> 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 ....
>
>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' ...
>
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