Go Down

Topic: Sensor to measure 10gms onwards to 2 kgs (Read 1 time) previous topic - next topic

NI$HANT

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?
"Real Men can Accomplish  Anything"
- <a href="http://www.winacro.com">  <b><font color="red">Website</font></b> </a> 
- skype : nishants5  

ਫ਼ਤੇਹ ਕਰੁਂ!

Erdin

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.

NI$HANT

Thanks a lot edrin.
Can you give me some links to the load cells you would recommend for my purpose straightaway.
"Real Men can Accomplish  Anything"
- <a href="http://www.winacro.com">  <b><font color="red">Website</font></b> </a> 
- skype : nishants5  

ਫ਼ਤੇਹ ਕਰੁਂ!

Erdin

#3
Aug 20, 2013, 04:16 pm Last Edit: Aug 20, 2013, 04:17 pm by Erdin Reason: 1
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 ?

NI$HANT

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
"Real Men can Accomplish  Anything"
- <a href="http://www.winacro.com">  <b><font color="red">Website</font></b> </a> 
- skype : nishants5  

ਫ਼ਤੇਹ ਕਰੁਂ!

Erdin

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.

NI$HANT

#6
Aug 21, 2013, 12:12 pm Last Edit: Aug 21, 2013, 12:16 pm by NI$HANT Reason: 1
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.

Quote
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  :P
"Real Men can Accomplish  Anything"
- <a href="http://www.winacro.com">  <b><font color="red">Website</font></b> </a> 
- skype : nishants5  

ਫ਼ਤੇਹ ਕਰੁਂ!

Chagrin

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.

Go Up