Units

[revchris]

Every fall we do an experiment in the undergrad lab class where we demonstrate superplasticity. This is done using a lead-tin alloy containing 62% tin. Last year we discovered that our alloy really is only 45% tin.

Doing the math today, the correction requires me to add one pound of new tin for every kilogram of existing alloy.

I'm not going to ask NASA for help, as I don't want to confuse the units issue any more than necessary.

In other news, I'm currently trying to use Ghost v14 to clone teaching lab computers. It's slow, as it's all going through the USB port on my office computer.

Comments

[misteropinion.livejournal.com]

Pounds per kilogram? Damn, I wanna see that page of ciphering!

[revchris.livejournal.com]

It's simple:

45 wieght percent tin means that for every 100g of original material, we have 45g of tin.

If we add 45g of tin to every 100g of original material, the composition shifts to (45g Sn + 45g Sn) / (100g base solder + 45g Sn) = 90g Sn / 145g total material = 62%.

One kilogram of original alloy (10 x 100g) requires 450g (10 x 45g) of tin.

454g is one pound, and if I add 454g of tin to 1000g of base material, I should end up with (904g)/(1454g) = 62.2% by weight of tin, and the 0.2% of tin is probably less error than I'm going to get from whatever material oxidizes during the melting and casting.