Electronic scales are generally rated for accuracy as a percentage of actual
load rather than full scale, plus or minus a few counts in the least
significant digit. Its difficult to say what your accuracy will be without
knowing the specs for your scale.
I use an OHAUS scale, model LS200. This has a 200g maximum with 0.1g
resolution. The error is 0.75% of actual load or +/- 3 counts, whichever is
greater. This gives 0.75% accuracy down to about 30g, where 0.3g is 1%. The
0.1g resolution is appropriate since one drop is just about 0.1g. Cost of
the LS200 was about $90 US.
I bought a 1qt poly bottle with a flip top over a 1cm opening in the cover to
hold the resin. This allows pouring rapidly by squeezing the bottle to get
the approximate amount desired. Then I use a discarded mustard squeeze bottle
to add resin one drop at a time until an exact gram amount is reached.
The PTM&W hardner is lower viscosity than the resin so a squeeze bottle used
to dispense drinking water when participating in sports works fine. This has
a flipover spout which seals. It allows dispensing the hardner in a stream
with firm pressure or drop by drop with lesser pressure. Again, its
relatively easy to dispense to the desired weight within 0.1g.
It is important that the poly bottles seal well to avoid moisture pickup
between uses.
I used a spreadsheet to print a table giving resin quantities for each gram
---From 20 to 140 vs the total including hardner in the next column. Just pour
epoxy to the approximate desired weight, top up to the next exact gram, add
hardner to the nearest 0.1 gm as read from the table, and off you go. It
takes less than a minute per batch.
So, for a total outlay of about $3 for plastic bottles, one can dispense
epoxy fairly efficiently. The scale, unfortunately, is my most expensive tool
so far.
A side benefit of using an electronic scale is that it works well for the 5
minute epoxy too even though only a few grams are mixed at a time - the mix
ratio is less critical here. And, if I ever get to the fuselage, it should
work well for the Redux. Another plus is that you develop a feel for the
amount required in grams per square foot to be covered so you seldom mix much
more than is needed. Now if I could only figure a use for all the epoxy I've
saved....
John Moran, A044
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