Make a Balance Scale Accurate to 1/2 Gram, Using a Ruler and 2 Old CDs




Introduction: Make a Balance Scale Accurate to 1/2 Gram, Using a Ruler and 2 Old CDs

This is my first instructable.  The reason I made this scale is that I am a photographer and I mix my own developers using household compounds like instant coffee ( Caffinol---a future instructable) and I need to measure quantities as small as .5 grams.  The electronic kitchen scales are not accurate down to that level, so I decided to see if I could make a balance scale that would do the job.

Step 1: Materials and Tools

You will need a plastic ruler, 2 old scratched CDs (new would do but why waste them),  6  6" fishing leaders with snap ends,  2 small pieces of plywood,  long finish nails and some wood screws.  You will also need a drill with an assortment of bits, a pencil and a US Nickel.

Step 2: The Ruler:

The ruler is the beam for the scale and must be as symmetric as possible.  The ruler I used was a plastic Wescott with a raised thumb hold ridge down the length of it.  It has a hole at one end to hang on a hook.  Pick a drill bit of the same diameter and drill an identical hole at the opposite end.  then drill a small hole next to both of the large holes. The small holes are for hanging the discs.    Next pick a drill bit that is slightly larger than the finish nail, find the mid point on the ruler, on this ruler it is right at the 6 inch mark.  Drill a hole through the raised holder, as close to the flat surface of the ruler as possible.

Step 3: The CDs

Stack both CDs evenly and drill 3 evenly spaced holes near the edge of the discs and clip 3 fishing leaders to each disc.  Take two paper clips and thread  each set of leaders onto them ( I did not have any paper clips so I used a finish nail bent into the shape of an elongated "C") and hang them on from the holes at the ends of the ruler.

Step 4: The Base

Take the vertical piece of plywood and measure and mark registration lines from the bottom up. It doesn’t matter how many lines you make…..I made 13 and then filled in the space of the center ones. This will be the balance  reference point.  Mine are at ¼ inch intervals.  Then mark a center line vertically.  Measure the distance from the cd to the ruler as it hangs.  Then measure that distance up from the shaded balance reference point, and make a mark on the centerline.

Next attach the vertical piece to the base piece with three screws through the base.

Step 5: Putting It Together

Insert a long finish nail through the center hole on your ruler and drive it into the backboard at the mark you just made.   You will want to do this with  the thumb hold on the ruler facing down .

Now if you measured and drilled everything perfectly, the ruler scale should be  hanging level with the CDs  both in the shaded area…just like mine isn’t.  Not to worry…..take a 1/4-20 nut and place it in the groove of the thumb hold of the high side and move it left to right until the CDs are both in the shaded area.  Then once the scale is balanced put a drop of super glue to secure the nut in place.

Step 6: Calibration

Now that the scale is balanced how do you know how much something weighs?    According to the U.S. Mint all modern U.S. nickels weigh exactly 5 grams, and U.S. pennies weigh 2.5 grams.  So that is the starting point.  Now you need to find something that is small and identical.  What I used here is finish nails for a nail gun….they came glued in a strip and can be separated by inserting a screw driver tip between the nails.  I was hoping to get a number of nails that was a factor of five to be able to have a 1 gram or 1/2 gram counter- weight.  But 10 nails were a tad too light so 11 nails made it balance almost exact.  So if you  divide 5 grams by 11, each nail weighs .45 grams. One nail is enough to tip the scale all the way down when the other side is empty.    I have since found small split shot weights for fishing that weigh exactly .5 grams each, so that is what I use for measuring small quantities.
For example if I have to weigh 1.5 grams of potassium chloride, i would put 3 1/2 gram split shot on one disc and add the potassium chloride to the other until the two sides are balanced.

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