Comment by jrapdx3 | Hacker News Reader

Comment by jrapdx3 | Hacker News Reader

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Having mixed, and used, quite a bit of 2-part epoxies, I concur that measuring by weight is far more satisfactory vs. attempting to measure volumetrically. Most industrial epoxies are mixed at ratios of 2:1 to 5:1 (resin/hardener). The 1:1 varieties are mainly consumer products. As a rule if the 1:1 ratio is a bit off it doesn't noticeably affect results.

Some manufacturers/suppliers provide volumes and weights for proper mixing. For example, West System instructs mixing 1 quart/2.4 lb of their "105" resin with .43 pint/.47 lb of "205" hardener. (Ratios: by volume: ~4.6, by weight: ~5.1) Of course the savvy user can mix smaller/larger quantities proportionately.

In some cases measurement by weight isn't mentioned by the producer. However it can be inferred from product density. The info is usually given in the product's SDS. The "205" hardener SDS states it weighs 8.85 lb/gal, whereas "105" resin is 9.6 lb/gal. With this info on hand, it's straightforward to compute the required quantities of the components.

Manufacturer of epoxies explicitly advise not to alter the mix ratios. IMO measuring by weight is the easy way to mix epoxy accurately.

i concur, though you seem to have more experience with epoxies than i do

it's easy and cheap to get an electronic strain-gauge balance that measures weights to ±0.05%, though weighing things out that precisely is quite challenging, especially if they are viscous liquids. (note that the rounding error in the densities you give is probably ±½%)

but measuring small amounts of viscous liquids by volume i think you'll probably get errors more like ±10%

powders, like for cooking or for resin fillers, are even less precise to measure by volume

> "...though weighing things out that precisely is quite challenging, especially if they are viscous liquids."

The "error tolerance" in mixing epoxy is probably at least 1% so inexpensive scales are quite adequate. The "secret" to mixing epoxy is measuring the resin and hardener in separate containers, then combining by pouring the hardener into the resin. I also put some of the mixture back into the container used to measure the hardener, mix thoroughly and add it to the main portion of mixed epoxy. This procedure assures all of the hardener is mixed with all of the resin.

BTW I use my kitchen scale every day. As you say, measuring powders, like flour, by volume isn't reliable. Then again, liquids, even water, are easier to weigh out. Besides, measuring cups aren't very accurate. Then again being only as precise as necessary for the job is what efficiency is about.

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