Pretty Good Postal Scale from Old CDs

Step 8 Calibration

Calibration marks can be made with a piece of frosted tape and a fine point marking pen.

I did an Internet search for the weight of a US Quarter ($0.25 piece). Those made since 1967 with the copper core weigh 5.67 grams each. I used a metric to English conversion calculator. In ounces that is 0.20003 ounce each. That means five post-1967 US Quarters weigh exactly one ounce. (No one at the Post Office will worry much about anything after four decimal places.)

A plastic Baggie for a sandwich has a negligible weight. I attached a Baggie to the clothespin and put five US Quarters into it. When the scale came to rest, I made a mark for 1 ounce at the point where the hanger wire for the clothespin crossed the frosted tape. It is the leftmost mark of the three you see. Then I added five more Quarters for 2 ounces. It is the second mark. The third mark is for fifteen Quarters or 3 ounces. You can add "1", "2", and "3" to your marks to avoid confusion later about their meaning.

The marks appear to be a bit to the side of the wire hanger/indicator, but they really are not. That is because I did not take the photo straight on to the CD. I wanted to avoid reflections that would make the photo less useful.

The more carefully you make and read your calibration marks, the more accurate your scale will be.

Comments

[Utahtabby]

why couldn't you just use a wooden ruler, lay it across a battery and put the 5 quarters in a stack at one end, like a see-saw / teeter-totter, and put the letter or object at the other end?

[Phil B]

You could do that, but then it would not be a new use for old CDs, nor would it be an adaptation of the design I linked in the Introduction. What you suggest could work in theory, but the ruler would not be fixed in a stable way, and that introduces a high risk of sudden error. Also, where do you balance or hang the mail while weighing it? I think what I have suggested is eminently more practical than a ruler on a battery. I hope you will soon publish your first Instructable.

[wizodd]

If you put a third hole on the other side of the support point, and hang a pointer wire, you can arrange it so that you have a much larger movement per unit. Note, the scale will be non-linear, and will be more accurate at the heavy end.

[Phil B]

Good observation. Thanks. Quite a few variations would be possible. For example, instead of wire for the handle and for the clothespin hanger, string or fish line could be used. Your suggestion would make the scale a closer facsimile to the original by Arvind Gupta that I linked in Step 1. One of my goals was to get double duty out of as many components as possible and keep the number of parts to an absolute minimum. As it is now, the range of movement between unit marks is about the same, perhaps a little more than what you would find on a commercial spring-operated postage scale.

[ATL94]

Are you geeks or what?

[Phil B]

I did not think I was that geeky. I have never thought I knew enough technical stuff to be a geek. I know enough to get by. I do have geekier things than this posted on Instructables. See http://www.instructables.com/id/Fix_a_non_working_electronic_fly_swatter/ I went onto the Internet looking for the answer to a problem and had to develop it myself.

Thanks for the compliment.

[ATL94]

Sorry I was bitchy that day don't take it too offensively... Sorry.

[Phil B]

I took it as a compliment, as if you thought I might know something useful. I hope you are feeling fine now.

[ATL94]

yeah I'm ok now and agian I'm sorry. It's cool that you want to know about technical stuff, my dad is a Machanic so I grew up around this stuff all my life.

[wizodd]

My favorite postage scale is the one made of a single piece of steel kinda 'L' shaped (well, squashed backwards 'C' with a pointer hanging from the suspensionpoint and the scal reading right to left. There is another, similar design with a 'button' weight which is about as good for letters, but not for weighing anything else (anything else bumps into the weight and throws off the readings.)

[Phil B]

I used to see what you describe in several stores. I have not seen them for quite a while. I wanted something that could be used easily without the letter or parcel coming into contact with part of the scale and skewing the reading. In your mind's eye you can easily impose the "L" or backwards "C" you describe onto the CDs. Although the CDs are actually a circle, the effect is the same.

[wizodd]

Right. I did find someone a couple years ago that was selling them online, and I should have a couple around somewhere.... The cutout shape was designed to move the weight to the outer edge to get enough movement on a scale that's only 3-4" long.

[Phil B]

I also saw some on-line. The price was pretty good, but the shipping and handling was several times more than the purchase price.

[baxterdog]

Balmuge means 5 grams U.S. Where 28.35 grams = 1 ounce.

[Phil B]

Regardless. No whole number of nickels weighed together equals one ounce, or two ounces, or three ounces, etc. The US Post Office wants to know if your letter or parcel is greater or lesser than one ounce, two ounces, three ounces, etc. Nickels are useless for anything but a very rough guess. Meanwhile, five quarters equal exactly one ounce. Multiples of five quarters are ideally suited for calibrations of a scale for US postage. Nickels are not.

[baxterdog]

Fair enough, good argument.

[balmuge]

a nickel weighs exactly 5 grams, so you can use that as well for some of your markings

[Phil B]

See my response to Swishercutter below. If you live in the US, you need a scale that reads in ounces and a Quarter is exactly one-fifth of an ounce. It is nice that a US Nickel weighs exactly five grams, but not at all helpful if your final measurements are in ounces.