Unexpected readings while acquiring barometric pressure

[HKubinhetz]

Hello!

First of all, even though I'm using LabVIEW, my question is about instrumentation. If I posted in the wrong place, I apologize. Also, where should I ask something like this next time?

I'm using a NI-9237 (connected to a cRIO-9144 in scan mode) to read barometric pressure. My transducer is a honeywell FPB that returns a Hg value and has a 10V excitation voltage. I have attached the calibration certificate with the conversion values and every relevant information about the piece of hardware at the end of this message.

When I try to read barometric pressure, the returning values are unexpected. The figure down below shows raw voltage obtained through a probe (number 5) connected right after the I/O node and the respective module's configuration:

First of all, the value is negative. By reading the calibration table, I expected a positive number. Besides, since the pressure today is around 27.5Bar where I'm at, I was hoping the voltage would be around 0,93mV/V. A measured value of -4e-7V/V means -0,0004mV/V (after converting V/V to mV/v) there is still a big difference.

What am I doing wrong? It is the excitation voltage, the transducer's configuration, something else?

Please notice that this calibration is outdated. Can such period change an acquisition behavior that much? Also, I am not converting these values to Bar neither converting raw data to mV/V. The number in the figure above is exactly what I am acquiring from my physical input.

I hope I was detailed enough in my post. Please let me know if something else is needed to write an answer and I'll be glad to fill in the blanks.

Regards,

Henrique Kubinhetz. 

[orlandiisabelle]

Hello Henrique,

Let me ask you something. What is your conector model of the NI-9237? In page 10 of the manual there are two models: DSUB (left) and RJ-50 (right). Which of them are your?

GETTING STARTED GUIDE - NI 9237:

http://www.ni.com/pdf/manuals/374186f.pdf

Another question: do you want to connect the transducer in the full-bridge configuration, right? Take a look at the page 12 of the manual to verified if your connections are right.

I will wait for your answers to better understand your issue, and be able to help you.

[HKubinhetz]

Hello, Isabelle! Thank you for your help.

I'm using a NI-9237 with a RJ-50 connector model and double-checked my connections, which are correct. The cables came with my entire order (composed by cRIOs, modules, transducers and cables) and these cables are from Honeywell, just like the transducers. I made some more tests that may help to troubleshoot. It looks like that when the excitation voltage is 3.3V and 5V, output values are coherent, but when raised to 10V, this tension drops abnormally. Down below you can see the pressure measurement VI I built. There was a transducer connected only to AI0 (other signals were noise), the upper field shows mV/V voltage and the lower one represents the final output based on excitation. The only changes I made between images are related to the excitation both on the VI control called Excitation Voltage (Volts) and module's configuration in project manager. Also, I tried different cables and transducers, but found the same behavior of the following results:

Excitation of 3.3V

Excitation of 5.0V

Excitation of 10.0V - Abnormal values

I am also attaching a VI to show the block diagram of this code in case my description was confusing. I'm multiplying the Inputs to represent V/V in mV/V then considerating excitation voltage:

Thank you for your availability on helping me!

Regards,

Henrique Kubinhetz

[JÞB]

That's easy 

Your system is inside a black hole.

Alternately,  since you  are posting outside the event horizon, your wiring is faulty.  Check your physical layer.