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#1
04-23-2012, 12:12 PM
 superfahd Guest Join Date: Apr 2012
Use of non metric units in the US for scientific purposes

Ok ok first post. dont mess it up!

Heres something thats been bothering me for ages. Not being a US native, I've done my studies uptil my BS outside the US. I'm a software engineer and have always been keen on scientific topics and courses. One of the most fascinating things for me was the use of the SI (metric) system of units which allows consistent and comparable formulas and calculations not matter what the domain of study is.

After coming here and realizing that there is a part of the world that doesn't use the SI system, I was stunned! I simply cant comprehend how the math would work. Do the formulas remain the same or do you have to put in some conversion to account for the wildly inconsistent units of measurement. as a minor example, consider the formula for energy change in a uniform object due to change in temperature (ignoring all other factors)

E = mc(Δϴ)

where E = energy change measure in Joules (J)
m = mass measured in kg
c = specific heat capacity measured in Joules per kilogram Kelvin (J/Kg K)
Δϴ = temperature change measured in Kelvin or Centigrade (difference is equivalent in both units)

Notice that all units are related and the resulting formula is clean and simple and there is no need for conversion of any sort (or even a need to come up with a unit of measurement for the result). Even Joule is just shorthand for (kg.m^2)/s^2 or other equivalent metric units.

Maybe I'm overthinking this but my brain just BSODs when I even try to think how you would calculate this using pounds for mass, Fahrenheit for tempurature and god knows what for energy or specific heat capacity. Help me out here fellas! I'm going crazy!
#2
04-23-2012, 12:27 PM
 CookingWithGas Charter Member Join Date: Mar 1999 Location: Tysons Corner VA Posts: 9,337
Now in the US science is generally done in SI. But when the English system is used the units are determined so that everything works out. For example, torque is calculated feet x pounds and the resulting unit is the foot-pound.
#3
04-23-2012, 12:33 PM
 Baracus Guest Join Date: Dec 2006
It isn't all that different. The conversion factor is built into the constant. So instead of heat capacity in J/kg-K it would be measured in calories/lbm-ºF, or something to that effect, and the value would be different to account for the different units.
#4
04-23-2012, 12:35 PM
 lazybratsche Guest Join Date: Feb 2006
That constant, c, will just take on another value that accounts for all of the relationships between Δ°F, pound-mass, and BTUs (or whatever hogshead per fortnight bullshit you want to use).

ETA: But most science is done with SI. Some engineering, depending on the field, sticks with English units.

Last edited by lazybratsche; 04-23-2012 at 12:37 PM.
#5
04-23-2012, 12:36 PM
 robert_columbia Guest Join Date: Oct 2009
Quote:
 Originally Posted by CookingWithGas Now in the US science is generally done in SI. But when the English system is used the units are determined so that everything works out. For example, torque is calculated feet x pounds and the resulting unit is the foot-pound.
American here. First of all, you just get used to there being two systems.

The English system is used for most casual measurements. The news reports pretty much everything using English measurements (e.g. the weather report gives temperature in F and reports rain or snowfall in inches). Hard science uses SI. Construction work and other trades historically used English but may use SI depending on what is being built, what parts are being used, what the customer wants, and/or local codes.

And, generally, Dimensional Analysis will allow you to use all your favorite formulas. For example, F=ma is as true in English units as SI units, you just have to make sure that the values on BOTH sides of the equation are in applicable English units.

Last edited by robert_columbia; 04-23-2012 at 12:40 PM.
#6
04-23-2012, 12:41 PM
 johnpost Guest Join Date: Jul 2009
you could memorize constants in metric and english units. you could just remember the metric constants and use english-to-metric conversions which you have to know anyway in a dual system environment. actually if you used a few equations a lot you would know the english constants for those.
#7
04-23-2012, 12:42 PM
 robert_columbia Guest Join Date: Oct 2009
Quote:
 Originally Posted by superfahd ... Maybe I'm overthinking this but my brain just BSODs when I even try to think how you would calculate this using pounds for mass...
First of all, if you are using pounds for mass you deserve an F. Pounds is a unit of force, similar to Newtons. If you want mass, try Slugs.
#8
04-23-2012, 12:42 PM
 Donnerwetter Guest Join Date: Apr 2012
It is a real problem:

