Set up SCC-68 for thermocouple reading

[BP1000]

I've got a SCC-68 with out any added modules and want to set up a thermocouple to measure temperature.  I've got pin 70 (CJC+) wired to pin 57 (AI 7) and pin 71 (AI GND) wired to pin 23 (AI 15) as explained in the SCC-68 user guide.  I have a type T thermocouple, what is the correct way to connect this to the SCC-68?  I read that I should put the positive lead of the thermocouple in channel AI 1 and the negative in AI 9.  Do I have to use these specific pins?  I also have the SCC-68 set to self power.  I plan to use 'Acq Thermocouple Sample.VI' to get the temperature reading. 

Thanks

[Henrik_Volkers]

Without digging any deeper....

My first approach would be to  open MAX , browse to your DAQ board and look at the properties to see if the SCC-68 is configured.

then create a new DAQ-Task , the assistant will also output the connection diagram.

[Matt_A]

Hello BP1000,

In order to use the built in CJC on the SCC-68, you will need to create an NI-DAQmx Global Virtual Channel to read the channel and scale the voltage to temperature. More information about this operation is provided in KnowledgeBase 25LJ14YF online here. Page 34 the SCC-68 User Guide discusses the formula used to scale the temperature sensor voltage to degrees Celsius. For convenience I have copied this information below:

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Temperature Sensor

Temperature sensor
Accuracy..........................................±0.3 °C over a 0 to 55 °C range
Type.................................................Thermistor

Convert the voltage to temperature using the following Steinhart-Hart
equation:



where A = 1.2873851 × 10–3
B = 2.3575235 × 10–4
C = 9.4978060 × 10–8



where R1 = 4,870 Ω
VT is the CJC voltage measurement

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As you have noted, the SCC-68 User Guide recommends that you connect the CJC to ai7, but you can create this CJC channel on any of the analog input channels on the SCC-68. Then, to use the channel in the Acq Thermocouple Sample.vi example program you will select Channel for the "CJC Source" and your channel name for the "CJC Channel". You can use any other channel for the thermocouple, just be sure to select that channel in the "Physical Channel" drop-down menu. For example, if you wanted to select DevX/ai0 for the "Physical Channel" and then wire your thermocouple to pin 68 and 34. If you wanted to use ai1 you would select Devx/ai1 and wire the thermocouple to pin 33 and 67. In these cases DevX represents the hardware device that you have connected to your SCC-68.

Message Edited by Matt A on 07-11-2007 04:54 PM

[BP1000]

I'm starting to get a grasp on this now.  I've got the CJC set up as a global virtual channel as an analog input voltage.  Do I need to set up custom scaling using the temperature sensor conversion equation?  I also have a thermocouple global virtual channel that is using the CJC channel but the test numbers I am getting don't make any sense.

[Matt_A]

Hello BP1000,

The CJC is a thermistor and not a thermocouple, so it behaves differently. That is why the data from your thermocouple task is not correct.

You were correct in your first implementation; you will need to add a custom scale to the analog input voltage task to convert the voltage from the thermistor to temperature according to the formula I provided above. KnowledgeBase 30BG5VMW, online here, discusses how to set up a custom scale for your NI-DAQmx Global Virtual Channel.

[BP1000]

I'm confused now.

How does the equation relate to the custom scale?  I keep reading the Temperature (C)= 100xVoltage.  Is this just a really bad example? 

When I test the CJC it keeps bouncing between -21.7 and +21.7.  Any idea why I would see this?

I get that the CJC is a thermistor but I have a sepperate thermocouple that I am using and refferencing to the CJC.  Isn't that right?

[Matt_A]

Hello BP1000,

The equation for the thermistor on the SCC-68 is the formula that I listed in my first post. Many thermistors follow the Temperature (C)=100*Voltage equation, however the thermistor on the SCC-68 is more precise and has a more complex equation for conversion.

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BP1000 wrote
:...

When I test the CJC it keeps bouncing between -21.7 and +21.7.  Any idea why I would see this?

...

---

How are you "testing" the CJC? Are you running a test panel on the NI-DAQmx Global Virtual Channel in MAX? Is it configured to be a voltage channel or a thermocouple channel? If you try to read the thermistor as a thermocouple, you will get bad data. They both essentially acquire a voltage and convert to temperature, but they do it in very different ways.

It sounds to me like you are attempting to acquire a thermocouple and use the CJC on the SCC-68 to compensate. Using the Acq Thermocouple Sample.vi example, you will specify the thermocouple channel as "Physical Channel" and you will use your NI-DAQmx Global Virtual Channel for the CJC as the "CJC Channel" (see the image below).

Message Edited by Matt A on 07-13-2007 02:29 PM

[BP1000]

I have pin 70 (CJC+) wired to pin 57 (AI 7) and pin 71 (AI GND) wired to pin 23 (AI 15).  If I look in MAX at the configuration diagram tab it shows 57 connected to 23, why is this?  I tried the test button in MAX and I am getting -10.6.  It is configured as a voltage channel.

I tried using Acq Thermocouple Sample.vi.  I have the thermocouple wired to pin 68 (AI 0) and pin 34 (AI 8).  What do I specify the physical channel as if there are 2 pins?

I also tried setting up a global virtual channel thermocouple in MAX and specifying the CJC source as channel and the CJC channel to my CJC global virtual channel I created.  I used the test button here and get bad results.

[Matt_A]

Hello BP1000,

Your connections appear to take up two channels because your CJC channel and your Thermocouple channel are connected with a Differential Terminal Configuration. You can read more about terminal configurations and other considerations in the document called Field Wiring and Noise Considerations for Analog Signals here. Basically, you are using two channels to carry your signal and then comparing the two to determine the signal level. This helps to eliminate ground loops and reduce the effect of noise.

Looking at the values that you are acquiring, I suspect that there might be a problem with your hardware. I am measuring the thermocouple on an SCC-68 using an NI-DAQmx Global Virtual Channel and I read approximately 2.5 V at room temperature. I assume when you say that your channel reads -10.6 you mean -10.6 V. If that is correct, then this value is way off and the fact that it is negative suggests to me that the device is damaged. Most of our data acquisition devices have a range of +/-10 V, so 10.6 V would be impossible. Can you confirm what device you are using to acquire the data? Are you able to read a known value (like a battery or a power supply) on channel ai7? If you connect the CJC to any of the other analog input channels, do you see the same readings?