIO-16E-4. If you are unsure of the pinout of your feedthrough panel, the KnowledgeBase linked below provides this information:
Pinout for the SCXI-1302 50-Pin Terminal Block
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DAQ signal accessory and thermocouple reading fluctuations

I have a DAQ signal accessory connected to a laptop via a 6024E DAQ card. I am attempting to connect a type K thermocouple to the thermocouple port on the signal accessory and take temperature readings. The VI that I have set up is a slightly modified 'continuous thermocouple reading' example VI. I am having problems getting accurate readings. The temperature fluctuates 30-40 degrees between readings. This is impossible do to the lack of thermal variations in the environment. Please help!

Nah. I think we've isolated the problems.
It sounds like noisy inputs to the DAQ board.
As I said, TCs are very prone to noise on the line, and normally there's a filter on the TC to eliminate the noise, and just get the DC component of the TC--but you don't have that.
Therefore, to get the same effect, you're going to have to average many readings, which will in effect average out the AC component.
Another thing--if your panel (TC junction) is in a controlled environment (same temperature all the time), I would average the CJC IC voltage, and just continuously use that, rather than reading it every iteration.
Try something like this attached file.
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How do I take a thermocouple reading with a pci6025E?

How do I take a thermocouple reading with a pci6025E? I have seen some postings with scxi-1112 modules which I do not have. I need advice from the ground up. Do I need external circuitry, can I just wire both leads of the thermocouple to Ain0 differential pins, what vis are necessary in the actual programming... If there is an example program or anything else that might seem relavent? I appreciate any help.

This is most probably a result of multiple factor that make the measurement imperfect:
- You're not using signal conditioning, so your resolution is definitely very reduced. Any temperature drift in the board could cause a big difference in the reading. I recommend you run your vi for approximately half an hour, unplug the accessory and perform a self calibration. This will have the board at the operational temperature you're expecting and will give you the best results.
-I personally would recommend using a different sensor type (RTD or Thermistor) as the CJC source, in order to obtain an accurate measurement, otherwise you have a bit of a "chicken and egg" problem, because you're using thermocouple 2 as the CJC for thermocouple 1, but what are you using as the CJC source of thermocouple 2?
- Are both your thermocouples of the same type and manufacturer? Due to the nature of the thermocouple itself, you can find slight variations between thermocouples from different vendors, due to the purity of the metals used. While this would usually be almost impossible to detect, it might be driving your voltage just enough to be in the next detectable voltage step (again, due to the reduced resolution).
- I've worked with temperature sensors for a while, and I'd still recommend that you try some sort of signal conditioning in order to increase your accuracy. Even a small amplifier and a passive lowpass filter will do great. I know you said you don't have SCXI, but if you want some lower cost ready to use applications you might consider looking at the SCC Carriers and modules we have.
I hope these suggestions help. Please let me know if you have any other questions.
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Thermocouple reading (w/ Built-in CJC Source) only works on random SCB channels

I am using LabVIEW 8.2.1, a PCI-6255 card, and an SCB-68 terminal block. I have a Type K thermocouple with a thermocouple calibrator which I am trying to measure. Here is the problem, first, using the attached VI, I have tried to read this TC using AI0. Everytime I use AI0 I get the error saying Built-In CJC is not available. When I use any other channel, this error disappears. The second problem, when I use a channel other than AI0, I am only able to successfully read the TC on random channels. For example, it only works on AI13 to AI15. For the channels that it does NOT work on, I am able to see the correct voltage through MAX, however the AI Read TC VI does not for some reason give an accurate reading.
Does anyone know why I would not be able to use AI0, and why I am only to use certain channels for this TC? This makes absolutley no sense and I have tried swapping the SCB, the DAC Card, and the cables. I have reinstalled drivers and LabVIEW also.
Please help this is driving me crazy!
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Solved!
Go to Solution.
Attachments:
test.vi ‏28 KB

bkb wrote:
So AI0 is the CJC value? And I have to specify CJC Channel when I read the TC? Why is it that I got this to work when specifying Built-In source, and wired the thermocouple to AI15? And why does it not work on other channels?
Not sure if this helps you but...
Thermocouple reading are often differential measurements since they are typically very small values. To get differential readings two AI channels are paired up so half of the channels go away when you set things up for differential measurements. So could you double check the documentation for your device to ensure you are wiring the thermocoules properly? Correct pair, proper polarity, and proper TC type specified.
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My thermocouples reading are incorrect

I am acquiring temperatures using type-J thermocouples with PCI-6033E and SCB-100. I have created a virtual channel for each thermocouple and I also have a channel for the CJC that I tested and it works ok. I am running the channels in differential mode which is the mode of the board. I have also 27K resistances between the negatives and the AIGND.
The thermocouples give incorrect readings. For example, if I move one thermcouple signal from one channel to another, it gives totally differnt readings.
Another example is that sometimes the readings are ok while other times the readings are wrong without modifying anything in the circuit. Am I doing something wrong or what?

I am using the board (PCI-6033E) to measure 24 signals, 8 of them are for thermocouples. All the remaining signals are working properly. So, I don't think there is something wrong with the board. I did also the on-line calibration test for the board and it turned out to be ok. It's only the thermocouples that cause me problems. Now, 7 of the thermocouples are giving me correct values while the 8th one is giving me unphysical values eventhough it was working ok some time ago. (same signal, same channel, same everything).
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Why is the thermocouple reading wrong?

Situation:
I have the PCI-6221 interface board with an TBX-68 connector block directly connected with T-Type thermocouples. Thus the temperature reading should only be between 0 and 400°C. I made a program to read 6 thermocouple channels.
Problem:
I only connected the first two channels. These are reading properly but the others give me signals which are completely nonsense! Like 600°C and so on. Do I need to install an condensator (like 10µF) at the conector block between the thermocouple contacts? How can I get a constant 0°C out of not contacted thermocouple channels?
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Attachments:
Thermocouples reader1.jpg ‏203 KB

The nonsense it because of the open terminals, and the bias currents off the inputs.
If you shortcut the inputs you should read the board/cold junction/room temperature.
Or wire a 1M from AI+ to 5V and another from AI- to GND (diff mode assumed). That would give a max/overrange reading, but might have some influence to the other channels due to crosstalk but worth a try.
The clean way is to disable the readings by software.
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
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Lowpass filter help for thermocouple reading

Hello
I have an old legacy E-Series AI-16SE 12bit 500KHz PCI card.
I am attempting to setup a temperature reading test program. I will have the E-Series connected to a SCB-68, which will have five T type thermocouples connected. I realize this is not the best test setup but it is an attempt at a quick setup on a bench so hopefully any noise interferance will be minimal.
I will be using LV 8.2 professional development.
I was wondering if I setup a software based lowpass filter if that would be helpful in this situation?
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Tim,
Rob's idea is probably what you should try first. It is in effect a low pass filter with about a 10 Hz corner frequency.
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Incorrect thermocouple reading

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