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I need to build some type of scale where one can place four 600mL beakers on it and measure out its weight down to a milligram level of accuracy. I'm doing this so I can dose a very small amount of liquid (2 microliters) into the beakers at a time. The problem is that load cells and essentially all resistance-based force sensors are only accurate down to ~.03 - .05% of their full-scale load (at best). I've looked into piezoelectric sensor solutions but it seems that they are ill-suited for detecting static loads.

I'm completely open to having 4 separate scales, but the problem of having an accurate enough load cell will still exist.

Is there any way to detect changes in weight to this level of accuracy? Any suggestions?

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  • $\begingroup$ Do you need to detect small additional increments to a 10kg load, or are you asking for "milligram accuracy" over the full 0-10kg range? $\endgroup$ Feb 25, 2018 at 22:32
  • $\begingroup$ Detecting small increments to a 10kg load would be acceptable. $\endgroup$
    – AndrewP
    Feb 25, 2018 at 22:37
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    $\begingroup$ What is level of accuracy your seeking? I understand it is milligrams, are looking for +/- 200 mg or +/- 100 mg accuracy, Can you share some of the research done up to date? $\endgroup$ Feb 26, 2018 at 2:21
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    $\begingroup$ Can you share information on you current list of force sensors? $\endgroup$ Feb 26, 2018 at 14:31
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    $\begingroup$ What about a balanced system of 2x 10kg weights - you can effectively subtract the 10kg, and then use a much smaller/more sensitive sensor to detect your incremental change? $\endgroup$ Feb 26, 2018 at 14:51

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Actually, 600mL of liquid (water?) is about 600g, not 10kg, so it makes selecting an off-the-shelf balance a lot easier. Likewise, 2ul is about 2mg so a 1mg accuracy should be good enough. Based on this something like this Sartorius balance should do it:

enter image description here

Disclaimer: although I have nothing to do with balances, I do work for a company which is part of the Sartorius group.

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Based on the information that you provided I would recommend looking at a Wheatstone Bridge Circuit type of arrangement. It is my experience that using similar type if circuitry can help increase accuracy (need conformation)

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For starters I would suggest taking a look at single point bridge.

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For your other electronic circuits I would suggest looking at instrumentation or difference amplifier circuit together with ADC which will feed a micro controller similar to the circuit block below.

enter image description here

For the sensing element a pressure sensor base weight sensing scheme or stand strain gauge system might still be you best option.

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Take a look at Load Cells, Force Sensors and Torque Transducers. There is good chance you will find a good solution.

I also suggest that you take a look at Tactile Pressure Sensor Technologies from Sensor Products Inc

Below are some good references for you to get started.

References:

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  • $\begingroup$ That's certainly going about it from first principles! Seems like a lot of work that's unnecessary $\endgroup$ Feb 26, 2018 at 19:52

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