Quote:
 Metric mishap caused loss of NASA orbiter CNN NASA lost a 125 million Mars orbiter because a Lockheed Martin engineering team used English units of measurement while the agencys team used the more conventional metric system for a key spacecraft operation, according to a review finding released Thursday. The units mismatch prevented navigation information from transferring between the Mars Climate Orbiter spacecraft team in at Lockheed Martin in Denver and the flight team at NASAs Jet Propulsion Laboratory in Pasadena, California. Lockheed Martin helped build, develop and operate the spacecraft for NASA. Its engineers provided navigation commands for Climate Orbiters thrusters in English units although NASA has been using the metric system predominantly since at least 1990.
http://articles.cnn.com/1999-09-30/t...tem?_s=PM:TECH
#9
04-23-2012, 12:47 PM
 Darth Panda Guest Join Date: Mar 2010
I can comment that the energy industry in the US uses English - btus, or mmbtus. And coal characterisitcs use English units as well, with heat content being given in btus/lb (or mmbtus/ton) and SO2 quantity given in lbs/mmbtu. Oil is usually priced by the barrel, but #2 heating oil is priced in \$s/mmbtu. And when we talk about heat rates at power plants, we use mmbtus/megawatt-hour.

If I had to switch to SI, I would be really, really confused for a while, so I can certainly understand why someone going the other direction would ahve a hard time of it.

Last edited by Darth Panda; 04-23-2012 at 12:48 PM.
#10
04-23-2012, 12:50 PM
 MikeS Charter Member Join Date: Oct 2001 Location: Williamstown, MA Posts: 3,156
It's not really that hard, so long as all your units are expressed in a self-consistent system. You might be interested in the Wiki pages on English Engineering Units and the FPS system. For example, in the latter system you would do the following:

E = energy change measured in foot-poundals
m = mass measured in pounds (or, for the nitpickers like robert_columbia, pounds-mass)
c = specific heat measured in ft-pdl/lbm*(°F)
Δϴ = temperature change in degrees Fahrenheit

It's all self-consistent, and the units still all cancel out. Granted, every one of the numbers you'll put into your calculator is different, but there's no particular advantage of a system of units based on the meter and the kilogram versus one based on the foot and the pound-mass.

Now, if you were given a volume in (say) gallons and needed to figure out the mass of that volume of a particular liquid, and you knew its density in terms of base units (lbm/ft3), you'd still need to convert it into cubic feet first. For that matter, if you're given a volume in liters and you know its density in kg/m3, you need to convert liters into cubic meters too. Here, though, metric has the advantage, since 1 gallon = 0.133680556... ft3, while 1 liter = 0.001 cubic meters exactly.

Finally, as noted above, almost all scientific calculations are done in SI units. (The big exception in my field is the electron-volt for energy.) Some engineers still occasionally use FPS-flavored units, though.
#11
04-23-2012, 12:50 PM
 akennett Guest Join Date: Feb 2003
Quote:
 Originally Posted by robert_columbia First of all, if you are using pounds for mass you deserve an F. Pounds is a unit of force, similar to Newtons. If you want mass, try Slugs.
Second of all, if you are going to correct someone, be correct. http://en.wikipedia.org/wiki/Pound_%28mass%29
#12
04-23-2012, 01:07 PM
 BwanaBob Charter Member Join Date: Feb 2003 Location: New York & Maryland Posts: 3,515
Quote:
 Originally Posted by akennett Second of all, if you are going to correct someone, be correct. http://en.wikipedia.org/wiki/Pound_%28mass%29
I was a math/physics major in the 80's. We never ever used the pound-mass. For mass we either used kg or slugs, for force either Newtins or pounds.

