Hi Jez,
Very intresting - especially as I decided to purchase the My-weigh i500 with
500g capacity and .1g resoution, but its out of stock at right on scales
http://www.rightonscales.co.uk/i500.htm, probably cos its 20 cheaper here
at 55 than elsewhere .
So therefore are you inferring that 0.1g is too low a resolution or
specifically the mechanism of the scales is at fault, i.e some scales react
this way whilst others do not??
Don't tell me to be assured of the correct accuracy, a balance with a lot
higher resolution at a lot more cost is required...
Cheers
Paul..
-----Original Message-----
From: Jeremy Davey [mailto:jeremycrdavey@btinternet.com]
Subject: Europa-List: Faulty scales
<jeremycrdavey@btinternet.com>
You may remember a thread some weeks ago started by Ron Parigoris about
problems with digital scales that ignored any very small, very slow
increments in weight? His test involved weighing a piece of Letter paper,
then tearing it up and reweighing it, putting the pieces on very slowly.
After a long search for a piece of Letter-sized paper to undertake the test
(we use A4 over here) I tried it and, to my surprise, my scales failed!
Now this was potentially a serious problem. I measure Ampreg 20 by weighing
a pre-decided weight of resin, then adding hardener with a syringe, slowing
to a drop at a time at the end. I'd often found that the last 0.5g or so (my
scales have 1 decimal place) took a lot more drops of hardener than
expected. Now I knew why!
Fortunately for me I had discovered a few days previously that a friend in
the village is a representative for a well-known laboratory scales company.
I'd had problems reading my scales from some angles ever since I tried to
wipe off some drops of resin/hardener with an acetone-soaked piece of paper
towel (D'oh!!) and got a semi-opaque screen. He'd offered me a redundant
sample from his large stock.
With my new scales, the sort that have a spirit bubble and are accurate to
0.001g, I found that a drop of hardener weighs 0.03 to 0.05g. So what was
the worst-case scenario? My best guess was that in the worst cases I'd added
10 extra drops, making the mix with 0.5g excess hardener at most. The
smallest mixes I'd made were 40.0g resin + 10.0g hardener in order to keep
the proportions within the accuracy (!!) of the scales. So these cases could
be up to 5% excess hardener.
I checked with a number of people for an opinion: my inspector, the Europa
Factory, Dr. Bill Brooks of Scrapheap Challenge and Pegasus Aviation fame (a
top bloke!). No-one knew for sure if I'd have a problem - and so far I'd
done the tail, ailerons, flaps, XS wings and Cockpit module. I didn't want
to ask SP Systems because I feared the litigation-aware response of 'if the
ratio is wrong, you must redo everything'.
Bill's suggestion was to call SP Systems, and he did have a point, so I rang
Martin Armstrong in their product support (who I knew was excellent from
previous experience with another question). To cut a long story short,
Martin suggested the eminently practical idea of making up a deliberately
worst-case-scenario batch of epoxy using my old scales and usual technique,
cure it for three days, then send it to them for testing.
The results were most interesting, and I'm in the clear - the sample was
strong enough (PLEASE DO NOT TREAT THIS AS AN EXCUSE TO BE LESS METICULOUS
IN YOUR EPOXY RATIOS!!). Strangely enough the sample gave indications of
being cured at 50C - I hadn't done this (I heat the workshop to a consistent
22-24C) so I assume the striking postal workers must have left it over their
brazier for a few days.
I wanted to tell this story in order to encourage people to check their
scales for the issue, and also to publicly thank SP Systems for the high
quality of their product support. I was a very worried chap for a while.
Once again I am glad I chose the kit I did, and that the various suppliers
of parts are fully behind their products.
Regards,
Jeremy
Jeremy Davey
Europa Monowheel 537M G-EZZA
Tail done
Standard XS wings awaiting mods and closing
CM ready for installation in fuse (with airbrakes fittings)
1100 build hours to date
Intended fit:
Rotax 914 turbo, Airmaster CS fully-feathering prop
Lots of lights, buttons, switches, gizmos, and alarms
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