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signal conditioning

i am trying to measure greenhouse temperature using thermocouples (3 air, 1 water) and LabVIEW 5.0 with the AT-MIO-16X I/O connector. i am ordering thermocouples from OMEGA, and i need to know about referencing the thermocouple. the rep said i need signal conditioning. if this is the case, what exactly do i need? does LabVIEW take care of this?
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If you are intent on using thermocouples, then I would highly recomend signal conditioning, as at room temperatures, the millivolt output of almost all thermocouples is very low (depends on the type of thermocuple). Thermocouple-specific signal conditioning will also provide for the required signal characterization (thermocouples are not linear devices).
The other problem you mention, calibration, is also critical with thermocouples especially if you are operating in the room temperature and lower ranges. You have to have a method of compensating for the reference junction temperature (assuming that it remains constant, which in an outdoor environment, is questionable).
Having said all of the above, I would highly recomend that you elect to use RTD's to
measure your temperatures. The first gain is that you can calibrate them with a fixed value resistor substitution. There is no reference junction ambiguity/possible-variable to deal with.
You will require signal conditioning with and RTD, but the cost is similar to that for a thermocouple, and you circumvent all of the problems of thermocouples. The second big advantage with RTD's is that you can wire to them with ordinary copper wire. With thermocouples, you must use specific thermocouple wire (read expensive) from the thermocouple terminals to the point at which you measure your reference junction. If there is some distance to your thermocouple, the cost of wire can outstrip the cost of the thermocouple itself!
Which ever path you decide to take, NI has good signal conditioners listed on their site/in their catalog.
Hope this is of some helpt to you.
Dave
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I'm wondering if these signal conditioning boards provided by NI are also useful in low power measurements. I've studied the signal conditioning tutorial of NI and realized that the signals (e.g. high voltage, noisy environment, extreme high or extreme low) which are difficult to be measured using DAQ boards could be measured using the signal conditioning circuits. I’m also aware of the fact that signal conditioning maximizes the accuracy. So, I’m interested to know if these signal conditioning circuits might be helpful in reducing the noise and for more accurate measurements of the low power in the range 1-30 milli Amperes. I still am not sure of how they might be helpful in low power measurements. I appreciate the ideas. Also, if you have prior experience working on low power measurements, please share your views.

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Hello Kay Kay,

What type of low power measurement are you looking to achieve (i.e. voltage, current, temperature, pressure)?  There are many reasons why you can have a noisy signal and signal conditioning may or may not be the answer for you.

There is a lot of useful information on techniques for minimizing noise in the Field Wiring guide.

Signal conditioning offers a few different techniques for improving signal quality including amplification and filtering.  There are many resources available to learn about using signal conditioning for different applications including the following: Improved Signal Quality via Conditioning and Increase Your Measurement Accuracy by 10X with Signal Conditioning.

Hope this is helpful.

Regards,
Jennifer O.

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I'm measuring the current consumption of a microcontroller unit (MCU) . I'm using a custom designed board which has an instrumentation amplifier. The output of this board will go to the NI PCI 6115 through BNC2110. So, the output from the custom board is the analog input to the DAQ board. And I've developed a current equation (in the range of current to be measured) for the custom board using different resistors of the desired range. For the custom board, there is a supply of constant voltage. MCU is connected to the custom board where the resistances are connected during the expriment. At a constant voltage, as the resistance (MCU current consumption) varies the current varies. Hence I observe these changes in LabVIEW.
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To measure your current consumption I assume you are using differential mode and measuring voltage through a known high-precision resistor.  Is this correct?  If you could be more specific as to the nature of the signal that you are trying to measure (voltage level) and any problems you are having in doing so, I could provide you with suggestions to improve your measurement.

You may also wish to work with the Accuracy Calculator to determine the absolute accuracy of your board.  You can also select Signal Conditioning products to determine how and if they will improve this accuracy.

Regards,
Jennifer O.

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Jennifer,
I'm trying to measure a signal with voltage level ranging from 0 - 2V. I have no problems using the PCI 6115 but I'm more concerned about how to beware of the external noise and the precautions to avoid the noise (if possible). And yes, thanks to the NI material on Noise and the link you've provided haelped me in understanding the basics that I should know. However, I'm not sure about the accuracy calculator and how to read it. And I thinkg for our board PCI 6115, there are no signal conditioning products available. Well, all I would like to listen from you (if you have more knowledge on small signal measurements) is the problems faced when measuring small voltages of the range specified. Thank you!

Message Edited by Kay Kay on 08-01-2006 11:08 AM

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Kay Kay,

You should be fine without signal conditioning if your signal range is 0 to 2 Volts. Remember to set your input level as narrow as possible (maybe -2 Volts to +2 Volts). Follow the guidelines in the Field Wiring diagram. Use short wires and make sure they are shielded. You should be fine.

Cheers,



Garrett H
National Instruments
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Garrett,             I need some help on a similar problem. How do I set the input level? I went into max, but am only able to view the settings and not able to change them. I am using MAX v3.1.1.3004

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Hi arbit,

In the future, you might get more/faster responses if you create your question in a new thread, rather than tagging it to the bottom of one that’s been inactive for several months.

When you create a temperature task (In MAX under Data Neighborhood » NI-DAQmx Tasks, right-click and select Create New NI-DAQmx Task…) you should be able to select the input range, thermocouple type, etc.

If this is not working for you, will you please be more specific as to what you’ve tried and possibly include results with screenshots, specific error message, etc.?

Mark E.
Precision DC Product Support Engineer
National Instruments

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