A Spring Scale

For quite a change from the normal pace of things arround here, I’ve decided it is finally time to add the “engineering” to this blog.

A home-made spring scale.

I’ve been wanting to weigh a book I have that I am considering selling so that I can calculate what shipping would be.  However, I didn’t have a scale to weigh the book in and I wasn’t willing so spend $15-20 on a scale right now.  So what does a guy with an electrical engineering degree do?  Build my own scale.

Well, spring scales are based on a physics principle called Hooke’s Law, which basically states that the force that a spring exerts is linearly proportional to the distance it has been pulled or pushed.  I had a spring that was used to hold the greenhouse door closed that has just been sitting arround after the door broke in high winds.  So, I took the spring, the eyelet screw attached to the spring, one of several five-gallon buckets I have, and some twine and made myself a scale.

The math I tried to use to calibrate the scale. Pretty much all of it is wrong.

To use this scale, I had to calibrate it against a known weight.  For that, I used two gallon jugs of water.  A gallon of water is 8.35 pounds and using two of them gives me two data points from which I can get a linear equation that will give weight from spring displacement.

In the end, I got the book weight, which is just over three pounds.  And the scale didn’t require anything more that what I had. I should be able to use this when I finally have a harvest to give me some idea of how much I get from the harvest.


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