Unit Conversion Help

[O|2ioN]

Hello,

 

I need your help with an argument I got myslef into. I may be wrong here due to old information but bare with me and see if you can help me out.

 

I am a designer for an engineering firm and I posted a response in a CAD forum in regards to converting inches to cm or mm. Since I did not know what this person was drafting I posted that 1" = 25.3995mm. This was found in an old math book from the 1950's that had a conversion table in the back of it (yes I typed old - I know old sources are never the best thing to quote - my mistake).

 

The industry standard seems to be 1" = 25.4mm and in the link here THIS LINK post's that "1in = 2.54cm (exactly)". I was baffled by "exactly", considering that I was under the impression that nothing is exact in this world so we just have to settle for dam close.

 

Is there any information that supports my post that 1" = 25.3995mm? Please help!!

 

Let me know if you need clarification...

[timo]

I have my doubts that many people here are able to help you. After all, this is a science forum and I think scientists usually tend to use the international standard units (or at least similar systems). Just out of interest: Do you have any CAD application in mind where this relative difference in the order of 1/10,000 would play a role ?

A rough guess of mine would be that noone does precision measurements in inches, so the 25.3995 just became accepted as 25.4 because the difference doesn´t matter anyways.

Not that I´ve ever used inches in my life, that´s just my two euro-cents.

[swansont]

1 mile = 1.609344 kilometer = 160934.4 cm

 

5280 feet in a mile x 12 inches in a foot

 

That's 63360 inches

 

I get 2.54 exactly.

 

It may be that your old textbook was using a value from before the slight redefinition of the meter in 1927, or perhaps from when it was redefined in terms of a Kr wavelength in 1960.

[timo]

Guess I was wrong ...

[Phi for All]

I got this link from Sayonara³ once upon a time. Hope it helps.

[O|2ioN]

Thank you very much folks. I do undertand my post was out of the ordinay for ya but I needed help. Im actually glad it's clarified. I must admit defeat.

 

Thanks again!

 

O

[Yggdrasil]

One inch is exactly 2.54 cm because the inch was redefined in terms of the metric system in order to make the two systems more compatible.

 

See http://en.wikipedia.org/wiki/Inches

The international inch is defined in terms of the metric system of units to be exactly 25.4 mm. This definition was agreed upon by the U.S. and the British Commonwealth in 1958.