Once again Wikipedia serves to confound the masses.
#13
04-23-2012, 01:15 PM
 akennett Guest Join Date: Feb 2003
Quote:
 Originally Posted by BwanaBob I was a math/physics major in the 80's. We never ever used the pound-mass. For mass we either used kg or slugs, for force either Newtins or pounds. Once again Wikipedia serves to confound the masses.
I was a mechanical engineering major in the '90s. We used both slugs and lbm.
#14
04-23-2012, 01:36 PM
 superfahd Guest Join Date: Apr 2012
Quote:
 Originally Posted by MikeS It's not really that hard, so long as all your units are expressed in a self-consistent system. You might be interested in the Wiki pages on English Engineering Units and the FPS system. For example, in the latter system you would do the following: E = energy change measured in foot-poundals m = mass measured in pounds (or, for the nitpickers like robert_columbia, pounds-mass) c = specific heat measured in ft-pdl/lbm*(°F) Δϴ = temperature change in degrees Fahrenheit It's all self-consistent, and the units still all cancel out. Granted, every one of the numbers you'll put into your calculator is different, but there's no particular advantage of a system of units based on the meter and the kilogram versus one based on the foot and the pound-mass. Now, if you were given a volume in (say) gallons and needed to figure out the mass of that volume of a particular liquid, and you knew its density in terms of base units (lbm/ft3), you'd still need to convert it into cubic feet first. For that matter, if you're given a volume in liters and you know its density in kg/m3, you need to convert liters into cubic meters too. Here, though, metric has the advantage, since 1 gallon = 0.133680556... ft3, while 1 liter = 0.001 cubic meters exactly. Finally, as noted above, almost all scientific calculations are done in SI units. (The big exception in my field is the electron-volt for energy.) Some engineers still occasionally use FPS-flavored units, though.
That explains things a lot. Thanks. The main issue is, as you said, to use consistent but non SI units are inconsistent by nature (and there are so many of them!) where as SI are designed to be internally consistent. e.g take the water volume to mass conversion. 1 liter is 1 kg so conversions are easy. even using a factor like 0.001 is a lot easier than 0.133!

Never heard of foot poundals before! Also learned that pounds are a unit of force not mass! Shows how much I know. Still I'm relieved. (I mean that in the "oh good it'll still be familier if I have to do it again" sense and not the "oh good there's hope for you bloody yanks yet!" sense!)
#15
04-23-2012, 01:37 PM
 superfahd Guest Join Date: Apr 2012
http://en.wikipedia.org/wiki/Pound_%28mass%29

http://en.wikipedia.org/wiki/Pound_(force)

ok now I'm really confused!
#16
04-23-2012, 01:43 PM
 constanze Guest Join Date: Aug 2006
Get either a programmable calculator or a software conversion tool or an online site like this one. You enter the result in one unit and get the result in the other unit. Do all calcuations first with the starting unit for accuracy - if your scale reads inches, then use inches per second instead of meters, and convert once you're done.

The formulas are all the same after all - it's distance over time, whether distance is feet or furloughs or meters. Just plug the numbers in.
#17
04-23-2012, 01:54 PM
 BwanaBob Charter Member Join Date: Feb 2003 Location: New York & Maryland Posts: 3,515
Quote:
 Originally Posted by akennett I was a mechanical engineering major in the '90s. We used both slugs and lbm.
Interesting. In actuality our professors shunned anything but SI units.
#18
04-23-2012, 01:58 PM
 akennett Guest Join Date: Feb 2003
Quote:
 Originally Posted by superfahd ok now I'm really confused!
Don't worry about that...this is a topic that gets everyone confused.

A pound, as you normally hear it, is a unit of force. In some instances, however, it can be a unit of mass. These are typically differentiated as pound-force (lbf) and pound-mass (lbm). There is also the slug, another unit of mass.

The simple way to think about it is that a slug is the most direct equivalent of a kg in terms of mass. That is, 1 lbf accelerates 1 slug at 1 ft/s^2) the same way that 1 N accelerates 1kg at 1 m/s^2.

A lbm = 32.17 slugs (32.17 m/s^2 being the imperial value for gravity on Earth). It is thus defined so that, on Earth, 1lbm weighs 1lbf.
#19
04-23-2012, 02:02 PM
 superfahd Guest Join Date: Apr 2012
For me its become a part of my life. For the longest time, I internally converted units in my head in order to know what they meant. Kilometers to miles was easy since I drive a lot now but I still sometimes convert fahrenheit to centigrade in my head to get an actual feel of temperature. Even after 6 years, I still cringe when I hear the Mythbusters use a non SI unit on their show!
#20
04-23-2012, 02:48 PM
 Chronos Charter Member Join Date: Jan 2000 Location: The Land of Cleves Posts: 50,877
Quote:
 Quoth CookingWithGas: Now in the US science is generally done in SI. But when the English system is used the units are determined so that everything works out. For example, torque is calculated feet x pounds and the resulting unit is the foot-pound.
And likewise, rotational speed is measured in rotations per minute, or RPM. And power, which is just torque times rotational speed, is measured in foot-pound-rotations per minute.

Oh, wait, no it's not. Power is measured in horsepower, and so the folks who use such units have to insert some crazy number into their calculations to convert from foot-pound-rotations per minute to horsepower.

