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NI PXI 4220 lead wire lead resistance uncompensated in hardware Quarter Bridge I

Hey,

 

there are different types of bridges for strain gauge measurement. I am wondering about the hardware compensation for the Quarter Bridge I, especially about the lead resistance at the Signal S+ pin (pin 1), which is not drawn at pin 1 for the other bridge types.

It says in the manual, that "The voltage drop across the lead resistance on a quarter-bridge type I configuration is uncompensated in hardware."

Since the QTR (pin9) connection is bridged to pin 7 by a completion resistor, the internal hardware is basically the same as for the other bridges - as far as I can see. The Signal wire has low current and it shouldn't make a difference (like for the other bridge types) in lead wire resistance change.

 

Is there an internal hardware difference between Quarter Bridge I and the other bidge types that isn't written down in the manual (can't find further explanation)? For my understanding, a lead resistance is already in every relevant wire of the bridge (->already compensated).

What am I missing for understanding the relevance of the lead wire resistance of quarter bridge I compared to the others? 

 

Best Regards,

Thomas

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Message 1 of 5
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Hey budist,

I was scrolling through the forum and run into your question. 

Did you manage to find out the answer yourself or are you still trying to figure it out?

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Hey Maciej_B,

 

I am still interested in finding more info about this special topic.

So far, we say our wire is so short, that it won't make a relevant difference (I have to look it up, but as far as I remember: below 10...20 m the lead wire resistance isn't relevant).

 

Regards

 

 

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Message 3 of 5
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Hi budist,

I passed your question to engineers from National Instruments. I will let you know when only they answer back.

 

Regards,

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Message 4 of 5
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Hi budist,

I consulted your question with engineers from NI and they responded that QB1 diagram is different from others because of the fact that there is only one resistor you have to complete the bridge with, outside the card itself. And because of that, the effect of the lead resistance grows since all other resistances are known, because they are inside of the card. 

Please let me know if it answered your question. Should you have more we will try to answer them as well.

 

Best Regards,

 

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