> digital postal scales, battery powered.
>Resolution is 1 gm which is plenty fine enough for Europa epoxy work.
That may be an over-optimistic assumption.
First, resolution is not the same as accuracy. To determine the
suitability of any electronic scales for resin proportioning you need
to know the amount of uncertainty associated with any displayed
value. In the manufacturer's specification, this may be stated in
terms of absolute or percentage accuracy, or by reference to
linearity and repeatability.
Earlier this year an SP Systems representative (supplier of the
Ampreg 20 resin used in the UK-sourced kits) told us that for Ampreg
20, acceptable strength is obtained with mix ratios between 101:24.5
(4.122:1) and 99:25.5 (3.882:1) by weight. Thus for optimum strength
the ratio should be within 3.06% (resin-heavy) or 2.94%
(hardener-heavy) of the theoretical value.
Assume 1-gram resolution scales are used to mix a nominal 125gram
batch of epoxy. If the scales are absolutely accurate (most unlikely)
the hardener displayed as 25g could be anything between 24.5g and
25.5g, and the "100g" resin 99.5g to 100.5g. This gives worst-case
mix ratios of 3.902:1 and 4.102:1 - both within the acceptable range.
Then we look at the scale manufacturer's specification sheet and find
that the accuracy is quoted as (say) +/- 1% or +/- 1 count, whichever
is greater. So any weighing up to 100g can be in error by up to 1g.
Cheap postal scales may have an even slacker spec than this.
So in reality the hardener portion displayed as 25g could be anything
---From 23.5g to 26.5g, and the "100g" resin could be 98.5g to 101.5g,
giving worst-case mix ratios of 3.717:1 to 4.319:1 - no longer in the
optimum range.
Of course, if you use the same dispensing technique for both
components (eg stop adding drops immediately the last digit changes
to the desired value), you are much less likely to get ratios at the
extreme ends of the range. And the linearity of your balance may be
such that errors are all in the same sense and therefore can be
neglected. In fact, if the scale is truly linear, and the
repeatability is good, then the absolute accuracy hardly matters! But
these are areas not usually well-documented by manufacturers at this
end of the market, so we are back to the accuracy specification.
My own choice was the Ohaus LS200 (200g by 0.1g for under 100 pounds
UK). The accuracy specification (+/- .75% or +/- 3 counts, whichever
greater) allows me to mix epoxy batches using as little as 20g
hardener / 80g resin and be confident they are within 2.5% of the
theoretical ratio. If I went down to 10g hardener / 40g resin then
the ratio could be in error by up to 4%, which I'd prefer not to risk
- and I may not need such small batches anyway. On the other hand,
larger batches of 40g hardener / 160g resin should be within 1.5% of
the theoretical ratio.
Some of you may now have no doubts about why, on a team-building
course in my former existence, the assessments of my personal
characteristics included the term "pernickety". But I console myself
that I've never heard anyone complain about me being too pernickety
as I pre-flight the rented aeroplane they are about to accompany me
in as a passenger ....
Having said all that about balances, I may finish up getting a resin
pump as well, after having enjoyed the convenience of one on the
Europa factory Weekend Workshop course.
Hope this helps.
Rowland
... that's Rowland with a 'w' ...
|