Temperature measurement of a wire carrying current

[David1982]

Hello Dear All,

I am not sure if I am writing my problem on right place, but I have searched a lot and could not find an answer. 

My problem is that, I want to measure the temperature of a metal wire that is carrying current. The subject wire has a voltage of around 3VDC and current of around 400 mA. I want to use K type thermocouple.  I am using PCI-MIO-16E-4 (PCI 6251) and BNC 2090 connector box. If I connect my thermocouple wire with  the wire whose temperature I want to find, then will this current affect the reading of the thermocouple ?and will this current damage my connector box and DAQ card? Is there a way to do this?

Thanks for the help.

[sahpe]

Hello David,

If I understand correctly you want to measure the temperature of a cable that is at 3VDC, and are worried about rejecting the common mode voltage.

You can use the thermocouple in differential mode so that no current flow is drawn into the data acquisition setup.

In regards to connecting it, there should be no electrical connection between the thermocouple and the wire anyway, so you can use a bit of epoxy resin of some sort, or maybe even super-glue might do the job. Essentially you need the wire to have a varnish coating so that it's insulated electrically but not thermally.

As a side note, if indeed you are using the PCI-MIO-16E-4, then your readings may not be accurate enough. This card has been revised and relabeled PCI-6040E.

If you are using the PCI-6251 instead though, then you have a great card and your readings will be fine.

I hope this helps a little,

Please do let me know your thoughts,

Kind Regards,

Michael S.
Applications Engineer
NI UK & Ireland

[David1982]

Thanks Michael for the reply.

I am using very thin alloy wires made from Nickel and Titanium. The diameter of the test wires is just 100 to 350 micrometer. Though I have found very thin thermocouple wires but I wonder if I will be able to coat my test wires with some varnish etc  (given the small dia of the test wires). In addition, which NI hardware I can use with my PCI MIO 16E 4 for the thermocouple temperature measurements?  I am already using BNC 2090 for displacement and load measurents.

Thanks

[Wayne.C]

David1982,

I do 'hot' thermocouple readings all the time.  A couple of ways to do it.  One is to use a 4-20mA loop isolator and a transmitter that converts the themocouple voltage to a 4-20mA signal.  The other way is to use something like an Analog Devices 5B37 module.  These provide isolation, are available for various types of TC's and have several options for ouput voltages etc.

Remember, any material between the actual metal wire and your thermocouple junction will act as an insulator and cause a temperature gradient.  We often solder the TC directly to the conductor being tested to help prevent errors in our readings.

[David1982]

Hello,

Thanks dear centrebolt for the reply. I am a bit confused now. In reply to same question Michael (above) said that I need to apply some epoxy or varnish to insulate thermocouple junction from the current carrying test wire but suggest that I use Analog Devices 5B37 module and DO NOT use any epoxy etc to insulate my test wire from the thermocouple junction?  I need som clarification please.

Thank you all  of you nice people.

[Wayne.C]

David1982,

You need to electically isolate your DAQ hardware from the potential connected to the wire under test.  This can be done a couple of ways.

One way to do that is to carefully apply epoxy to the wire that will insulate the TC junction from the wire.  The problem is that this will also create an error in your temperature measurements because of the temperature gradient ( delta T ) across the insulating material.

If you use the isolated 5B37 module or the 4-20mA loop isolator you will no longer need to isolate the thermocouple junction from the wire.  This will allow you to mount the TC junction directly on the wire and will result in more accurate readings.

[David1982]

Thank you very much centerbolt. I have checked for the 5B series thermocouple modules and carriers for the modules.  I am already using laser sensor and load cell and am taking measurements through PCI 6040E and BNC 2090. How can I use 5B series thermocouple module in existing set up? Do I have to give up BNC 2090 ? In that case, do i have to purchase 5B series modules for load cell and laser sensor (analog input modules)? Is there a way I can use to continue my existing set up of BNC 2090 and still use 5B series thermocouple module?

Thank you very much for helping. 

[Wayne.C]

David1982,

You should be able to wire up output of the 5B37 to the BNC 2090.  You just need a cable with BNC on one end and bare wires on other end.  If it were me I would buy a BNC to mini-grabber cable from pomona ( 3787-C ) and just remove the mini-grabbers.

[j..d]

Thanks dear for the help. But I have come to know that 5B37 is not compatible with NI DAQ mx (http://forums.ni.com/ni/board/message?board.id=250&message.id=5554&query.id=141705#M5554). Is there any simple solution to this problem, like purchasing a ready made system that is compatible with my system?

Thank you very much.

[Wayne.C]

The 5B37 can be connected to an DAQmx analog input set up to do a voltage measurement.  You can then use the '5B73.vi' ( I've attached a copy ) from NI to convert the measured voltage to temperature.