Actually, some of us actually do cut off the ends of the stir sticks so they
will be flat--with the trusty band saw--just to avoid to "missed stirring
opportunities" you mention. It's easy to do 10-20 of them at a time, and then
you have one round end and one flat end to use as one might wish. The
underlying point however of preferring to have 0.1g accuracy is well taken I
think. On small epoxy mixes (which many many will be), one needs reasonably
fine accuracy (or good resolution anyway) to be able to get them mixed to
within 1-2% accuracy, or even much better. And, if you do happen to overrun a
target mix by just a gram or two, with either resin or hardener, it is easy
enough to add the very small "make-up" amount IF you can measure to 0.1g. So
I'm satisfied with my Ohaus Scout SC4010 for those reasons...
Jim Graham
A-101 (still on horizontal tail)
GERAINT L OWENS wrote:
> > On the other hand, larger batches of 40g hardener / 160g resin should be
> within 1.5% of
> >the theoretical ratio.
>
> This is all very well Rowland, and I would believe it if you were then going
> to use a mechanised mixer for 5 mins to mix the goo; but given that most of
> us mix with a round-tipped spatula on a flat-bottomed container (when the
> spatula can't get into the 90 degree corner) for one to two minutes at most,
> I suspect a lot of your perceived accuracy of mix goes right out of the
> window. It would be interesting to do a scientific analysis of the mix
> from different parts of the pot, for a number of homebuilders. I suspect
> the results might be a little different from the manufacturers theoretical
> ratio. IMHO : -)
> Lloyd Owens
>
> -----Original Message-----
> From: Rowland & Wilma Carson <rowil@clara.net>
> Date: 17 November 1999 19:33
> Subject: electronic scales [was: NEW BUILDER - SCOTLAND]
>
> >
> >> 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>
> >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 ....
>
> >etc....
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