do not know gage factor

[rpulsifer]

My client has provided me witha  home grown loadcell for which I know the following items - Capcity 5kg, Full scale 2.6304 mv/v, input resistance: 1000 NOM. How can I fill in the DAQ assistant paramters (DAQmxTask in Measurement Studio) from these numbers?? Especially as I do not know the gage factor...

Thanks,

RPulsifer

[e-Cash-Money]

Hello Rpulsifer,

I did some research into calculating the gage factor and it appears that the value must be arrived at empirically and cannot be determined anaylitcally from the parameters you have given.  The gage factor specifies the sensitivity of the strain gage and relates the change in electrical resistance to the change in strain.  I do not think this can be determind based on capacity, full scale, and input resistance (though there might be some method I am unware of).  If you cannot get any more information from the client, you might have to setup a test system to determine the gage factor through trial and error.

[rpulsifer]

I looked up the definition of Gage Factor as well and as far as I can see it would only come into effect regarding the linearity of the output signal. i.e. the higher the strain the farther off the resistances wouldbe from nominal and thus the output would be more and more skewed as you went through the full range of the device.

If I have described this correctly (this is where you folks can jump in..;-)), then the fact that that calibration report from the instrument validation company shows a linearity of .05% would mean that I could simply stick a 1.0 in as gage factor and not worry about it.

Please tell me I'm right, 'cuz I'm getting nowhere with the "factory".

thanks,

Rpulsifer

[Logan_K]

What you are using is not a strain gage - it is a load cell, so it doesn't make sense to try and scale it's output using one of our strain gage equations. I would recommend creating a DAQmx Custom Voltage with Excitation Task, because you have all the information that you need to scale your signal into kilograms based on the raw voltage reading. Let me explain...

At full scale your voltage reading will be 2.6304 mV / V, where V is refering to the excitation voltage input. So, if you use 10V for your excitation voltage, then at 5kg (which is full capacity) you will read 2.6304mV/V * 10V = 26.304mV.

Vex = Excitation Voltage (V)
Max Load = 5kg
Vr = Voltage Reading (V)

Now, since your load cell is very linear, you can compute your slope of the y = mx+b equation by saying that m = (max load / (Vex * 2.6304)), which yields the equation:
Load = (Max Load / (Vex * 2.6304)) * Vr * 1000 (the 1000 accounts for the conversion from your reading which is in Volts to mV)
or: Load = (1000 * 5 / 2.6304) * (Vr / Vex)
or: Load = 1900 * Vr / Vex

I hope this helps!

On a side note - if you were to use the Full Bridge type I strain gage type, then the strain reading is scaled using the equation (-Vr / (GF * Vex)) so if you just say that your gage factor is -1/1900, then the output of the task should be scaled properly in kg and you could save a step in your calculations. I would start by scaling your output manually to verify my numbers, and then see if you get the same results when using the strain gage task.

Logan K

[rpulsifer]

Thank You!! Finally, someone who's knows what they're talking about!

When I started this thing, I never looked down the list of tasks, I saw Strain gage near the top, clicked it and went on to throuroughly confuse myself. While, of course, the load cell uses a strain gage method internally , the output is simple mV! Duh!

Thanks Again,
Rpulsifer

[rpulsifer]

BTW, E LEE, I was so excited that my app worked, that it wasn't until after I posted my thank yous that I noticed that it was a different person that clarified my cross connected braincells. So, no offense from my first "Finally" remark (I thot it was you) Many thanks to both of your efforts.

RandyP