It gets even worse. Some folks use slugs for mass, pounds for force, feet for distances, and seconds for time, and that works out OK. Other folks use pounds for mass, poundals for force, feet for distances, and seconds for time, and that works out OK, too, as long as you know which is which. But then there are some folks who use pounds for mass and for force, and still use feet and seconds, which means they need to cram an extra factor of something that resembles g all over the place into equations where it has no business being.
#21
04-23-2012, 03:14 PM
 installLSC Guest Join Date: Jun 2006
I work in a toxicology lab, and everything uses SI--chemicals, glassware, instruments, etc. The only thing I can think of that isn't primarily in SI is paper for obvious reasons.
#22
04-23-2012, 04:13 PM
 Sunspace Charter Member Join Date: Jun 1999 Location: Back in the GT eeehhhh... Posts: 25,594
And don't forget that the US Customary System of Measurements is not the Imperial System of Measurements used by the British Empire after the early 19th century, and neither is the old "English" system that the Imperial system superseded.

A comparison between the US and Imperial systems.

As a Canadian, I have to deal with ALL of these, except possibly the old English measurements. And it pisses me off.

Canada botched its conversion to SI measure by never finishing it, and many fields including publishing, residential house design, and cooking still tend to use Imperial measure. Then there are the differences between US and Imperial measure: when a Canadian ad mentions a car's fuel usage in "miles per gallon", is that "miles per US gallon" or "miles per Imperial gallon"? Especially since we buy fuel in litres and measure fuel usage in litres per 100 kilometres? IMHO half this stuff is either comparisons to US models, or sops to the older generations who still stubbornly cling to the old measures.

When I was investigating house drawings, I was dragged kicking and screaming backwards from the SI world. I would do my heat and volume calculations in joules and kelvins and millimetres, and convert to BTU/hour and degrees Fahrenheit and cubic feet at the end... especially since our regulations are in SI but our small-scalle typical-handyman construction still tends to take place in feet and inches.

Not that I was ever far from it; Canada may be the only country outside the US that still dimensions all its paper in inches as well. So even in ly old job, we usually designed our manuals and publications in inches, often to fit US "letter-size" paper: 8.5 inches x 11 inches.
#23
04-23-2012, 04:47 PM
 superfahd Guest Join Date: Apr 2012
Quote:
 I was dragged kicking and screaming backwards from the SI world
I know that feel bro!
#24
04-23-2012, 05:13 PM
 minor7flat5 Charter Member Join Date: Sep 2002 Location: Trenton, NJ Posts: 3,456
Quote:
 Originally Posted by Chronos But then there are some folks who use pounds for mass and for force, and still use feet and seconds, which means they need to cram an extra factor of something that resembles g all over the place into equations where it has no business being.
That would be my shipmates and I studying in Navy Nuclear Power School ages ago. We did everything in English units, including the thermodynamics, chemistry, materials science, reactor theory/nuclear physics. I suppose some of our work was in SI units, but the vast majority was in BTUs, Horsepower, and even lb-m and lb-f.
I remember peppering our equations with the "proportionality constant" which was the cheater constant that allowed us to work in lb-f and lb-m at the same time.

I recall that all of the charts and graphs were in English units (e.g. steam tables), so maybe that had something to do with it. I wonder if they have gone metric in recent years.
#25
04-23-2012, 05:34 PM
 ZenBeam Charter Member Join Date: Oct 1999 Location: I'm right here! Posts: 7,725
I work in electromagnetics, and I've never used them with English units. When I was first taking physics as an undergrad, we used cgs (centimeter,gram,second), and called it Gaussian units (but I've since learned there are more than one set called "Gaussian"). When I began taking electromagnetics in electrical engineering, we used mks (meter,kilogram,second), and different Maxwell's equations (the fundamental equations of electromagnetics). When I took graduate-level physics, the units were mks, but the equations were more like the ones we used in cgs. Somewhere in there, we used both exp(+j omega t) and exp(-j omega t) time conventions (i.e. whether lossy material will have a positive or negative imaginary part). If you're counting, that's three sets of equations/units spread over two time conventions, and none of them were in English units.

At work, we use the electrical engineering equations and mks units, which maybe are the SI units, I'm not really sure. I always use meters in any codes I write or work with, immediately converting any inputs in other units. When I have to interface with manufacturing, they usually work in inches or maybe feet, so I know those conversion factors. I still run into both time conventions, and frankly, that's more confusing than converting between inches and meters.
#26
04-23-2012, 05:52 PM
 Nunavut Boy Guest Join Date: Dec 2003
Quote:
 Originally Posted by installLSC I work in a toxicology lab, and everything uses SI--chemicals, glassware, instruments, etc. The only thing I can think of that isn't primarily in SI is paper for obvious reasons.
It may be SI, but for some reason American clinical labs insist on using SI-but-different-from-everyone-else's-units. For example, US measures glucose (among others) in mg/dl. Everyone else uses mmol/L.

