Strain Measurements with NI 9219

[Engr_tech]

Hello to all, 

i just join this forum and i would need bit of advise on strain measurement using NI 9219 with Lab view.

i have followed all the procedure and settings to configure a strain virtual channel in NI DAQ mx global virtual channel.

i need few clarifications on the strain gauge measurements:

1.if i am using quarter bridge 1 configuration do i need completion resistors for NI 9219?

2.i have ordered strain gauges but can i mimic the calculations of strain gauge with labview by using a variable resistor in place of active gauges?like what i understand is strain gauges change the resistances when force is applied.based on that concept if i use variable resistor on one leg of wheatstone bridge and plot the data? all i want is to play around with lab view till i get the real strain gauges.(will take a week to get deleivered)

3.by setting the Vex to internal i think i wont need to apply any voltage to the terminals of bridge as shown in strain setup window.i would just need to connect the to ends of wheat stone bridge to pin number 3 and 5 in NI 9129.please comment on that..

i really thank and appreciate all of you for taking time out and reading/commneting on my post.

Regards,

Ali

[Spicy_Mystery · Accepted solution]

Hi Engr_tech,

The 9219 has built-in resistors for half-bridge completion, but for quarter-bridge measurements, it must rely on 2-wire resistance measurements.  It does not support 3-wire quarter bridge measurements.  I am not familiar with using a variable resistor in place of an active gauge so I can't comment on that.  It sounds like your idea in #3 would work, although I am not 100% certain.  It will be unsupported but it could work.

[Engr_tech]

Thanks for your comment

as you mentioned for quarter bridge measurement i can only rely on two wire measurements methods.i am not sure which pins would i connect my gauge to in ni 9219?

[Spicy_Mystery]

Hi Engr_tech,

Check out this document, I think it may have the answers you're looking for: http://www.ni.com/pdf/manuals/374473b.pdf

Page 8 seems to give a good table that may be useful for you.

[Engr_tech]

thanks the document really helped me alot.

i have my strain gauge setup to the NI 9219 and after setting up the channel i could get data out of it but the error percentage in very high (around 6000) when i do the caliberation.

even though resistance of my gauge is 120.3 ohms.i am not sure why am i getting high error percentage.

i am using quarter bridge one configuration with strain gauge connected to pin 3 and pin 5 of ni 9219.

and when i start running the channel it does plot the strain gauge value(very high value not i was expecting it to be close to zero in no strain condition) but the program keeps on plotting even if the gauge is disconnected from the Ni 9219. 

please comment on that 

Regards,

Ali

[Spicy_Mystery]

Hi Engr_tech,

How are you verifying the accuracy of it?  Make sure you are following the instructions under the "Thermocouple Accuracy Verification" section, starting on page 8: http://www.ni.com/pdf/manuals/372264a.pdf

[Henrik_Volkers]

a quarter bridge OR two wire ohms measurement ... mh ... prone to errors

How does your strain gage looks like?  Do have access to the contacts close to the sensing element? 

Use a second strain gage (@ first one, but don't apply strain) to build a temperature compensated half bridge) 

[Nayara]

Hello!

Would you have a copy of the file inserted in this link that you sent? Because I'm afraid that the page related to this file is not available anymore.


Best Regards, 

 Nayara Montes.

[luda.mattski]

Hi Nayara,

Here is the link to the page you are looking for regarding verifying thermocouple calibration in a 9219:  http://www.ni.com/pdf/manuals/372264a.pdf.  Check out page 8 for more information.