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See if I could measure the correct weight with a "scale" made from a strain gauge.

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I did an experiment to see if the "scale" and the program I made the other day could measure weight correctly.

Here's the "scale" I made the other day 👇👇.

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Prepare weights

For the scale I made, the maximum measurement weight of the strain gauge is 500g, so I will prepare weights from 1g to 300g.

Use a 1 yen coin for 1 gram. For 20g or less, use a 1 yen coin as a weight, and for 30g or more, fill a PET bottle with water as a weight. For more than 200g, fill a beaker with water and use it as a weight.

The resolution of the TANITA scales for weighing water is 0.5g increments. Therefore, the weight of the water weight will have an error of about 0.5g.

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It seems that there are 0.1g units now, but since I bought it a long time ago, it is a scale with 0.5g increments.

Placement of the weights

In order to find out if the correct value can be measured no matter where the weight is placed on the scale, I decided to place the weight on the left, center, and right of the scale and measure the weight.

Start measurement

Start with one one-yen coin and increase to two, three, five, eight, ten, or twenty.

I also increase the amount of water in the plastic bottle to 30g, 50g, 80g, or 100g.

Finally, I filled the beakers with water and measured 200g and 300g.

Results

The result looked like this. The measurement is almost always correct no matter where the weight is placed, left, center, or right. Incidentally, the measurements of the scale always varied by about ±0.2g with no objects placed on it.

If I graph it, you can see that it is a nice straight line. Over 100g is lower than the correct value when the weights are placed on the left and right. This is because the top board and bottom board were in contact with each other as shown in the picture below.

天板の左端が底板と接触してしまっている

In the case of placing it in the center, this did not happen, so we were able to measure correct values up to 300g.

In addition, during the experiment, when I was placing and replacing weights, the error went from ±0.2g without weights to an offset of about ±2g in severe cases. Each time, I removed the offset and then measured the weight. Apparently, if the scale is moved even slightly by placing or removing an object, the state of the strain gauge will change and the offset of the change will be added to the measured value. I found out that strain gauge scales are quite sensitive.

To summarize what I found out.

  • From 1g to 300g can be measured with an accuracy of less than 1g error
  • When the top and bottom plates touch, the weight is measured as lighter than the actual weight
  • Measured values vary by about ±0.2g
  • If I touch the scale, it may have an offset of a few grams

In the next article, I would like to investigate the stability of the measurements over the long term.

Added on March 23, 2021 The rest of the article is here.

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