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 :)
--
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> >> > 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)