Dear god, why? If you're going SI, why not go with the same units that the rest of the world has agreed on?
#27
04-23-2012, 09:24 PM
 Lazlo Guest Join Date: Oct 2000
Quote:
 Originally Posted by akennett I was a mechanical engineering major in the '90s. We used both slugs and lbm.
Same, but I used lbm last month at work. Gotta love good ol' Crane TP-410.
#28
04-23-2012, 09:32 PM
 jz78817 Member Join Date: Aug 2010
Quote:
 Originally Posted by superfahd Ok ok first post. dont mess it up! Heres something thats been bothering me for ages. Not being a US native, I've done my studies uptil my BS outside the US. I'm a software engineer and have always been keen on scientific topics and courses. One of the most fascinating things for me was the use of the SI (metric) system of units which allows consistent and comparable formulas and calculations not matter what the domain of study is. After coming here and realizing that there is a part of the world that doesn't use the SI system, I was stunned! I simply cant comprehend how the math would work. Do the formulas remain the same or do you have to put in some conversion to account for the wildly inconsistent units of measurement. as a minor example, consider the formula for energy change in a uniform object due to change in temperature (ignoring all other factors) E = mc(Δϴ) where E = energy change measure in Joules (J) m = mass measured in kg c = specific heat capacity measured in Joules per kilogram Kelvin (J/Kg K) Δϴ = temperature change measured in Kelvin or Centigrade (difference is equivalent in both units) Notice that all units are related and the resulting formula is clean and simple and there is no need for conversion of any sort (or even a need to come up with a unit of measurement for the result). Even Joule is just shorthand for (kg.m^2)/s^2 or other equivalent metric units. Maybe I'm overthinking this but my brain just BSODs when I even try to think how you would calculate this using pounds for mass, Fahrenheit for tempurature and god knows what for energy or specific heat capacity. Help me out here fellas! I'm going crazy!
I work in the (American) auto industry. We work pretty much exclusively in SI units. Well, aside from horsepower and torque; dimensionally its all in SI. Fasteners are all specified with SI dimensions,etc. We might conversationally talk about 6.5" speakers, but we're really dealing with 165mm speakers.

Last edited by jz78817; 04-23-2012 at 09:37 PM.
#29
04-23-2012, 09:35 PM
 Diceman Guest Join Date: Mar 1999
The US automobile industry uses SI for all its design work. Every part on the car has its dimensions spelled out in millimeters -- even big parts, where centimeters or even meters might seem more logical.
#30
04-23-2012, 09:52 PM
 Dr. Strangelove Guest Join Date: Dec 2010
Quote:
 Originally Posted by Sunspace Not that I was ever far from it; Canada may be the only country outside the US that still dimensions all its paper in inches as well. So even in ly old job, we usually designed our manuals and publications in inches, often to fit US "letter-size" paper: 8.5 inches x 11 inches.
I'm not sure this complaint makes sense. What does it mean to dimension in a unit? If you're talking about round numbers, then US letter size qualifies, being 11x8.5". But A4 paper does not, being approximately 297.301 x 210.224 mm. Well, they do actually round it down to 297x210, but even so these aren't nice-looking numbers, given that they derive from multiples of sqrt(2).

The only thing even semi-metric about the A-series paper is the area, which for A4 is 1/16 m^2. Which itself looks quaintly Imperial, what with the power-of-2 fraction instead of a power of 10.
#31
04-23-2012, 09:55 PM
 Dr. Strangelove Guest Join Date: Dec 2010
Quote:
 Originally Posted by Diceman The US automobile industry uses SI for all its design work. Every part on the car has its dimensions spelled out in millimeters -- even big parts, where centimeters or even meters might seem more logical.
My understanding is that the metric system is used in architecture in the same way: millimeters for everything, and unlabeled (except in the legend). Seems like it has lots of advantages, though there's nothing really specific to the metric system there (you could do the same with inches).
#32
04-23-2012, 10:05 PM
 Civil Guy Guest Join Date: May 2004
Civil engineer. Hey, give us time. Of course, for the rest of time, we'd be forced to figure out how wide the sidewalks will be if we widen the road from 10 to 12 m, given a 60 foot right-of-way.
#33
04-23-2012, 10:09 PM
 GameHat Guest Join Date: Dec 2007
US doper here, and an engineer (chemical):

I graduated college in 2004. All the way through college, the science professors made us use SI (it is a better system, and internationally recognized.)

