Since most of what I work on is small (bikes) i seldom have to mix more then 30- 40g of resin. To dose properly I don’t want to buy an expensive scales that can accurately show 0.1 g intervals. Someone over on the R&G forum put together a table showing the number of drops from an eyedropper needed to reach a certain weight. I put this info in a printable table and thought it might be useful to others.
I’ve found quite cheap and accurate scales (along with much more) at Alibaba http://www.aliexpress.com/wholesale?SearchText=scales&catId=0
I bought a cheap accurate scale, but did not like it. It is good for weiging things, but for adding small amounts, it runs into an unforeseen problem:
For instance: I have 100 grams of resin (plus cup) and I want to add a very small amount of cobalt.
I zero the scale, to add for instance 0,2 gram of cobalt. I drop in a drop of cobalt. The scale jumps to 0,1 gram, then thinks “I need to auto-zero” and goes back to zero. I can drop in slowly what I want, but the scale keeps zero-ing. Very annoying. And very useless.
If you want to be sure your weight is correct, a scale with thousandths or 10s/thousandths is the way to go. Especially because tolerances are tight when producing a consistent resin.
Depends on the batch size. You are right when adding small amounts to small batches. However our lab scale is measuring in 0,01 gram increments, up to 5 kgs, which never proved a problem, for making 100 gram batches.
I am more used to making 100 kg batches, or even more. Then accuracy never should be a problem.
A trick to for instance add 0,03% of a substance to a resin:
Mix 3% of the substance into a small batch of resin. (say 100 grams / 3 grams) which is do-able on a scale with moderate accuracy. Now add 1% of that mixture into the resin you are promoting. Presto! High accuracy with crap equipment.
Cheap scales can be a pain. I have one that needs min. 10 g to show anything. The other one starts at 1 g but sometimes doesn’t show that until I have a good portion in. Then there’s the auto-off after 2 minutes. Shuts off right in the middle of adding hardner! Very annoying!
We had a set like that. Found out that they couldn’t handle the 3ton crane test weights. Such a shame. The shop attendants look at me weird when I stand there timing the display scale’s auto off.
Thank you for the table, it is very handy for small mixes. I always had problem with my cheap scale.
I use this cheap scale all the time for making small batches. It goes to .01 grams. I make 4 and 6 gram batches for alot of the small parts that I have to produce. I can get the hardener within .02 grams consistently. I wouldn’t use a scale that only goes to .1 grams for batches that are 100 grams or less.