thermocouple 9205

[lkdfjaio1241]

Hello,

I am trying to use a 9205 module with a type T thermocouple. I would have thought setting up the 9205 module would have been as simple as the 9211 module, however I have not found that the case. I have the positive TC wire plugged into ai0 and the negative in com (also tried + = ai0 and - = ai18). Then using the DAQ assitant, add a channel and choose thermocouple. From there choose 9205 and a0 as the channel, then setting CJC as a constant to 25. Pretty similar to the 9211 module set up. However the readings I recieve are obviously wrong. Perhaps might I need scaling of some sort? or maybe would my process in setting up the module be incorect?  Seems to me should be simple error, but has taken me some time. All help is greatly appriciated. 

[JamesBeams]

I'll try to help here but I'm more familiar with cRIO, and it appears that you must be using cDAQ.  First of all I'm curious why you are using the 9205 for a thermocouple measurement instead of a thermocouple module (9211 or 9213).  With the 9205 you don't get the cold junction compensation, and the range accuracy is ~4x worse than a module designed specifically to read thermocouples.  Maybe you aren't looking for that accurate of a measurement though.

I'd expect that you'd want to use the differential pair connection (ai0/ai18) and not use COM except for shielding. Also make sure the module is configured for differential pair measurement.

As for the CJC channel, I know the 9211 support VI's in cRIO require the CJC value as a voltage measurement provided by the module and this is converted to a temperature.  Possibly the DAQ assistant also requires this?  I don't have DAQ installed so I can't speak more for the configuration except suggest you take a careful look at the required units.

Also instead of just bypassing the CJC channel completely I'd suggest using a second pair of inputs and connect a very short piece of thermocouple wire twisted by the connector.  This will at least provide some sort of cold junction compensation to provide a more accurate measurement, though the best solution is really to use a thermocouple module.

[EEsRULEtheWORLD]

Hi,

In looking at page 12 of the 9205 manual, you are going to need to connect your type T thermocouple to ai0 (pin 1) and ai8 (pin 19).  In your post, you had stated that you connected the negative terminal to ai18, which would be pin 11.  Unless this was a typo, this will most likely solve the issue.

Also, refer to Table 1 (Floating Signal Source) of this DevZone article for information on applying bias resistors, in order to eliminate the chance of any erratic signals that you may experience.

[IshanG]

hey guys..

i had a doubt.. the specs for 9205 which says that the voltage range is from 200mV to 10V +-.. but aren't most of the thermocouple readings less than 200mV for temperatures which are close to room temperature? so is it a viable option to use 9205? 

[EEsRULEtheWORLD]

Hi, IshanG.

±200mV to ±10V reflects the minimum and maximum voltage ranges, respectively.  ±200mV means that the input voltage can be between +200mV and -200mV, and ±10V means that the input voltage can be between +10V and -10V.  Page 22 of the NI 9205 specifications lists the four selectable voltage ranges as:  ±10V, ±5V, ±1V and ±0.2V.  If the signal's maximum and minumum values are not the extremes of one of these ranges, then the driver will select the nearest range that the signal fits within, in order to maximize the resolution capabilities of the hardware.

It is important to note that you can programmatically select the maximum and minimum values of the voltage range that you expect to use.  These maximum and minimum values do not define the voltage range that the hardware's ADC will use.  The DAQmx driver will select the range and gain settings that it will use behind the scenes, based on the inputs.  There are two ways to set these expectations:

1.  DAQmx Create Channel VI allows you to set the range with the Minimum and Maximum Value terminals

 

2.  DAQmx Property node allows you to set the range with the AI.Min and AI.Max properties.

Let me know if you need any more information.  I hope you're having a great day!

[JamesBeams]

IshanG,

I agree that the 9205 is not the best choice for a thermocouple measurement.  Most thermocouples produce a voltage between +/-80mV across the entire temperature range.  The 9205 also doest not have a CJC which is critical to precise thermocouple temperature measurement.  Without this, the measurement will only be an estimate of the temperature assuming the cold junction (where the TC wires are connected to the module connector) is whatever static value that is input into the VI (25C was suggested).  If the module's connector is hotter or colder than this value then there will be more error to the measurement.  The 9211 and 9213 are also made specifically to measure thermocouples and have better accuracy for this type of measurement. 

While the 9205 is not the best choice, you can still do some temperature estimation and get an idea of the general range of the temperature of the thermocouple connected.  For example, if it is 'hot', or 'cold', but not if it's 77.2C vs. 68.7C, etc. 

[IshanG]

Thanks people.. by the way, I just went back to think over james' idea of using a very short thermocouple to measure an "approximate" cold junction temperature.. I seems like a viable option only if it suffices few conditions.. (please comment on the logic i am providing in the following lines):

If we refer to the basics of a thermocouple then the relation between the measured (unknown) temperature and cold junction temperature is given by: (Tx - Tc)(Sa - Sb) = VTC

where Tx is the unknown temperature, Tc the junction temperature and Sa and Sb are the seeback coefficients for the thermocouple wires. VTc is the differential voltage obtained. Now if we use a short thermocouple and keep it as close to the terminals as possible, we should get a near zero voltage, since the temperature Tx is nothing but the Tc itself (or very close to it).. and then we can judge on whether it will be a good idea..

I will perform this experiment soon and let you know..

thanks for your replies..

Ishan 

[JamesBeams]

Ishan,

I thought a bit more after posting about using the second channel with a short thermocouple wire.  This of course will not be a replacement for a CJC (which provides an absolute measurement of temperature), but it may be useful in providing an offset of the ambient close to the connector vs. the terminals themselves, to be used more as an offset or pseudo-autozero channel.  In a thermally static environment this may not provide much use if the temperature across the connector is static, and the measurement uncertainty may produce a higher amount of error in your measurement given the 9205's accuracy vs. an actual thermocouple module.  It may be worth a try if you have the spare time or expect temperature gradients across the connector, but its use in general may not be as beneficial in your setup as I initially thought.

[Le_Gross_Bill]

I have also been attempting this and am using an LM35 as a temperature sensor for the purpose of CJC. Works fine in theory as an LM35 does not require CJC. BUT all my TC measured readings at room temperature are 8-9 degrees Celcius below the LM35 reading so definitely not working properly.

In my case, the student I am programming for has fast response thermocouples and wants to sample them at 1KHz so NI9213 and NI9219 are not options. May have to go with an external thermocouple amp and read voltages and scale.

Any other suggestions would be greatfully accepted.

Regards,

Dean

[Henrik_Volkers]

---

@Le_Gross_Bill wrote:

I have also been attempting this and am using an LM35 as a temperature sensor for the purpose of CJC. Works fine in theory as an LM35 does not require CJC. BUT all my TC measured readings at room temperature are 8-9 degrees Celcius below the LM35 reading so definitely not working properly.

In my case, the student I am programming for has fast response thermocouples and wants to sample them at 1KHz so NI9213 and NI9219 are not options. May have to go with an external thermocouple amp and read voltages and scale.

Any other suggestions would be greatfully accepted.

Regards,

Dean

---

Well, TC EMF voltages are in the µV range, offset and bias easely generate such errors.

first round: assuming a good thermal CJCs, without a TC connected, apply a shortcut with a short copper wire.  The EMF voltage should be ideally zero, so calibrate to it. 

The quite academic approach to read TCs with 1ms resolution would need very thin TC wires and/or a very good thermal coupling 😄 , why not add mK resolution 😉