Sg-140 strain gauge resolution

[Vchavez]

I am using an FP-SG-140 as well as a cFP 2100 to take strain gauge measurements from 8 load cells.  The load cells have a max capacity of 5kg.  i need to be able to make measurements from 0 to 150 grams with an accuracy of .1 of a gram.  I have written my LabVIEW program and i am receiving measurements in mV/V as well as grams which I scaled through LabVIEW.  My problem is I am receiving inaccurate data. 

EX.  Say i put a 100g weight on the load cell, at a steady state I'd receive a reading of 100.862g and 99.487g switching between either of the two numbers but nothing inbetween.  The mV/V would switch between a reading of .010311761 and .01019255 with no numbers in between.

Is there a way to improve the resolution or would i need to purchase new load cells with smaller capacity?

In other words, the readings are accurate to .001 mV/V would it be possible to get readings accurate to the .0001 mV/V?

Averaging in Labview doesn't give more accurate results.

Changing input range in MAX does nothing.

Message Edited by Vchavez on 07-13-2006 11:21 AM

[Matthew_Williams]

From your gram and mV/V resolutions above it looks to me like your gages are
giving you about 0.433mV/V at 5kg; sounds very small.  However, if you calculate
back, assuming the +/- 3.5mV/V input range and that gage sensitivity, your
gram resolution should be about 1.23 g so I think the .433mV/V sensitivity may
be true.

All that being said, I think it will be a real challenge to get 0.1g accuracy from your
gages.  We have spent many months working closely with a competent gage
manufacturer to get a 0.1g accurate measurement from 300g gages that give us
~ 2 mV/V @ 300g.  Your gages are many times less sensitive than the ones we
are using.  Our system yields a gram resolution of ~ 0.02 g.

My suggestion would be to get a gage that gives you as close to one of the standard
SG-140 input ranges (+/- 3.5mV/V, +/- 7 mV/V, etc.) as possible at 150 g.  For example,
a 3 mV/V @ 150g gage will give you a gram resolution of 0.005 g.  Much easier
to get a specific accuracy if your resolution is smaller 😉

Hope this is useful.

Matt

[Vchavez]

I will take that into consideration, thank you.  I am currently in the process of building an amplifier for the load cell, but chances are i will be purchasing new load cells.

[Jarrod_B.]

Hello Vchavez,

Just to clarify a bit about the resolution of the FP-SG-140, the mV/V readings you are seeing are exactly as expected for this module with a range setting of 3.5 mV/V.  If you look at the Specifications section in the FP-SG-140 and cFP-SG-140 Operating Instructions, you will see that the true range at 3.5 mV/V is 3.90625 mV/V with overranging.  If you divide this by 2^16 - 1 (16-bit resolution), or 65535, you will find that the smallest change that can be detected is 1.19211 x 10 ^ -4 mV/V, which is exactly the difference between your measurements of 0.010311761 and 0.01019255. 

All of that being said, with your max capacity of 5 kg, and desired resolution of 0.1 g, you would need to be able to detect 50,000 increments within your total range.  As Matthew described, this means that your full scale reading needs to be pretty close to one of the input range limits to help maximize your resolution. 

I hope this information helps!

Best regards,