Flexlogger Actual Rate

[JudgeMental]

I am running Flex Logger on a cDAQ-9174 with the following modules:

9211, Thermocouple

9227, Current

9205, Voltage

Flexlogger seems to have a minimum Sample rate of 1,612.9Hz, even in the "Slow(1Hz)" setting.

I would like to sample as slow as 100 Hz. Is it possible?

[tic.not.tock]

Hi JudgeMental,

I am assuming you are experiencing this with the NI 9227. Is that correct? That specific piece of hardware has a minimum sample rate of 1600 samples / second. This module is a Dynamic Signal Acquisition (DSA) module, so it needs a minimum sampling rate to accurately get measurements.

What are you trying to measure at 100 Hz? We may be able to point you to a module that better fits your needs. 

[JudgeMental]

Hi tic.not.toc,

Thanks for the reply.

We want to replace our rental strip chart recorders, and are evaluating cDAQ and Flexlogger.

We sometimes need to sample at rates as low as 10 samples per second. At 1600s/s we will get way to much data. TDMS files are about 500MB per hour! We can run up to 10 days (that's 120GB). All broken up into 1G files, still a nightmare to review, even in DIAdem.

Is there another cDAQ current measurement module (2 channels or more, 5.0Adc) that is not a DSA?

[tic.not.tock]

Hi JudgeMental,

Admittedly I have more of a software focus, so I am not sure what you mean by 5.0Adc.  There are 2 Current Input C-series Modules in FlexLogger that can have 100 Hz and below sampling rate. They are the NI-9203 and the NI-9208. You can compare the Current Input C-series modules available here, and you can see which ones are available in FlexLogger by checking the FlexLogger 2018 R2 readme in the  Supported Hardware section under C Series Current Input Modules.

[JudgeMental]

Hello again tic.not.tock,

The 9227 can measure up to 5.0 Amps (rms). The two C-Series devices you mentioned can only measure +/- 20 milli Amps.

In the 9227 data sheet it indicates that if using an external time base, rates as low as 390.625 S/s can be achieved. I will look into that.

Thanks

[Jarrod_S.]

I agree that it will probably be best for you to find a C Series current module that is not a DSA device that can measure at 100 Hz or slower.

However, also note that you can set each module in your system to run at different rates. This will at least allow you right now to have the thermocouple and voltage measurements for the 9211 and 9205 running at 100Hz or slower.

To do this, set the the 9227 to run in the Fast sample rate group. The Fast group will coerce to a minimum of 1.6kHz due to the constraints of the 9227. Then set the 9205 and 9211 to run either in the Slow or Medium sample rate group, depending on your needs. Putting these modules in a different sample rate group will let them measure at the desired rate. If you want to change the rates for Slow, Medium and Fast, select Configure in the sample-rate drop down menu to bring up the Configure Data Rates dialog.

More details: http://www.ni.com/documentation/en/flexlogger/latest/manual/configuring-data-rate-values/

[Jarrod_S.]

Another option to consider:

If you have an external shunt resistor, you can do the current measurement through your 9205 voltage device. You can configure a Current measurement as shown below using a Current Shunt sensor type. You'll need to enter the Shunt Resistor Value for your external shunt:

[JudgeMental]

Hi Jarrod_S,

The "Common Mode Voltage" is to high for that solution, our source is up to +36Vdc. The 9205 is limited to +/- 10V.

Thanks

[Jarrod_S.]

Thanks! Good to know. Note that FlexLogger currently only supports sampling measurement data at the rates at which the DAQ boards support. However, we are evaluating features for a future release to resample recorded data to the rates which people specified (100Hz vs 1.6kHz).

[BriBear]

Actually, the NI 9205 can measure up to +/10 V accurately, but it actually can handle +/- 30 V between an AI channel and COM.  I just want to make sure you're aware of the capabilities of the module.

While your assessment that your source voltage is too high for this approach due to common mode voltage could still be correct, depending on the relative value of the shunt vs the load, the measurement (and potentially on whether the load value could be variable), the current shunt approach could still be an option using the NI 9205. For additional context, take a look at this Knowledgebase.