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Improve scale accuracy.

Posted: Tue Dec 30, 2008 9:17 am
by OnkaBob
Had trouble with mixing small batches for butt joints today so made an accessory for my scale to double its accuracy.
This is basically just a beam that gives a 2 to 1 mechanical advantage to whatever is being weighed. Applied to a 1g resolution electronic balance it will give effectively 0.5 g resolution.

This was knocked up out of scraps and tape to try out but will be progressively turned into a permanent gadget. (Glued up the fulcrum tonight with leftover epoxy).

Important points:
1) the beam has a groove and the fulcrum has a knife edge that fits into the groove to maintain the 2:1 ratio.

2) The tape at the left hand end is only to hold that end down and should be vertical. There should be enough tape to allow some lateral movement.
3) Cup must be centred over the 2:1 position which is the same distance from the fulcrum as the taped end. I cut out some tape to locate the cup as in the pic.
4) The beam must be very close to level to avoid effects of gravity that may cause weighing errors.
5) Fulcrum position on the balance plate is not critical - centred by eye will be fine.
6) Place cup on beam before zeroing the balance.

If all is set up as it should be then anything weighed on the beam will show double the actual weight. I found this enabled me to measure 6 gram batches much more consistently.

If any of this doesn't make sense it may be because I'm tired and falling asleep on the keyboard.

Re: Improve scale accuracy.

Posted: Thu Jan 01, 2009 12:12 am
by Kayak Jack
Devious buggar, ehh what?

Re: Improve scale accuracy.

Posted: Sat Jan 03, 2009 12:00 am
by hairymick
Very clever mate. I just use a plastic spoon :oops:

Re: Improve scale accuracy.

Posted: Sun Jan 04, 2009 10:16 pm
by OnkaBob
Thank you very much for that alternative Mr Mick :roll: .

I worked in an industrial laboratory for 25 years using precise burettes and volumetric flasks etc so I consider that sufficient excuse for being a geek :lol: .

I'm using a 5:1 epoxy system and suspect this may be a little less forgiving than a 3:1 or 2:1. Not sure how much would stick to the spoons but it is something I would worry about. Also can use this set up and the same mixing cups for all size batches.

Re: Improve scale accuracy.

Posted: Mon Jan 05, 2009 5:51 am
by _AL_
So is the weight of Epoxy hardener & resin the same? ie, does the mix ratio remain the same between measuring by weight or by volume?
I've been looking for ways to reduce waste such as measuring cups etc & this may fit the bill.


Re: Improve scale accuracy.

Posted: Mon Jan 05, 2009 6:54 am
by hairymick
G'day Bob, :D

I'm just a rough old booger :lol: but I use the West Systems 3 to 1 stuff and never had a problem. While the manufacturers of resins might harp on and on about the necessity for accuracy, I think that once the chemical reaction kicks in a few percent either way will make little difference.

I use a fresh spoon for each mix and use that sppon to stir it.

Likewise, I use a small plastic cup (30ml capacity) for measuring bigger quantities. about 100ml of the mixed stuff is about as much as I can manage at a time and have used the same cup throughout this build without a problem. :D

for me, this is about my pinnacle of accuracy. :lol:

Re: Improve scale accuracy.

Posted: Mon Jan 05, 2009 9:05 pm
by OnkaBob
Mick, I know what you mean and ultimately it's a case of "horses for courses" 8) .

Al, you will probably find that the specific gravity of the hardener is slightly different to that of the resin but it is usually only a few percent so volume and weight end up being accurate enough in most cases. If you check you tech or safety data sheets the SG should be on there somewhere.

One exception to this is West 207. West quote SG for 105 resin as 1.15 and for 207 hardener as 1.00. The data sheet says to use 3:1 by volume or 3.5:1 by weight. So in this case it makes a difference but for the other West hardeners (205, 206) it says 5:1 by volume or weight even though the SG is almost identical.

The Bote Cote MSDS only has a single figure of 1.12 for the complete system (ie resin/hardener together). You might need to check their tech data sheet or more info if using that.

The SG figures for the brand I use are 0.97 for resin and 1.00 for hardener.

Re: Improve scale accuracy.

Posted: Mon Jan 05, 2009 9:41 pm
by Oldsparkey
All I use are the measuring pumps on the two containers and for some reason I have managed to make 9 boats using them.

WHERE did I go wrong. What did I do wrong , How did that happen ........ Please do not tell me since they still work for me. I guess it is just me , since I like the K.I.S.S. idea ( the Keep it simple stupid) one.

Anyway as long as it works for me making boats , I will keep doing it. After all making wood boats is suppose to be fun and relaxing , not complicated. :roll:

One from this one and two from that one for a batch , Half a one from this one and only one from that one for a half batch .... Or a Two spoons from this one and one spoon from that one for touch up work. :wink:

I appreciate all this advance knowledge but just being a simple person ... I will keep doing it the simple and easy way since the boats I make float.


The above is just my humble idea and mine alone , no one else and I do not suggest that you do it , what works for me might not work for you. I only know what works for me since no one else makes my boats. After all this is not Rocket Science , anyway for me. :D


Re: Improve scale accuracy.

Posted: Tue Jan 06, 2009 1:57 am
by hairymick

Ya need more beer mate :P

Re: Improve scale accuracy.

Posted: Tue Jan 06, 2009 5:35 pm
by Oldsparkey
hairymick wrote:Chuck,

Ya need more beer mate :P
Sure do ... might have to and get one.

I was only trying to point out a simpler way of doing it since I am a simple person. That can be translated into an easy word.......... LAZY.
The easier it is to make a boat , then the enjoyment of doing it increases. Life creates to many problems for us so something simpler ( or easier ) makes it better , anyway for me.