Posted for "Paul Davis" <pdavis@bmc.com>:

Several folks (Marv and Don come to mind) have recommended scales in lieu of epoxy pumps.  Appeals to me, so I've been shopping.

Marv seems satisfied with the Ohaus Scout (300G capacity with 0.1 resolution).

I'm also looking at the Ohaus CS200 (200G capacity with 0.1G resolution and 0.1G accuracy (for $68.00).

I think most of these scales do "tare to capacity by subtraction"
which I understand to mean you can tare up to the capacity of the
scale in multiple steps.  So I could put an empty container on the
scale, tare to 0, weigh resin, tare to 0, weigh hardener, all without
removing the container from the scale -- so long as the total of
container, resin and hardener is less than the capacity of the scale.
Is that the way "tare to capacity by subtraction" works?

We have the fast-build Legacy and did the one week workshop so most
of the big volume steps are complete.  And of course we could always just measure several cups individually and then mix.

My question is what capacity scale do we really need?
My instinct is to go with "just big enough" on the assumption that I'll get best accuracy.  Any thoughts, recommendation or regrets?

-------------------
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A great man can come from a hut
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(Just a couple things... first of all, the easiest method I've found for measuring the epoxies is to put the container on the scale, zero out the tare, put in the required amount of resin, multiply its weight by 1.22 and add catalyst until you reach the number on the calculator.  For Hysol, do the same thing but multiply the resin weight by 1.445.  Couldn't be simpler.

If you do decide to purchase a scale, tape some clear plastic (pieces of baggies work great) over the control panel and wrap the platform with plastic as well.  No sense gooping up a nice piece of equipment.

While we're on the subject of mixing epoxy, just a few other important things to keep in mind... don't ever forget that mixing the two components together starts a chemical reaction, and if you mix a large quantity and leave it in a compact mass it's going to exotherm...  think smoke <g>.  Any time you mix more than an ounce or two you should try to transfer it to a larger container so it spreads out into a thin layer.  The folks at Jeffco Products suggest using a tinfoil lined cookie sheet, and that works for me.  You won't be mixing lots of large quantities, thankfully, but when the time comes don't forget about this.

Trying to figure out how much epoxy to mix for a given layup can also be problematic... when in doubt, weigh the glass or carbon fiber you're going to lay up and then mix a similar amount of epoxy.  They say that the ideal ratio of reinforcement to resin is about 60:40, but that's hard to achieve unless you're vacuum bagging.  Using as much epoxy as reinforcement (in our rather crudely controlled layups) insures that you get a good wet out, and you can draw out the excess by stippling through a paper towel before applying the peel ply.  Additionally, don't draw further resin out of the layup into the peel ply... stipple it down with some additional resin on top so you don't starve the layup.

I really enjoy working with composites, as the possibilities are so great. Try several different methods of doing layups and use whichever works best for you.  As in most things, practice makes perfect.       <Marv>          ]