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Author Topic: Sensor to measure 10gms onwards to 2 kgs  (Read 841 times)
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What should I choose to measure extremely low weight items majorly from 10gms onwards to 2kgs or even 4 kgs?
a) Load cells(they need resetting or they become inaccurate?)
b) Force sensitive resistors but are they highly innaccurate?
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Use a load cell.

They are very accurate. They don't have a threshold for very small weigths.
They should be linear when correctly manufactured.

You can buy a table model pocket jewelry scale from Ebay for 5 dollars. There is a load cell inside those.

To get more accuracy, you need to reduce the noise and incread the number of ADC bits. A ADC of 12 or 16 bits with I2C would be a good solution.

Assume 4kg max, 10g resolution. That is even less than the 10 bits of the Arduino. So you could use a load cell with an INA125 OpAmp. But I would go straight for the an external ADC with more bits.

When a load cell is not overbent by a very strong force, it stays accurate. You could use a 100kg load cell with a 16-bit chip and still be more accurate than 10g.
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Thanks a lot edrin.
Can you give me some links to the load cells you would recommend for my purpose straightaway.
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This one: https://www.sparkfun.com/products/10245
Those are used in person weigthing scales.

A load cell is typically a aluminium beam with holes.
http://www.ishida.com/technologies/loadcell/html.html

Load cell on Ebay: http://www.ebay.com/sch/i.html?LH_BIN=1&_sop=15&_osacat=0&_from=R40&LH_PrefLoc=2&_nkw=load+cell+aluminium&_sacat=0
Most have 4 wires, I think that is better against temperature influence and noise.
But since they are on Ebay, I don't know how accurate they are.

Perhaps in your country is a webshop for load cells.

You could also buy a digital persons scale at a garage sale. I think 80% chance it has a load cell.


Why is resetting to zero a problem ?
Do you want absolute weight measurements at any temperature without the need to reset it to zero ?
« Last Edit: August 20, 2013, 09:17:53 am by Erdin » Logged

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Why is resetting to zero a problem ?
Do you want absolute weight measurements at any temperature without the need to reset it to zero ?

yes at any temperature weight measurement would be needed , why because the weight Would be stored the in measuring bin for like Sometime but the container would be emptied & filled up at times unknown
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I see.
The changes in resistance of a load cell is very little. In a wheatstone bridge with an opamp, the signal needs to be amplified a lot.

The load cell with 4 wires (2 stress gauges) are less influenced by temperature.
I don't know how it behaves over a long time.
The scales are reset every time when swiched on.

You may have to run some tests.

There are other options, like a spring with displacement detection (radio waves with coil, or a magnet with hall sensors, or optical). But a simple load cell is very accurate.
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I would like to run with load cells from sparkFun as atleast they are completely finished goods so it would retain less efforts needed my project to turn finished.

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Perhaps in your country is a webshop for load cells.

may be but I buy mostly from the US of A or China

Quote
This one: https://www.sparkfun.com/products/10245
Those are used in person weigthing scales.

I already had that in my sparkFun account shopping Cart  smiley-razz
« Last Edit: August 21, 2013, 05:16:08 am by NI$HANT » Logged

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You can find load cells on eBay or try sourcingmap.com. You should avoid the Sparkfun strain gauge; it's a cheap construction and Erdin already explained the disadvantages of three wire strain gauges. Also, try to find a load cell that is close to your ~4kg max requirement to keep the required amplification as low as possible.
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