Posted by *Eeyore* on April 17, 2009, 11:42 am

daestrom wrote:

*> > Curbie wrote:*

*> >> > nobody@xmission.com (Scott) wrote:*

*> >> >>*

*> >> >>>The calculations are vastly simpler in metric SI units.*

*> >> >>*

*> >> >>Snob.*

*> >> >*

*> >> >But he's right. BTU's went out with the ark, even those that still*

*> >> >use feet and pounds in the UK no longer use BTU's.*

*> >> About what?*

*> >*

*> > For anything.*

*> >*

*> >> It is simpler to add two metric units, than it is to add two imperial*

*> >> units?*

*> >*

*> > SI requires no 'conversion factors'. That's why it's simpler.*

*> Actually, it *does* use conversion factors, it's just that they all happen*

*> to be 1-something (1J = 1 N-m, or 1W = 1 J/s)*

Those aren't conversion factors. You're simply stating the same thing using

different dimensional units.

*> If you don't keep track of the units being used, it's just as easy to screw*

*> up something in the SI system as any other.*

Dimensional analysis applies to every sytem of measures.

http://en.wikipedia.org/wiki/Dimensional_analysis

Graham

Posted by *daestrom* on April 18, 2009, 1:47 pm

*> daestrom wrote:*

*>> > Curbie wrote:*

*>> >> > nobody@xmission.com (Scott) wrote:*

*>> >> >>*

*>> >> >>>The calculations are vastly simpler in metric SI units.*

*>> >> >>*

*>> >> >>Snob.*

*>> >> >*

*>> >> >But he's right. BTU's went out with the ark, even those that still*

*>> >> >use feet and pounds in the UK no longer use BTU's.*

*>> >> About what?*

*>> >*

*>> > For anything.*

*>> >*

*>> >> It is simpler to add two metric units, than it is to add two imperial*

*>> >> units?*

*>> >*

*>> > SI requires no 'conversion factors'. That's why it's simpler.*

*>>*

*>> Actually, it *does* use conversion factors, it's just that they all *

*>> happen*

*>> to be 1-something (1J = 1 N-m, or 1W = 1 J/s)*

*> Those aren't conversion factors. You're simply stating the same thing *

*> using*

*> different dimensional units.*

Well, by your definition, then saying that 1 btu = 1055 Joule isn't a

conversion either. It's just 'different dimensional units'. After all,

both BTU and Joule are simply units of energy.

*>> If you don't keep track of the units being used, it's just as easy to *

*>> screw*

*>> up something in the SI system as any other.*

*> Dimensional analysis applies to every sytem of measures.*

*> http://en.wikipedia.org/wiki/Dimensional_analysis *

Exactly right. And not carrying them through because all the 'conversion

factors' are just 1 means you run the risk of not getting the right result.

Just because the conversion factors in some systems of measure are 1.00 and

in others they are something else besides 1.00 doesn't make one system

superior to the other.

If anything, the one that always has unit conversions of 1.00 will lull you

into complacensy and screw up by forgetting (in your case even denying) that

you're doing unit conversions.

daestrom

Posted by *Jim Wilkins* on April 18, 2009, 2:09 pm

*> If anything, the one that always has unit conversions of 1.00 will lull you*

*> into complacensy and screw up by forgetting (in your case even denying) that*

*> you're doing unit conversions.*

*> daestrom*

Eliminating conversion factors was useful back in the era of

multiplying & dividing with slide rules and adding & subtracting

intermediate results on paper. We ran three parallel calculations; the

values, the exponents and the units, and sometimes an order-of-

magnitude sanity check as well. Calculators are sooo much easier but

also more likely to lead you astray.

Jim Wilkins

BS Chem 69

Posted by *Eeyore* on April 18, 2009, 3:54 pm

daestrom wrote:

