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10k sensors


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#1 ampicoj

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Posted 17 December 2012 - 02:42 PM

I am a rank amateur with the LabJack. I have purchased a UE6. It will be used, initially, to monitor some 14 points, using Goldline 10k sensors. Can someone point me toward material on the best way to connect the sensors to the LabJack/Multiplexor? The decision for differential or single-ended seems very unclear. Also where are the low-level programming functions documented for C#? Many thanks, AmpicoJ

#2 LabJack Support

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Posted 17 December 2012 - 05:00 PM

Sounds like you have thermistors:

http://labjack.com/s...erature-sensors

Consider using silicon-type sensors instead if you can.

Easiest way to monitor thermistors is using the 10UA current source on the U6:

http://labjack.com/s...users-guide/2.5

You put all thermistors in series so that the 10uA current goes through each. By measuring the voltage across each thermistor, you can calculate the resistance of each (R=V/I). Rather than just using 0.000010 for the value of the current, you should use the actual value measured during calibration which you can see in the test panel in LJControlPanel, or better yet use a fixed resistor (Y1453-1.0K from digikey) to measure the actual value in real-time.

Do you have a Mux80? If not, you need to do all single-ended as shown in Figure 2.5-2. If you do have the Mux80, then you can use differential measurements which will perform better for this application, so use Figure 2.5-4 where R1 & R2 are thermistors and R3 is the fixed resistor to measure current.


One concern here is that you will have a lot of signal impedance with all these 10k thermistors. I can do some tests to determine what settings will work best. What is the highest resistance you expect from your thermistors (typcially at the lowest temperature)?


To talk to the U6 from C# you will use the UD library which is documented in Section 4 of the U6 User's Guide:

http://labjack.com/s...6/users-guide/4

I suggest you just start with calls to eAIN.

See the U6_Efunctions example here:

http://labjack.com/s...examples/dotnet

#3 ampicoj

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Posted 18 December 2012 - 10:50 AM

Thanks for a lot of good info - I'll digest. My temperature ranges are 32F (32.6K ohms) to 200F (829 ohms). Most are in the 100F-200F range. Is it possible to split the sensors into 2 (or more?) groups to help with the impedance issue? Are external current sources a help? I don't need 5 digit accuracy. Using 16 bit mode if I can get temps to xxx. I will be very happy! Thanks again for excellent prompt customer service! rtt/

#4 LabJack Support

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Posted 20 December 2012 - 08:54 AM

32F to 200F is in the range of silicon sensors, so if you can use those instead you will find that they are better, easier, and cheaper. So the max expected resistance from your thermistors is about 33k, and Section 2.5 notes that the max resistance the 10UA source can drive is about 300k, so 9 would be the maximum number of thermistors you can use. An external current source capable of driving more resistance would take care of that, but at that point you might just consider signal conditioners that take care of all issues. Such signal conditioners might cost ~$100 per channel, but spare you the expense of buying current sources. Another option is to use voltage dividers rather than the current source. Consider Figure 2-4 where R1=10k and R2=thermistor. You could repeat this 14 times and use a DAC to provide the Vin excitation. Actually you need to save 1 channel to measure the actual DAC excitation, so if you don't have the Mux80 you can measure 13 voltage divider signals.


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