NI9237 measurements fluctuate

[M.M.R.El]

Should I connect the half bridge as shown in the Labview program which is recommended by your Company !!!

[KatieC]

M.M.R.E.I, 

What LabVIEW program are you looking at? 

In terms of what bridge type you want, that depends on your application and your hardware. Here  and here are some links that may assist you in determining what type of bridge you would like for your strain gage application.

Have a great day!

Katie

[M.M.R.El]

Dear Katie ,

I really know what I want to measure , but the problem I am using a half bridge I and when I was connecteng it I was wondering should I use that diagram appearing in Labview .... or should I hear from those who sold me such instrument in my country whose advice was to use extra resistors to complete the bridge .... really this makes me crazy as What is the importance of such module .... I can use an ordinary data aqusition card !!!  To understand me more .. they sent me this file !!!!

[KatieC]

M. M. R. EI, 

I am still not sure what diagram you are talking about in LabVIEW; could you please clarify?

On page 8 of the manual, it states "When you connect a half bridge to the NI 9237, you must connect the AI+, EX+ and EX–, and RS+ and RS– signals, but you do not need to connect the AI– signal because it is connected internally." This is the programmable half bridge completion.

However, the advice of whomever sent you the picture is suggesting you do bridge completion externally and then take a full bridge measurement. While it is not necessary to do so, you may want to if you are concerned about factors such as wind, temperature, etc. If it is exceptionally hot outside, that will affect the strain measurements. Since the programmable half bridge completion is occurring internally, it can't account for this high temperature. By completing the bridge externally, then when you compare the two halves of the bridge it will eliminate the error caused by the change in temperature as both sides are equally affected.

Whether you want to do bridge completion internally or externally is dependent on your application.

Lastly, this article discusses some of the benefits of using this module over a DAQ.

Let me know if I was clear!

Katie

[M.M.R.El]

Katie ,

Tahnks for your replies , I will try your recommendations soon in the laboratory . But I have one more question  I have a couple of 120 Ohms strain gages how to know the excitation voltage if it wasn't written on their box .... Thanks in advance .

MMR

[KatieC]

M.M.R, 

I do not know of a way to determine the accurate excitation voltage without looking at the specification sheet. What model number and brand is it? I can try and look up that information for you!

Katie

[M.M.R.El]

Katie ,

Thanks a lot ... About strain gages ... here you are the available data ...

Tokyo Sokki Kenkyujo Co.,Ltd.

Type : FLA-6-11-1L

Guage length: 6mm

Guage Factor: 2.1

Resistance : 120

Please let me know if more data is needed !!!

MMR

[KatieC]

MMR, 

I took a look at their website and didn't have any luck. I emailed the company the information you gave me and hopefully they will get back to me with the excitation voltage. You may also try calling them as it might be faster. 

I will let you know if I get a response. 

Katie

[KatieC]

MMR, 

I heard back from them! They were very prompt. He said "The factory recommends limiting the current to 25 milliamps."

Katie

[M.M.R.El]

Katie,

I appreciate your help , but I have to ask how can I limit this current as I can only give voltage so what should be the resistance !!! in fact I still need help in this calculation as the inputs in labview puts me in a sharp corner .... as the strains are not known yet ... 

Thanks alot

MMR