Finding weight of an unknown mass

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Discussion Overview

The discussion centers around devising a method to determine the weight of an unknown mass using a self-contained device. The mass can range from 10g to 2000g, and the device must operate independently after the mass is placed on it. Participants explore various approaches, including mechanical balances and strain gauges, while considering constraints such as power supply and the incorporation of a mystery material.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes using a traditional balance with a known weight and a motor to adjust the known weight until the balance angle is less than 30 degrees, seeking guidance on calculating the unknown mass based on angle and distances.
  • Another participant suggests using a kitchen scale, but this is dismissed due to the requirement of creating a custom device.
  • A different participant recommends building a kitchen scale using a strain gauge, arguing it may simplify the process compared to the motorized balance approach.
  • One participant expresses curiosity about how strain gauges work, noting the challenge of incorporating a mystery material into the design, which complicates the simplicity of the strain gauge solution.
  • A technical explanation of strain gauges is provided, detailing their operation and integration into a Wheatstone bridge for measurement, highlighting their common use in kitchen scales.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to create the device. Multiple competing ideas are presented, including the use of a balance and a strain gauge, with differing opinions on their feasibility given the constraints.

Contextual Notes

The discussion includes limitations related to the requirement of using a mystery material, the need for the device to be battery-operated with a maximum of 12 volts, and the challenge of ensuring the device operates independently after the mass is placed.

gallimaufry
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The goal is to find the weight of an unknown mass by building some sort of device. The mass given can be anywhere between 10g and 2000g. Once the mass is given to you, the only thing you can do is place it on your device and that's it. You are not allowed to touch the device afterwords. The device must work on its own and do what it does, but the person is allowed to look at the device and do any calculations needed. For example, if you need to know a distance, then a meter stick would be mounted on the device so that you do not have to touch the device. The idea i came up with, is a simple old style balance with a fulcrum in the center and a known weight on one side and an unknown weight on the other side. The known weight will be moved by motor until the angle the balance is at is less then 30 degrees. There will be a sensor in the middle where the fulcrum is that will tell me the angle it is at on an lcd. I need to know how to be able to calculate the unknown mass when the balance is not balanced and the things i know are the known mass, the angle the balance is at, the distance from the fulcrum of the unknown mass, and the distance of the known mass. If you can explain how to solve for the mass, that would be great help. Thanks for the help.
 
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Why not just use a kitchen scale?
 
You cannot use any pre-made scales and the scale must also be able to run on batteries no more than 12 volts. Thanks for the reply!
 
OK, then I would recommend making your own kitchen scale using a strain gauge. That will be a lot easier than setting up a motor with a feedback loop and figuring out some way to measure the torque.
 
How do strain gauges work, this might be a good idea except i left out the trick to the challenge. When you show your device you will be given a mystery material that you will have to incorporate into the scale. So the more simple, the harder it is to incorporate a random material. I will look into the strain gauge and maybe even try it out. Thanks
 
gallimaufry said:
How do strain gauges work
A strain gauge is made of a material whose resistance changes as a function of its length. You attach the strain gauge to a cantilever beam, build it into a simple Wheatstone bridge, and amplify it. Most kitchen scales work this way. You can make it more accurate by periodically reversing the excitation voltage on the Wheatstone bridge and taking the difference between the forward and reverse voltages.
 

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