The engineering professors all made us use English units ("yeah, we know SI is better, but industry in the US still uses English units, and it's not great, but that's the way it is.)

So I got through a chemical engineering degree using (about half the time) grams and joules, the other half using lbs (mass) and BTU.

And yep, the formula work the same either way, except there will be a "conversion" constant usually on the English unit formulas.

Having been in the chemical industry for nearly 8 years now, yeah. Our manufacturing departments use pounds. Our QC/Analytical/R&D departments use grams and kilograms.

Our smaller "shop" customers usually still use pounds.

But we sell into Canada as well, and they mostly use kg. Even the smaller shops.

So I jump between units every day. I wish we'd all use SI, because it is more intuitive, but institutional inertia is strong.

Last edited by GameHat; 04-23-2012 at 10:10 PM.
#34
04-23-2012, 11:18 PM
 Sunspace Charter Member Join Date: Jun 1999 Location: Back in the GT eeehhhh... Posts: 25,594
Quote:
 Originally Posted by Dr. Strangelove My understanding is that the metric system is used in architecture in the same way: millimeters for everything, and unlabeled (except in the legend). Seems like it has lots of advantages, though there's nothing really specific to the metric system there (you could do the same with inches).
When I was in architecture school, they told us that buildings would be dimensioned on a 100-mm 'module': that a basic grid of locations at multiples of 100 millimetres would be defined, and things would be referenced to it. Plywood would be 1200 x 2400 mm, studs would be on 400- or 600-mm centres, etc.

But.

That was thirty years ago. Small-scale residential construction continues in feet and inches. Plywood remains 4 feet x 8 feet (1219 mm x 2438 mm). Studs remain on 16-inch or 24-inch (406 mm or 610 mm) centres.

I've seen plans for larger constructions dimensioned in metric; for instance, this railway underpass (Figure 12 on the fourth page of the linked PDF) has clearances and elevations dimensioned in millimetres, and appears to use metric throughout, with a few Imperial measurements added for reference. (For railways which use miles and miles per hour internally for their own operations...)
#35
04-23-2012, 11:30 PM
 WarmNPrickly Member Join Date: Oct 1999 Posts: 5,439
In reality the advantages of SI are over rated for most purposes. When I need to know the distance from here to Mars in millimeters, then just being able to shift decimal places will be really handy, but for the most part English units are perfectly suited for the dimensions they are intended. Calculators make converting units trivial and I never have had to convert units on the fly.

The one exception is an oven I have at work that works in Fahrenheit, and that drives me batty.
#36
04-23-2012, 11:49 PM
 Chronos Charter Member Join Date: Jan 2000 Location: The Land of Cleves Posts: 50,877
The units being related by powers of 10 is handy, but you're right that it's ultimately not that big a deal. What is a big deal, though, is that everything uses the same units. The unit of force is equal to the unit of mass times the unit of length divided by the square of the unit of time. The unit of energy is equal to the unit of force times the unit of distance. The unit of rotational speed is one over the unit of time. All of these are things that do show up regularly in real-world calculations.

Here's a pop quiz: Suppose I have a fish tank that's 2 feet long by 1 foot wide by 1 1/2 feet high. How many gallons does will it hold? Explain how you got your answer.
#37
04-24-2012, 12:24 AM
 Dr. Strangelove Guest Join Date: Dec 2010
Quote:
 Originally Posted by Chronos Here's a pop quiz: Suppose I have a fish tank that's 2 feet long by 1 foot wide by 1 1/2 feet high. How many gallons does will it hold? Explain how you got your answer.
May I assume the tank is a rectangular prism? Inside or outside dimensions? If outside, how thick is the glass? How far below the top edge is the water level? How much gravel is there? How big is the tank heater? Rocks? Also, what country do you live in?

