sutherla@qtp.ufl.edu (scott sutherland) (08/03/89)
Charles Brown writes that, in order to determine the amount of lubricant on a platter, do the following: 1) Weigh the disk. 2) Spread the lubricant. 3) Weigh the disk again. 4) Leave the rest as an exercise (with a smilie :)). Okay, let's look at this: WARNING, THIS IS AN EXCERSIZE IN RELATIVE ORDERS OF MAGNITUDES. HIT N IF YOU COULD CARE LESS. (PROBABLY 99% OF YOU) ;^) This is a back of the envelope (literally, I used my paycheck envelope) caculation to get a feel for what one is in for using the above steps to figure out how much lubricant is on a disk. Assumptions: Disk is 130 mm in diameter (~ 5.25"). Layer is 5 molecules thick. Molecule is a fits in a cube of volume 1 cubic nm. Molecular weight is 1000 g/mole. 1) The area of the plate is pi*r*r, which is ~ 500 square cm which is 500e14 square nm. 2) The volume of the layer is the area times the thickness, which is 500e14 times 5 molecules (or 5 nm) which is 25e16 cubic nm. 3) Since 1 molecular volume is 1 cubic nm, there are 25e16 molecules. 4) The number of moles of molecule is 25e16/6e23 (dividing by Avogadro's number), which is ~4e-7 moles. 5) The mass of the layer is simply the M.W. (in g/mole) times the number of moles (from #4), giving 4e-4 g, or 0.4 mg. 6) Let the weight of the platter be ~ 3 oz, or ~ 400 g. (I don't have any idea if this is a reasonable guess for the platter weight.) 7) Then the weight of the platter without lubricant is 400.0000g and the weight of the platter with lubricant is 400.0004g. Thus you are trying to measure one part in one million by using the difference between two large numbers. This is a statistical nightmare! (Any statisticians care to comment?). If the error in your scale is 0.0002g, you have a 50% error in your measurement. NOT GOOD. So, this ain't as easy as you might have first guessed. Please note that I am in an analytical chemistry lab, so this type of calculation haunts me in my dreams. I couldn't resist posting this. It was a compulsion. ;^) Scott Sutherland "More lasers, more fun!" "Orders of magnitude are my life!"
ez004560@vega.ucdavis.edu (0040;0000009034;0;250;143;) (08/03/89)
One hundred apologies for adding to this stupidity, but: In article <625@orange6.qtp.ufl.edu> sutherla@orange6 (scott sutherland) writes: > > >7) Then the weight of the platter without lubricant is 400.0000g and > the weight of the platter with lubricant is 400.0004g. Thus you > are trying to measure one part in one million by using the > difference between two large numbers. This is a statistical > nightmare! (Any statisticians care to comment?). If the error > in your scale is 0.0002g, you have a 50% error in your > measurement. NOT GOOD. > ... > >"Orders of magnitude are my life!" > Attach a helium balloon with a 400g lift (or a weight in the atmosphere of -400g.) attach this to the platter before and after adding the lubricant. hopefully this will reduce your error to less than 50% ------------------------------------------------------ Quantum _\/_ 545 Sycamore CellBlock 207 Bruce (6502 RULES!) Rogers |\ Duck ( 0 0) Davis, Ca 95616 Quantum Duck Software, |\ \______/ / \\\ 916-757-2925 ez004560@vega.ucdavis.edu |\ < < | \/ "Cool as a body on ice, hotter than the rolling dice." \________/ Quark! ------------------------------------------------------ Quantum _\/_ 919 Drake CellBlock 146 Bruce (6502 RULES!) Rogers |\ Duck ( 0 0) Davis, Ca 95616 Quantum Duck Software, |\ \______/ / \\\ 916-753-1169 u586182058ea@vega.ucdavis.edu |\ < < | \/
0040;0000009034;0;250;143;@mamab.FIDONET.ORG (0040;0000009034;0;250;143;) (08/06/89)
-- Fidonet: 0040;0000009034;0;250;143; via 1:363/9 Internet: 0040;0000009034;0;250;143;@mamab.FIDONET.ORG Usenet: ...!peora!rtmvax!libcmp!mamab!0040;0000009034;0;250;143;
alh@hprmokg.HP.COM (Al Harrington) (08/11/89)
>/ hprmokg:comp.sys.amiga / 0040;0000009034;0;250;143;@mamab.FIDONET.ORG (0040;0000009034;0;250;143;) / 10:17 am Aug 5, 1989 / > > >-- >Fidonet: 0040;0000009034;0;250;143; via 1:363/9 >Internet: 0040;0000009034;0;250;143;@mamab.FIDONET.ORG >Usenet: ...!peora!rtmvax!libcmp!mamab!0040;0000009034;0;250;143; >---------- > This Fidonet garbage has to stop. There has been 5 or more of these null messages coming through w/in the past few days. Would someone please try to figure out why the Fidonet -> Usenet is so screwed up. Thank you. +-------------------------------------+------------------------------------+ | -Al Harrington /// | | | /// | "Do I look like I'm joking?" | | alh@hprmo.HP.COM \\\/// | | | ..{hplabs,hp-sde}!hprmo!alh \XX/ | - The Joker | +-------------------------------------+------------------------------------+ | My comments in no way reflect the views or opinions of HP | +--------------------------------------------------------------------------+
tomb@hplsla.HP.COM (Tom Bruhns) (08/14/89)
sutherla@qtp.ufl.edu (scott sutherland) writes: > Lots of other stuff deleted, but this caught my eye: >6) Let the weight of the platter be ~ 3 oz, or ~ 400 g. (I don't have any > idea if this is a reasonable guess for the platter weight.) It's not a reasonable conversion! 1 oz is about 28 grams. > > >7) Then the weight of the platter without lubricant is 400.0000g and > the weight of the platter with lubricant is 400.0004g. Thus you > are trying to measure one part in one million by using the > difference between two large numbers. This is a statistical > nightmare! (Any statisticians care to comment?). If the error > in your scale is 0.0002g, you have a 50% error in your > measurement. NOT GOOD. I don't think it's the measurement that's a problem. Clearly, you'd use the same balance for both the "before" and "after" measurements. Rather, it's making sure that nothing else of similar mass is transferred on or off the platter between weighings.