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Keith Hickling wrote:
"I expect that using digital scales would take longer, as you have to
accurately measure quantities of both resin and hardner, whereas with the
ratio balance you just pour roughly the amount of resin you want, then add
exactly the right amount of hardner. "
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I have always used the digital scale. It is an Ohaus, with a resolution of
0.1 gram and does not suffer hysteresis inaccuracies like some do. Absolute
accuracy is not important, since the accuracy error will factor out in a
ratio, as long as repeatability is good. No need to bother with using the
sample weights that come with the scale to make it accurate.
With the scale, you never need 2 cups to mix the resin back and forth, and
only have to carefully measure the hardener. I put resin pumps on the
containers of hardener and resin, and leave them there(they can be obtained
---From the local epoxy supply, such as Tap Plastics, or probably Aircraft
Spruce or Wicks), with a plastic bag over them when finished. Upon needing a
batch, just grab both out of the temperature-controlled storage area, pull
off the plastic bags, and start pumping. For example:
Epoxy:
Put a disposable cup onto the scale. Turn it on, and it will auto zero,
excluding the weight of the cup from the equation. Dump in whatever amount
of resin you feel like. It does not need to be measured, a wild-ass guess
will do, but write down the final amount of the weight of the resin.
Multiply this amount by 0.27 (Aeropoxy), and you have your hardener amount
to add. Add that number back to the initial amount of resin measured, and
you get the total amount of resin plus hardener needed. Simply pump hardener
into the same cup until you get close to the final amount, then go drop by
drop until you nail the reading to a tenth of a gram (resolution of the
scale), and you got it! Its very easy and takes almost no time, and no need
to mess around with pouring between the resin cup and hardener cup to use
all of the material -just keep a calculator handy, and a pencil to tap the
numbers to keep the calculator clean. Mix until you see a homogenous
mixture, and you are done. It took a little while to explain, but it goes
really fast. As a final check, keep a mixture lookup chart nearby and right
before adding the hardener, verify that the calculated hardener amount to
add is close to proper value.
Araldite:
Araldite won't pour, and needs to be scooped out with a Popsicle stick. It
will adhere grossly to the stick. Hence in this case, put the stick and the
cup on the scale before it zeros the value, to cancel out the weight of the
popsicle stick too. Make sure it is always in the cup during measurement.
Scoop out the araldite needed, and put the scooped amount along with the
stick, into the cup. The measurement reading taken will be for the araldite
only, since the stick was cancelled out. Make the calculation (multiply by
0.4, similar to the epoxy, above) Then pour in the hardener from a
ketchup-like bottle which has the ability to dispense from stream to drops,
depending on squeeze pressure (I purchased one from the local epoxy shop,
and it is similar). Verify calculated amount with the araldite mixture
lookup sheet (found on one of the web sites) to verify that the calculated
amount is close, then mix.
In the build manual, I write the resin amount measured, and hardener amount
to be mixed, along with the total amount of the two, which is the important
number, since you will add hardener until the total amount is achieved, then
write how close I actually got to the calculated total amount. Almost
always, the final resin + hardener measurement (actual) is the same as what
is calculated.
It is amazing how accurate the ratio scale can be if it is built properly,
like using the fishing line technique that Fred described. And it is
cheaper. For me though, the $100.00 digital scale which you can sometimes
find for 35 or 40 works really well, is fast, and if carefully done and
verified, will always make the proper ratio.
Have fun,
~Greg Fuchs
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