The Avogadro Constant And The Epoxy Putty (2018) | Hacker News Reader

The Avogadro Constant And The Epoxy Putty (2018)

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40 pointsoptimalsolver15 comments

Interesting to see the connection of moles and epoxy putty but wish this article was more in depth. The two parts of putties are typically different colors so one can visually tell when they're sufficiently mixed. It would be nice if epoxies in general (putty and liquid) data sheets had more information on their constituents. For example if an epoxy is 1:1 by volume and you have a scale available what would an equivalent 1:1 by mass/weight be? It's possibly to find out somewhat visually/by trial but would be nice if we could more easily calculate the mass/weight ratios

edit: fwiw in recent experience with 1:1s there's not much of a difference between equal weights/volumes but it could made a large difference on industrial scales

Some epoxy manufacturers make it pretty easy. For example, West System sells special hand pumps[1] for their resin and hardener, which attach directly to the product's cans and provide the right ratio for you--just use the same number of pumps from each product and you're good. Maybe a lot of manufacturers offer their own ratio pumps--I'm only really familiar with West.

1: https://www.westsystem.com/the-105-system/metering-pumps/300...

That's a good point. I've found the pumps to be handy for West System especially the 1:5 (or whatever odd ratios they have) but don't like how they are messy and tough to transport. Have had the hand pumps get pressed in the back of the truck and stuff and it can be a nuisance - but they can also be quite convenient when working as intended.

Those pumps are awful. If you get an air bubble in the line or need a small amount you risk messing up the ratio and having a poorly cured batch. Plus the hardeners react with the air so if its been sitting a little while the first part of your pump is bad hardener. Highly recommend everyone use a scale.

He says the 25-fold was the point of visual homogeneity but the putty didn't reach full strength until 60 folds.

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.

The reputable brands of liquid epoxy have data sheets included or available on their site with weight mixing ratios. The best way to mix is to use a paint scale (mettler toledo and sartorius make some) which lets you get both amounts precisely. They are made for automotive paints which require very precise ratio measurements (like 30:20:1).

Source: I mixed up about 20 gallons of various brands/ratios last year.

Estimating Avogadro's number to within 2 orders of magnitude from kneading putty seems very precise.

Avogadro's constant is just a band-aid arising from the arbitrary units we've chosen as the SI standard. There is nothing significant or magical about the particular numerical count. In an ideal world we'd all use natural units with Boltzmann constant rather than empirical Gas Constant, which removes the arbitrary "scaling factor" that is Avagadro's constant.

The practical benefit of Avogadro's constant is to manage error bounds.

The point is that we can measure the weight of atomic-scale objects very accurately in Daltons (1/12 the mass of a Carbon atom).

The weight of macroscopic objects can be measured very accurately in kilograms.

Avogadro's constant is the conversion factor between Daltons and grams, and currently we can't measure it as accurately.

So if you have an accurate weight measurement in kilograms, then you can lose significant accuracy by expressing the same weight in Daltons. Same for the other way around.

edit:

Huh, I did not know about the 2019 redefinition of Avogadro's constant, so this analysis might not stand:

https://en.wikipedia.org/wiki/Avogadro_constant#SI_redefinit...

Or at least there is still a conversion factor between atomic weight unit (Da or u) and grams, it's just no longer the Avogadro's constant?

edit2:

Hmm, interesting:

> In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram.[1][2] Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (kB), and the Avogadro constant (NA), respectively.

Anyway, accuracy was a major concern for retiring the old kg standard. This effectively means that now in SI macroscopic weight measured in kg has one-to-one correspondence to measurement in atomic weight units. But this development was quite recent.

https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...

interesting, i didn't realize that the redefinition meant that avogadro's constant was no longer the ratio between daltons and grams. unsurprisingly this is controversial: https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...

i think it's good to be specific about what you mean by 'very accurately' though; maybe i misunderstand but i think the uncertainty of measurement is about 10⁻¹⁰ in all three cases

if we were designing the metric system today instead of in 01795 we would probably define the unit of mass as something like 10²⁴ atoms of silicon-28 (about 47 grams)

> I didn't realize that the redefinition meant that avogadro's constant was no longer the ratio between daltons and grams

No, I was wrong about that. It's still the ratio, but now this ratio is defined as an exact number. Basically grams defined in terms of daltons.

i thought they defined grams in terms of an integer number of atoms of silicon-28, but the mass of a silicon-28 atom in daltons is not an exact number; it's an empirically measured number, because of the binding energy mass defect, and because the dalton is referenced to a different atom, the carbon-12 atom

but all in all it's very confusing

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