How Can i acquire right frequency?

[e.aydeniz]

Hello everyone,

I'm new to LabVIEW. I have a problem. I want to measure pressure fluctuating and Sound Pressure  in LabView with daqmx. Data acquation card is PXIe-4481. It is in chassis that PXIe-1071. I am using Dynamic pressure transducer (Kulite HKS-LP-375 M) , TELEDYNE RESON TC4034 (hydrophone probe) and its amplifier is TELEDYNE RESON EC6081. I added specifications.

I have connected the pressure transducer and hydrophone connected with amplifier to the PXIe-4481. I made my connections using a coaxial cable (copper). Also I want to see frequency reactions. To do this, I used 'Voltage -Continuous Input', a sample of LabVIEW in daqmx. And I added FFT and frequency indicator in program.I can see that the voltage values change, but the frequency response is steady. 50 Hz. It does not change. This is the same in the two sensors I mentioned. 

As a final question, according to the Nyquist equation. Sample rate should be at least 2x more than the maximum frequency. How will we know the maximum frequency? What is the maximum frequency?

I will be happy if you describe it clearly🙂

Thank you so much for your return already.

[Kyle97330]

Do you live somewhere where the AC line voltage is 50 Hz?  If so, I would suspect that the frequency of your line voltage is what's being measured due to some sort of grounding error or line noise interference.  

[e.aydeniz]

Yeah. I live in a place with ac line 50Hz. So do you have any suggestions on how to solve this? 

Thanks for the return.

[Henrik_Volkers]

The pressure sensor seems to a bridge type sensor.

The 4481 doesn't seems to support direkt connection of a bridge sensor.

(even in differential mode, there is a 50 Ohm to GND connection, as noted in the spec)

So you need a bridge amplifier

Samplerate: What is the maximum frequency you want to look at?

The hydrophone and the amp works (give signals)  up to ~500kHz (and higher with lower level),   so if you are only interested in signal up to 100kHz  I would suggest to set the amplifier lowpass to  250kHz and always use 1MSPS .

The hydrophone spec a lower range down to 1Hz .. do you need that lower range?

For the 50Hz signal:

A schematic of all connections (including your tank/volume, exitation , power supplies ) would be helpful

Basic strategy: test one component at a time . look at the signal, look for correlations

first only the amp/sensor isolated

connect it to the tank 

power up exitation 

could be electric hum (always first guess at line frequency :)) or mechanical hum (vibration into the tank by neaby engines...)

[e.aydeniz]

Henrik_Volkers Thanks for fantastic answer. 

Yes, I would say Pxie-4481 does not support whetsone bridge. So If I use bridge amplifier, I can probably  fix it.

But, for the Pressure transducer, Frequency ranges are not unspecified. There are only natural frequencies of the transducer. The natural frequency is not the same thing as the measuring frequency range? 

For Hydrophone,

in addition to question

How Can i convert Voltage to Sound Pressure ? Receiving Sensitivity multiply to Voltage output? But sensitiviy unit is V/uPa ?

[Henrik_Volkers]

Have a look at some appnotes on your sensor 🙂  .. .

Found this one starting at Kulites web page ....

https://www.kulite.com/assets/media/2018/01/Piezoresistive_vs_Piezoelectric.pdf

see page 12  for frequency range...

Do you want to do things like this?

https://asmedigitalcollection.asme.org/GT/proceedings-abstract/GT2014/45752/V006T06A031/235776

If you have a S=V/µPa  calibration factor and you want µPa and have V .... . what's the problem?