Epoxy Ratio Scale
by Joe Tribulato
Mixing epoxy must be done to with the correct amount of resin and hardener. Measuring may be done either by volume or by weight. I have tried several methods but my preferance is measuring by weight using a simple homemade balance beam. This tool gives consistantly accurate results by following a few simple steps.
I am using Systems 3 epoxy in this example. It requires a resin to hardener ratio of 2 to 1 by volume. Their manual states that the ratio by weight is 100 to 44. You will have to use the weight ratio for the systen you use. These are the numbers used for the balance beam. You may remember that the weight times distance from the pivot point on one side of the beam is equal to the weight times distance on the other side. This is the principle at work here.
To build the scale start with a 1/2 inch board 18 inches long and wide enough to accomodate your mixing cup. My daughter has a cup of yogert for breakfast each morning so that's what I use. Mark a line across the center. The base is 1/4 ply with a 1/16th inch metal strap bent in a U shape and drilled for small finish nails hammered into the center edges of the board. Fasten it to the base. Glue a stop on the left end of the board. Mark a bold line about an inch from the other end. Now the key step. Measure the distance from the center to the bold mark. Let's say it is 200 mm. This distance represents 144 units of resin and hardener combined. The distance for resin alone would therefor be 100/144 x 200, which equals 138.9 mm. Mark a second bold line that distance from the center. This represents the weight of the resin while the distance from here to farther mark is for the hardener.
Now that the scale is built here are the steps to use it. With the yogert cup against the end stop, zero balance the beam with a weight near the center. Nudge it until the cup end just begins to drop. Get several big washers for 1/2 inch bolts. Let's use just 3 of them for a gluing job. The hole will help to put them in the right place. Center them over the resin mark on the beam. Pour resin until the cup just drops. I bounce the beam gently to get a feel for when to stop. Then move the washers over the farther mark. Pour hardener, slowly so you don't pour too much. That's it. Mix and use. Add fillers if needed. The sketch below should make it all clear.
The scale in the photos is well used. It has helped me well in building 35 boats so far. I feel that this scale is better than pumps , graduated cups or any other system I have seen for accuracy and ease of use. It's cheap too. That appeals to me. I would like some feedback after you have used it.