Strain Gauge Meassurement: Difference NI PXIe-4330 and HBM QuantumX MX840A

[Andband]

Hi guys,

I need to meassure strain gauges that are wired as quarter bridges.

Option A:

I get an NI PXIe-4330 and a PXI - System

Option B:

I get an HBM QuantumX MX840A with additional quarter bridge cables

On the first glance the QuantumX MX840A is able to do much more (for example alternating current excitation)... but acutally I don't know what either of those devices really does.

I always thought, using a meassurement bridge means, to simply amplify and acquire a bridge's output voltage.

I know that the HBM has an integrated amplifier - has the NI one, too? I haven't read about it anywhere. This leads to the assumption, that the Ni 4330 is just a normal 24-bit A/D converter, trying to convert those low brdige output voltages ? Than, why do I need such an expansive device as that 4330 is? Can you explain that to me ?

[Paul-D]

Hi Andland,

First, I wanted to point out that, depending on your application, buying a PXIe-4330 and a chassis would probably not be the most cost effective or simplest method of achieving your bridge measurements.  I would first recommend our cDAQ chassis and modules, which offer USB or Ethernet Connectivity, are more rugged and portable, and can be easier to work with.  Depending your channel counts and the sample rates that you require our NI 9219, NI 9235, NI 9236, or NI 9237 may meet your needs.  Ultimately choosing the right hardware for you should be done with your local Field Sales Engineer. 

I think it may benefit you to look through our Measuring Strain with Strain Gages tutorial to better explain how bridge measurements work, and how that plays into your application.

I'm not sure what you mean about what you mean by an "integrated amplifier", but our bridge devices do offer internal excitation, and some also offer the option for external excitiation.  Our devices work by that DC voltage excitation being put across your quarter bridge device, and then measuring the voltage across that device as explained in the above tutorial.

To answer another question, our bridge devices do not feature normal ADC converters.  They actually feature Delta-Sigma converters, also known as Dynamic Signal Acquisition (DSA).  DSA offers a much more accurate measurement through oversampling.  You can can out more about the benefits of DSA in our Benefits of Delta-Sigma Analog-to-Digital Conversion tutorial.

Let me know if you have any further questions.

Thanks, 

[Andband]

Ok, thanks a lot Paul,

I decided that I just want to meassure the bridge voltage of  a "real, existing" bridge. So I won't need any bridge-modules.

But to do that, I need very accurate meassurements - I guess 🙂

So again: there is the MX840A from HBM with 8 Channels and 24-bit @20 kHz - which means a resolution of 35 nV @± 300 mV on the one hand - and sadly nothing comparable (speaking of resolution in combination with sampling rate) from NI in my price range around 6000 €

There only is the PXI-62989 which has a resolution 0.8 µV @± 100 m.

And actually I really would like to get a PXI-System, that my successors have something to work with 🙂

So what can I do ? 🙂

[Paul-D]

A PXI system will be a good solution for you if you plan on using it's modularity to your advantage in the future.  I just wanted to show that there were other options available besides the PXI.

We actually do offer a few solutions in the range of measurements you are requesting:

We offer a selection of Digital Multimeters (DMM) that offer very accurate measurements.  Specifically, the PXI-4071 beats your stated specification by offering +/-100mV range for an effective resolution of 10nV (per the 4071 User Manual).  The major caveat with DMMs is that to get this accuracy, they acquire much slower than other data acquisition devices.  See the user guide for the data rate specifications at different resolutions.

I suspect that the 3rd party device you are referencing is also a DSA device.  We also offer other DSA Devices that will acquire at a higher data rate, and offer high resolutions.  For instance, or 446x series offers a voltage range and accuracy very close to the other device you mentioned.  I would recommend you check out our DSA manual to find out more about what each family offers.

Thanks,

[Andband]

I already 'found' the PXI-4071 - but as you have mentioned: It is just too slow for my application. But the PXI-4462 sounds very promising. Is there any information about accuracy (as there is for the PXI 6289 e.g.) ?

And with the 4462 it is possible to meassure four independent differential signals? So the 'BNC-shielding' is not bridged within all channels as it is with every oscilloscope? This would be really important, since the output of the meassurement bridges will be floating.

Thanks 🙂

[Paul-D]

DSA boards are designed to make measurements with high linearity, flatness, alias rejection, and dynamic range.

Absolute accuracy of DSA products is not as good as DAQ boards and especially Digital Multimeters (DMMs). DSA products are not designed to be used in applications where Absolute/DC Accuracy is an important requirement.  What the DSA boards do offer is high precision.  Please see Absolute Accuracy of Dynamic Signal Acquisition Devices for more information on the accuracy of DSA measurements, and more about what those values mean in the Dynamic Signal Acquisition Fundamentals tutorial.

The 4462 supports differential measurements on all channels, meaning that none of your channels will sharing a ground, meaning they will be independent.