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I have a project where I need to be able to measure the volume of remaining liquid in a bottle based on weight. The idea is to use a Force Sensing Resistor:

(model FSR402-Short Tail: http://www.interlinkelectronics.com/FSR402short.php)

image
(source: interlinkelectronics.com)

My issue is that I can't seem to get an accurate consistent reading on the pressure exerted on the device when any kind of shifting (such as replacing / refilling the bottle, moving the device location, etc) would create a huge variance in the data points. It can vary from 20-150 points easily... which is too large of a margin since 150 points could equate to about 500ml of liquid.


The idea was to loosely hang the bottle & connector to the pressure plate (see second image) using two centrally located screws. the pressure plate would have a set of two rubber feet perpendicular to the screw placements to allow the pressure to be focused on the rubber footing with the FSR.

This produced two main issues that I did not foresee with the CAD design:

  • over-tightening/under-tightening of the screws
  • inaccurate or inconsistent readings of the pressure from very minor shifts or bumps.

What can I do to improve my design, shown below?

1) Top perspective view of pivot mechanism. Top perspective view of pivot mechanism.

2) Side perspective view, translucent with descriptions. Side perspective view, translucent with descriptions.

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  • $\begingroup$ "I can't seem to get an accurate consistent reading on the pressure exerted on the device if the bottle gets bumped slightly or if the water sloshes and then settles again" So, don't touch or interfere with the bottle. $\endgroup$
    – Solar Mike
    Aug 1 '17 at 5:36
  • $\begingroup$ What was meant by that was any kind of shifting (such as replacing / refilling the bottle, moving the device location, etc) would create a huge variance in the data points. I will update the text to this for clarification. $\endgroup$
    – Jason
    Aug 1 '17 at 6:41
  • $\begingroup$ What was meant by my comment - think of you standing on your bathroom scales - do you stand still to get an accurate reading or do you dance a jig???? Perhaps you should consider a delay before taking a reading.... $\endgroup$
    – Solar Mike
    Aug 1 '17 at 7:12
  • $\begingroup$ How about changing the system and have a calibrated container and use a depth sensing circuit - still need a delay though... $\endgroup$
    – Solar Mike
    Aug 1 '17 at 7:14
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    $\begingroup$ What are the accuracy and precision required by your process? You need to start with what's required and design to that. Otherwise, you're just flinging poop at the walls and hoping enough sticks. $\endgroup$
    – DLS3141
    Aug 1 '17 at 12:58
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This is not an appropriate sensor for what you are trying to do. This type of flat pressure sensor is sensitive to localised pressures. For example, from a piece of dirt or an uneven surface. You should use a load cell which is designed for your application. For example, something like: Omega LCMKD-20N would be more suitable.

You can also consider this brief introduction to designing a weighing system.

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  • $\begingroup$ The LCMKD is more suitable, but also 40x more expensive... but I will look for alternatives in the 20 range. Thank you. $\endgroup$
    – Jason
    Aug 25 '17 at 7:53

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