I joke, but the point is that a simple multiply by 7.48 (assuming US) is an infinitesimal fraction of the entire calculation. The hard parts are the same in SI or Imperial.
#38
04-25-2012, 07:44 AM
 thelabdude Guest Join Date: Feb 2011
Quote:
 Originally Posted by MikeS but there's no particular advantage of a system of units based on the meter and the kilogram versus one based on the foot and the pound-mass.
This is true. When I took a job at the Ashland Chemical research headquarters in the70's, I was appalled to find their resin and plastic group working in pounds and Fahrenheit. I quickly adjusted. I see little utility in water being spg 1.0 when usually you are needing to pounds per gallon or specific gravity.
#39
04-25-2012, 07:49 AM
 BigT Guest Join Date: Aug 2008
Quote:
 Originally Posted by superfahd http://en.wikipedia.org/wiki/Pound_%28mass%29 http://en.wikipedia.org/wiki/Pound_(force) ok now I'm really confused!
It's no different than the common use of kilograms for weight.
#40
04-25-2012, 07:50 AM
 Smeghead Guest Join Date: Apr 2000
American biologist here. All SI, all the time.
#41
04-25-2012, 01:19 PM
 Chronos Charter Member Join Date: Jan 2000 Location: The Land of Cleves Posts: 50,877
Quote:
 I joke, but the point is that a simple multiply by 7.48 (assuming US) is an infinitesimal fraction of the entire calculation. The hard parts are the same in SI or Imperial.
No, multiplying by 7.48 isn't hard. The hard part is knowing that 7.48 is the number you need to use. And that's just one of dozens of different random-looking numbers you have to keep handy, and remember which one is which.
#42
04-25-2012, 02:08 PM
 psychonaut Guest Join Date: Apr 2001
It's curious that people here are referring to the American units as "English", since the English system of measurement is actually significantly different; the units have the same names but some have different values. For example, an American pint is 473 mL, but an English pint is 568 mL. Also different are fluid ounces, cups, quarts, gallons, and tons. And some customary English measurements, such as the stone, are used rarely or not at all in America.
#43
04-26-2012, 02:17 AM
 Stranger On A Train Guest Join Date: May 2003
Quote:
 Originally Posted by Donnerwetter It is a real problem: http://articles.cnn.com/1999-09-30/t...tem?_s=PM:TECH
Although this invariably the example that is cited whenever the issue of SI versus US Customary units, the reality is that this was a problem of requirement verification, not an innate problem with the units system. Spacecraft, even those built natively using SI units, have a wide array of different unit conversions and coordinate transformations that have to be performed correctly. Engineers designing structures will use different units or scales than those working on guidance, navigation & control, which will used different units than those in software. Factors and conversion functions will be applied for different conditions and responses, and if any of these are wrong, things don't work. In this case, in an effort to be "faster, better, cheaper" they eliminated verification testing to make up schedule, and thus didn't catch the error.

Quote:
 Originally Posted by jz78817 I work in the (American) auto industry. We work pretty much exclusively in SI units. Well, aside from horsepower and torque; dimensionally its all in SI. Fasteners are all specified with SI dimensions,etc. We might conversationally talk about 6.5" speakers, but we're really dealing with 165mm speakers.
Quote:
 Originally Posted by Diceman The US automobile industry uses SI for all its design work. Every part on the car has its dimensions spelled out in millimeters -- even big parts, where centimeters or even meters might seem more logical.
This is because the auto industry sources so many components internationally and often even has systems or whole vehicles assembled in nations where US Customary units are not used (thus, making tools and tooling more difficult to acquire). The auto industry started going through this conversion in the early 'Eighties, and it was by all accounts a painful process, though more straightforward than for, say, construction or aerospace insofar that the modern auto industry is large enough to order mill runs and full lots to spec of materials and fasteners, and completely redesigns product lines on a roughly five to eight year cycle, meaning that they only need to maintain tools, tooling, and spares for a period of twelve to fifteen years. A manufacturer that doesn't have that kind of purchasing capability and tooling turnover has to bear a much higher cost of obtaining SI-compatible tools and materials, which can be considerable indeed.

Quote:
 Originally Posted by WarmNPrickly In reality the advantages of SI are over rated for most purposes. When I need to know the distance from here to Mars in millimeters, then just being able to shift decimal places will be really handy, but for the most part English units are perfectly suited for the dimensions they are intended. Calculators make converting units trivial and I never have had to convert units on the fly.
Relying on a calculator to perform unit conversions is lazy and prone to generating unintended errors. Wherever conversions are used in engineering or scientific calculations the conversion should be an explicit step with the appropriate factors for verification. In the case of using decimal-based units, it is a fairly trivial matter to confirm that the exponents are correct.

