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9237 problems with dynamic pressure measurement

Hi everyone. I have a 9172 CDAQ and a number of in/out cards. Recently, I got a 9237 card to use with Kulite dynamic pressure transducers. They are full-bridge transducers, 0-500 psi, 10V exc, 0.200 mV/psi (note: the manufacturer does not spec the sensitivity as a ratiometric value relative to excitation, but these are actual calibration values). The nominal resistance across the exitation leads of the transducer is 2525 ohm and across the output terminals is 1504 ohm. I connected up the transducers and took some measurements which produced strange results.

 

First, I created the appropriate pressure AI bridge task, selected 10V excitation, full bridge, and left the default nominal resistance of 350 ohm. I created setup a 2-point linear calibration with a point at 0 mV/V = 0 psi and 10mV/V at 500 psi (0.2 mV/psi * 500 psi / 10V = 10 mV/V). I took a measurement of a dynamic event which should have produced a reading of ~100-200 psi, and it peaked out at maybe ~2-2.5. In addition, the second transducer did not read at all. I played with the calibration values some and then removed all scaling completely, in order to see the V/V output (I changed the task from a pressure bridge measurement to a generic AI bridge). It was on the order of 4E-5 for the same dynamic event, which should have produced maybe 4E-3 V/V. So about a factor of 100 off. I will note that the measured excitation voltage at the transducer was 10.83 V. But that isn't going to cause a factor of 100 difference. Still, I figured it would be closer to 10V exactly.

 

Since I have other modules, I connected the transducer output leads to my 9205 AI module and created a linear calibration for 0-0.1 V = 0-500 psi. And sure enough, it read correctly. Also, the second transducer read correctly as well.

 

Any thoughts as to troubleshooting the 9137 for this problem? My initial thought would be some sort of grounding issue? Also, is the nominal bridge resistance value actually used in the measurement? Because changing it had no effect on the output scale.

 

Thanks and let me know if there are any other details I should include.

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Hi c_bedick,

 

The NI 9237 has a power limitation of 150 mW. With the resistance of your full-bridge transducer being fairly high, you will need to use the external excitation to properly take measurements, especially with two transducers. Page 16 of the NI 9237 manual discusses the calculations for this.

 

I believe the factor of 100 scaling issue is due to the configuration settings. Are you creating a task in MAX? If so, can you post a screenshot of settings? 

Briana P.
Licensing
Sr SW Engineering Manager | NI R&D
ni.com/support
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Well, the high resistance implies a lower current and power consumption at a given voltage. P = V^2/R = 10^2/2525 = 0.039 W = 39 mW. I only have two sensors, so ~80 mW total consumption. I don't believe that is the issue (not saying it isn't for sure though...). Especially since I can power the transducer from the 9237 module, and acquire the output signal from my 9205 module and get correct readings. But I'll try digging up a 10V supply and powering it directly to confirm.

 

I have configured the 9237 in MAX and just programmatically through Labview, with the same results. I set the excitation to 10V, select internal excitation, full bridge, and then I have done the scaling a few different ways with the same results. I have tried using the generic AI bridge and AI bridge (pressure) blocks. If I disregard the actual scaling values and just spit out the raw V/V values, they are down by about a factor of 100 still.

 

Is there anything different about how the 9237 inputs are grounded relative to the regular 9205 AI modules?

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The main difference of the NI 9237 is that it is ratiometric module. Could you please post your configuration and the manual to the model of your transducer?

Briana P.
Licensing
Sr SW Engineering Manager | NI R&D
ni.com/support
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The following link has the spec sheet for a transducer similar to what I am using. (input/output impedences are slightly different....indicated in my original post) But it is a fully active 4 arm wheatstone bridge transducer.

 

I think I posted all of the details regarding my configuration. Please let me know if there is anything else specific you would like to know.

 

And thank you again!!

 

http://phubner.eng.ua.edu/Files/Kulite%20XT-190.pdf

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Please try a "Custom Voltage with Excitation" task and post the results.

Briana P.
Licensing
Sr SW Engineering Manager | NI R&D
ni.com/support
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