Dummy strain gauge (unbonded)

[Derby2P]

Hi all,

I'm about to experiment with various configurations of strain gauge. I've never really used quarter bridges before.

In a quarter bridge configuration (type II in NI's terminology) - with the dummy gauge for temperature compensation - NI and other sources mention, for obvious reasons, leaving this dummy gauge unbonded. Please can someone enlighten me as to how this dummy gauge is attached to the specimen?

Many thanks in advance.

Mark.

[J_Oneill]

Hi Mark,

How are you attaching your first strain gauge? The dummy gauge is attached to the specimen in the same way the other gauge is. The only (and important) difference is that it is placed transverse to the applied strain so that the strain does not affect it.

You may have come across this before; it is a useful resource when working with strain gauges:

https://www.ni.com/en/shop/data-acquisition/sensor-fundamentals/measuring-strain-with-strain-gages.h...

Hope this helps,

James 

[Derby2P]

Hi James.

Many thanks for reply. It's thrown me a bit this!

99% of the time I use M-Bond 200 (CA) from Vishay/MicroMeasurements.

Surely if I attach the "dummy" gauge in the same way as the primary gauge (albeit at 90 degrees) then I have a half bridge (half bridge type I to use NI's terminology) with two active elements, where the second gauge is a "Poisson gauge"?

The NI notes clearly say the dummy gauge (quarter bridge type II) is "unbonded" - or I guess this could be bonded to an identical part of the structure guaranteed to be unstressed. This is what NI says about the dummy gauge in a type II quarter bridge:

        "The dummy gauge is mounted in close thermal contact with the strain specimen but not bonded to the specimen".

I'd very much like to know how this gauge is mounted, but not bonded. Can't get my head around it! Unless the idea is to hold this dummy gauge securely in place with tape so only temperature changes in the metal are carried across. Just never come across this!?

Thanks again for reply. I dearly hope I've not overlooked something blindingly obvious!

Regards,

Mark.

[J_Oneill]

Hi Mark,

This is rather confusing and the documentation doesn’t help to clear up the confusion very well. If the dummy gauge is to be effected by temperature it must be in contact with the material. Therefore I believe the term “unbonded” refers to it not being bonded to the part of the specimen affected by the strain. Either at 90 degrees to the first strain gauge or at a different point on the specimen where you know it will not be affected by the strain.

Regards,

James.

[Derby2P]

Hi James - thanks, it is a confusing bit of writing.

Thanks again for your input.

Regards.