*> > daestrom wrote:*

*> >> > Curbie wrote:*

*> >> >> > nobody@xmission.com (Scott) wrote:*

*> >> >> >>*

*> >> >> >>>The calculations are vastly simpler in metric SI units.*

*> >> >> >>*

*> >> >> >>Snob.*

*> >> >> >*

*> >> >> >But he's right. BTU's went out with the ark, even those that still*

*> >> >> >use feet and pounds in the UK no longer use BTU's.*

*> >> >> About what?*

*> >> >*

*> >> > For anything.*

*> >> >*

*> >> >> It is simpler to add two metric units, than it is to add two imperial*

*> >> >> units?*

*> >> >*

*> >> > SI requires no 'conversion factors'. That's why it's simpler.*

*> >>*

*> >> Actually, it *does* use conversion factors, it's just that they all*

*> >> happen to be 1-something (1J = 1 N-m, or 1W = 1 J/s)*

*> >*

*> > Those aren't conversion factors. You're simply stating the same thing*

*> > using different dimensional units.*

*> Well, by your definition, then saying that 1 btu = 1055 Joule isn't a*

*> conversion either. It's just 'different dimensional units'. After all,*

*> both BTU and Joule are simply units of energy.*

Same dimensions, different measurement systems.

*> >> If you don't keep track of the units being used, it's just as easy to*

*> >> screw up something in the SI system as any other.*

*> >*

*> > Dimensional analysis applies to every sytem of measures.*

*> > http://en.wikipedia.org/wiki/Dimensional_analysis *

*> Exactly right. And not carrying them through because all the 'conversion*

*> factors' are just 1 means you run the risk of not getting the right result.*

*> Just because the conversion factors in some systems of measure are 1.00 and*

*> in others they are something else besides 1.00 doesn't make one system*

*> superior to the other.*

*> If anything, the one that always has unit conversions of 1.00 will lull you*

*> into complacensy and screw up by forgetting (in your case even denying) that*

*> you're doing unit conversions.*

Well, I disagree and I've been there, and it seems you have not.

Realistically, the world isn't going to change back to old UK or US units. Thank

God for that too, although since I gew up with Imperial measures, I still use

feet and inches for everyday use ( and metres too ) , but any technical drawing

will be metric. I have no trouble with that.

Graham

Posted by *Mike* on April 17, 2009, 3:52 pm

wrote:

*>> SI requires no 'conversion factors'. That's why it's simpler.*

*>>*

*>Actually, it *does* use conversion factors, it's just that they all happen *

*>to be 1-something (1J = 1 N-m, or 1W = 1 J/s)*

*>If you don't keep track of the units being used, it's just as easy to screw *

*>up something in the SI system as any other. Can't tell how many times I've *

*>caught students trying to say 1m + 1 N = 2J. Ever try to add units of *

*>length with units of force? Keeping track of the units and using even SI *

*>conversions is essential to doing the right calculations.*

*>Where did you Europeans ever get the idea to measure force using kilograms *

*>anyway? *

We use Newtons, not kg

*>While the proper unit for pressure is the Pascal (or kPa or MPa), *

*>why do you have pressure gauges that measure 'kg / cm^2'? What's up with *

*>that?*

The Europeans might, the Brits (general public when inflating tyres)

commonly use psi or occasionally bar ( = 100 kPa) Real world

engineering uses psi, bar, Pa depending on which way the wind is

blowing and the specific application.

Many years ago I heard an Australian weather forecast that used Hecto

Pascals this was the first time I had ever seen the hecto multiplier

in use (with the exception of hectares!) after 20 years of using SI

units and I vaguely recalled the first day I saw a list of SI

multipliers. I knew the numbers (circa 1000) they quoted were in the

same region as those for millibars, but until I sat down and really

thought about it I was totally convinced that by using Hecto Pascals

they were out by a factor of 10.

*>Not that I'm a big fan of the US imperial system, but there are a few tricks *

*>that work well. One lbm always exerts one lbf due to gravity. Heating up *

*>one lbm of water one degree F takes just one BTU. Maybe a few others.*

Substitute 1 kg, 1 deg K, 1 Joule, 1 Newton as necessary

It's ok I do know about metric water :)

--

> > Curbie wrote:> >> > nobody@xmission.com (Scott) wrote:> >> >>> >> >>>The calculations are vastly simpler in metric SI units.> >> >>> >> >>Snob.> >> >> >> >But he's right. BTU's went out with the ark, even those that still> >> >use feet and pounds in the UK no longer use BTU's.> >> About what?> >> > For anything.> >> >> It is simpler to add two metric units, than it is to add two imperial> >> units?> >> > SI requires no 'conversion factors'. That's why it's simpler.> Actually, it *does* use conversion factors, it's just that they all happen> to be 1-something (1J = 1 N-m, or 1W = 1 J/s)