Strain gauge reading difference in SCXI-1520 + SCXI 1314

[S_Kanchap]

I am using PXI 1052 chassis with PXI-6221 DAQ card with SCXI 1520 and SCXI 1314 terminal block.

1. I have pasted 3 strain gauges (120e) on a long metal strip adjacenty (side by side).

2. I have connected one strain gauge as quarter gauge-I configuration in one channel.

3. I have connected two strain gauges as R1, R3in full bridge-I configuration with two dummy gauges as R2, R4 which were not pasted any where.

Problem :- I am getting 2 times strain in quarter bridge configuration than full bridge.

[lrallen]

Surender,

Have you performed a strain calibration on all of your channels.  Also, you should be sure that your connections and resistance values match those specified in the document linked below.  Please verify this and let me know if you are still having problems. 

http://www.ni.com/pdf/manuals/372583d.pdf

Regards,

lrallen

[S_Kanchap]

Thanks Irallen,

As for channels calibration issue, i have check my all channels but found same behaviour.

I am sure about connections and my straing gauges are all of 120 ohms.

I have problem still also.

Surender

[GDE]

Surender,

Take a look at this KB, it looks very similar to the issue that you are having. -GDE

[Logan_K]

The problem is that you aren't technically conforming to the Full Bridge I configuration. The Full Bridge I configuration assume 4 active gauges (no dummies), which should in theory would quadruple the sensitivity over a quarter bridge configuration. When you take two of the gages and make them dummies, you are reducing the sensitivity of the bridge by half - which would explain why the quarter bridge results are double the full bridge results. If you look at the SCXI-1520 user manual, it describes the different supported configurations, and if you plan on having two dummy gauges then I would recommend using a half-bridge II configuration. In your case, since the gauges are on the same side of the metal, you  can keep your current configuration and just use the Half Bridge type II equation when creating your channel. The only difference between these equations is that the half bridge II multiplies the readings by a factor of 2.

Logan K
National Instruments
SCXI R&D

[S_Kanchap]

Thanks Logan,

I am agree with you but if i need to take reading in the same fashion then what should i do?

Shall i use wheatstone bridge equation in place of ni supported 7 user modes q-i, q-ii, half-i etc. if yes then how?

Surender

[GDE]

Surender,

I am a bit confused by your last post and what you are asking. I believe Logan pointed out that in the configuration you described in the first post really falls under the category of Half Bridge type II, and thus that is the equation you should use. If that configuration is not what you meant in your last question, then please clarify with which configuration you mean.

-GDE

[S_Kanchap]

GDE,

I mean when we are performing any measurement, we need to use "DAQmx create virtual channel.vi"  and select type of measurement and got respective options.

When we select measurement typem strain then we have options of "bridge information" & "strain information". In the "bridge information" we have 7 options and their should have some equations respectively.

If we want to conduct different test then i want to know what calculations/equations i need to use.

Thanks

[GDE]

Surender,

I think I may see the confusion. When you choose strain/bridge type then the equations are taken care of for you. That is what the DAQmx driver is doing behind the scenes. So choose what kind of reading you want to make, and the driver will apply the appropriate equations, and give you back a strain measurement. Take a look at this tutorial, it may shed some more light on strain measurements in general. -GDE