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cFP-AI-112 measuring heat-flux gage voltage output

I have some Gardon gage (thin-foil) type heat-flux gages that produce a mV output. The leads of these gages are made from nickel-plated copper. Currently, I have a cFP system with TC-120 modules to measure the gage output. We run into the issue of whether one should classify the heat-flux gages as thermocouples (and in need of cold-junction compensation) or as simple transducers with mV outputs. My question is will the AI-112 modules suit my purposes better? The gage leads will obviously conduct heat, so will this be an issue for the AI-112 modules in terms of thermal conductance affecting the mV readings? Any help would be greatly appreciated.
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Hi Jim,

I'm not familiar with Heat Flux gauges. But if you provide more information about them I can try to research the best way to use them.

My question is will the AI-112 modules suit my purposes better?
The 112 will work to read the voltage output, however the TC 120 will give you a better resolution at smaller mV ranges. So, you should consider whether you need the resolution or larger input swing. What kind of voltage swing do you see from the gauge? What kind of resolution do you need?

The gage leads will obviously conduct heat, so will this be an issue for the AI-112 modules in terms of thermal conductance affecting the mV readings?
Compact FP is a robust design and both devices mentioned have the same temperature operating range, so as long as you are operating either device within -40 to 70 °C then you should have no problem. What temperature extremes are you planning to measure?



Message Edited by Sappster on 07-15-2008 07:58 PM
Sappster
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Sappster,

Thanks for the reply. I would say that the voltage range I will probably see lies between +/- 2 mV, so a somewhat small resolution (~ 1 uV) would be ideal. These thin-foil type gages rely on a material with known thermal conductivity that separates the two copper leads. The voltage/temperature gradient comes from this separation and not from the junction of two dissimilar metals like thermocouples. One lead is simply positioned at a "hotter" location relative to the other lead. I do not expect the temperature of the leads to exceed 70 °C at the module inputs, so the ambient temperature range is not of a huge concern. The reasons I am looking into replacing some of the TC-120's are because 1) they have only 8 channels whereas the AI-112's have 16 channels and these would allow me to measure every gage I have, 2) I have been having problems with getting unexpected negative readings from the gages, which may be some physical phenomenon or something with the TC-120 modules (I have the revision B modules), and 3) I am not certain if the cold-junction compensation on the TC-120 modules might be interfering with my mV readings. Thanks again for the reply.

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HI Jim,

Thanks for getting back to me with more info on your gauge. Here is some information about your concerns:

1) they have only 8 channels whereas the AI-112's have 16 channels and these would allow me to measure every gage I have
The AI 112 has more channels but has less resolution: 3 uV at lower frequencies (50 - 60 Hz) and 25 uV at high frequencies (~500 Hz). In contrast, the TC 120 has higher resolution, 1.5 uV, but a hardware sampling rate of .88 Hz (1.13 sec refresh). So you need the weight the ability to have all your reading on one module and control the sampling rate vs. resolution.

2) I have been having problems with getting unexpected negative readings from the gages, which may be some physical phenomenon or something with the TC-120 modules (I have the revision B modules)
I'm not sure what could be causing this but if you explain more or can provide a sample of data I might be able to troubleshoot it.

3) I am not certain if the cold-junction compensation on the TC-120 modules might be interfering with my mV readings.
You can disable the cjc reading on the 120 and then scale the input as you would with the 112. However, the AI 112 is much more configurable than the 120.

Let me know if this helps with your decision of if you have more concerns.

Sappster
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Sappster,

Thanks again for your help. We are getiing closer to making a final decision. I do have one more question about the AI-112 modules. I was wondering what the resolution would be at very slow sample rates such as 5-10 Hz. If the resolution decreases with increasing sample rate, then would the reverse apply (i.e. increasing resolution with decreasing sample rate)? Thanks for the information you have provided.

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The resolution is dependant on what level of filtering you select (50/60Hz or 500Hz), not the sampling rate. Resolution is also dependant on the full scale input range that is selected. See page 10 of the user's manual here for full details:

http://www.ni.com/pdf/manuals/371330a.pdf

Hope that explains your question.

-AK2DM



Message Edited by AnalogKid2DigitalMan on 07-23-2008 08:57 AM
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