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USB 6001 does not accept thermocouple input

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I am trying to input a K type thermocouple into one of the differential analog inputs of the USB 6001. I've used the DAQ assistant to do so, but the read out shows around 3oC at room temperature. Has anyone else encountered this problem with the USB 6001? (When I input a voltage, that works fine, so the unit is working fine).

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Accepted by topic author AllanNish

Allan,

 

When you run a thermocouple task on one of our Analog Input devices the device will read the voltage on that channel then convert it to temperature using the thermocouple's known relation. The K-Type thermocouple has a sensitivity of ~41 uV/C.  The USB-6001 has a 14-bit resolution with a range of -10V to +10V.  This gives us a sensitivity of ~1.2mV or ~29.8 C.  This does not factor in that the USB-6001 has an absolut accuracy of 6 mV which would throw the measurement of further.

 

A thermocouple measurement needs to read very small voltage changes at very low voltage levels which is why we have devices speifically made to read thermocouples.

Matt J | National Instruments | CLA
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I think this should be stated in the specifications of the USB 6001, and the DAQ assistant should at least give a warning (if not even to make the thermocouple mode inaccessible). 

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Both sides of the argument have their merits.  From the developer side, thermocouple measurements are really just reading in an analog voltage and applying a custom scale in MAX so the option should be available to any devices able to read analog input signals (my thought as I do 0 development work here).

 

On the other hand if you have anything less than 16-bit resolution and a range more than ±1V the reading isn't going to give you much useful information.  This might be a good idea to post to the Data Acquisition Idea Exchange if it is not already proposed.

 

http://forums.ni.com/t5/Data-Acquisition-Idea-Exchange/idb-p/daqideas

Matt J | National Instruments | CLA
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Dear All

 

I bought my NI USB a few days ago. I tested it against my T-type thermocouple, but it does give really funny readings alternating between positive and negative (either voltage reading temperature reading). However, the additional piece of software includes thermocouple configuration, which means that the necessary correction of converting the voltage to temp reading is considered. If you using a multimeter hooked to a thermocouple, you will get a solid voltage reading.

 

One more thing, NI marketing this product to measure voltage output sensors including thermocouple through four differential channels, please see this link page#4 (http://www.ni.com/pdf/manuals/374259a.pdf). 

 

Any advice in this regards. I will start the negotiation to return USB-6001 as it does not work comparing by the guidance from NI documents and the prior purchasing over phone chatting with NI engineering. I think this is my verdict for NI-6001 as an engineer working with different DAQs of different brands.

 

Best Regards

Ahmed

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Page 4 of the pdf you linked to doesn't mention thermocouples; pages 14 and 15 talk about how to connect different signal types, including thermocouples, but nothing in the pdf suggests that the USB-6001 will be good at reading thermocouples. The device is only as good as its specifications say, and while the output of a thermocouple is an analog voltage, it's so small that it's not a good match for this device. Same would be true if you tried to measure the output of a Wheatstone bridge; you'd be able to do it, but probably not precisely or accurately enough to be useful. If you've worked with other DAQ devices, you're probably aware that those intended for use with thermocouples or strain gauges will be specifically designed to read that kind of input, and their specifications will be a better match to the measurement task.

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wrote:

Allan,

 

When you run a thermocouple task on one of our Analog Input devices the device will read the voltage on that channel then convert it to temperature using the thermocouple's known relation. The K-Type thermocouple has a sensitivity of ~41 uV/C.  The USB-6001 has a 14-bit resolution with a range of -10V to +10V.  This gives us a sensitivity of ~1.2mV or ~29.8 C.  This does not factor in that the USB-6001 has an absolut accuracy of 6 mV which would throw the measurement of further.

 

A thermocouple measurement needs to read very small voltage changes at very low voltage levels which is why we have devices speifically made to read thermocouples.


Yes but without a proper cold junction reference thermocouple measurements taken this way are of little value.

 

https://www.maximintegrated.com/en/app-notes/index.mvp/id/4026

http://www.ti.com/lit/an/sloa204/sloa204.pdf

 

========================
=== Engineer Ambiguously ===
========================
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Hi Ahmed

A year or so ago, I did a post in which I asked if anyone had tried using the USB6001 for thermocouple readings (I use the type K). I had purchased about 6 of these for a introduction to LabView programming course. I had the same issues like yours: the readings were unstable. I called NI help line, and they could not help me. I ended up returning them. USB-TC01 works perfectly; so I ended up getting them instead.

Allan

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Here is another solution that I used with my set up using a DAQ usb-6001 and a AD8495 Breakout. The breakout is a voltage amplifier mostly used with Arduinos but will work with the DAQ's Its made by adafruit. The pin out and the set up is printed on the board.

 

https://www.adafruit.com/product/1778

 

 

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