But there is an additional reason to prefer decimal-based units, and that is for the ease in applying and using logarithmic scales, Although analog calculators such as slide rules are no longer in common use, the base-10 log scale is still very useful for power and field factors that function on a power law, reliability and hazard calculations that fit a lognormal distribution, dealing with floating point calculations, and linearizing scaling factors in astronomy, climatology, biology, et cetera, that nicely fit to factors that are multiples of ten. Technically speaking, factors of 10 are arbitrary; it could as easily be an octal, hexidecimal, or base-13 scale, but being able to scale to a coefficient of an exponent is the basis for many methods of reduction is of inestimable value, even with digital computers and calculators that can perform unit conversions automagically.

Quote:
 Originally Posted by WarmNPrickly The one exception is an oven I have at work that works in Fahrenheit, and that drives me batty.
Actually, both Fahrenheit and Celcius are problematic, as they are units based on relative scales that only work if you are performing calculations on a differential factor (difference or rate of change). Temperatures should properly be measured and stated on their absolute scales.

Stranger
#44
04-26-2012, 02:51 AM
 Dr. Strangelove Guest Join Date: Dec 2010
Quote:
 Originally Posted by Chronos No, multiplying by 7.48 isn't hard. The hard part is knowing that 7.48 is the number you need to use. And that's just one of dozens of different random-looking numbers you have to keep handy, and remember which one is which.
That's what the internet is for. It's not like I don't already have to keep track of hundreds or thousands of numbers anyway, even with SI, when doing any remotely serious calculations--for a fish tank, that might mean everything from the density of seawater to the mean packing factor of gravel. Of course, if all you want is to know how many jugs of distilled water to buy at the supermarket, then "7 or 8 per ft^3" is perfectly adequate. For that margin of error, it's not even hard to *guess* based strictly on a mental image.

Of course, none of this is to argue that SI isn't superior for many or most purposes. I just think many of the claimed benefits are overblown (especially for the "man on the street"), or have nothing to do with SI.

Also, quick: how many kilowatt-hours is 100 megajoules?
#45
04-26-2012, 02:59 AM
 Dr. Strangelove Guest Join Date: Dec 2010
Quote:
 Originally Posted by Stranger On A Train But there is an additional reason to prefer decimal-based units, and that is for the ease in applying and using logarithmic scales
Units aren't decimal (what would that even mean?). Prefixes are decimal-based, but within a calculation you shouldn't be mixing prefixes anyway: a calculation that contains joules, kilojoules, and megajoules as units is inviting diaster. Use scientific notation (which has nothing to do with the metric system) instead and stick with one unit.

The only thing remotely decimal about metric units is the difference between MKS and CGS, which of course differ only by factors of 10, but you really shouldn't be switching between those systems.
#46
04-26-2012, 08:01 AM
 WarmNPrickly Member Join Date: Oct 1999 Posts: 5,439
Quote:
 Originally Posted by Stranger On A Train Relying on a calculator to perform unit conversions is lazy and prone to generating unintended errors. Wherever conversions are used in engineering or scientific calculations the conversion should be an explicit step with the appropriate factors for verification. In the case of using decimal-based units, it is a fairly trivial matter to confirm that the exponents are correct. .... Stranger
My point is that we almost never need to convert at all. If there is a large difference in the same measurement, we'll use exponents. The only conversion I have to do is from grams to moles, where I have to look up the molecular weight anyway. There are advantages to metric units, but they are overstated.
#47
04-26-2012, 09:14 AM
 johnpost Guest Join Date: Jul 2009
the ability to scale quantities more easily makes the metric system valuable.
#48
04-26-2012, 09:21 AM
 minor_details Guest Join Date: Feb 2009
Chemist here...almost everything is in SI units, but some literature still uses calories (or kcal/mol) rather than joules (or kJ/mol). Yes, it's simple enough to convert, but after a while you have an intuitive feel for what a reasonable value for, say, a bond strength is in kJ/mol, and it's hard to apply that to values in other units - it's a bit like someone from the US knowing that 72 F is comfortable, but not having much of a feel for 18 C.

Theoretical chemists (and many theoretical physicists) use atomic units.
http://en.wikipedia.org/wiki/Atomic_units
They are another internally consistent set of units which has the property that many of the common constants (mass of electron, speed of light, hbar=Planck's constant/2pi, 1/(4*pi* permittivity of vacuum)) are equal to one. It also avoids using extremely small numbers. For example, hbar*hbar shows up a lot. In SI units, it's approx. 10^-68 J*s. Years ago, some computer systems turned numbers this small into zero... The atomic units for length, mass, energy and even time are all different from their SI counterparts.
#49
04-26-2012, 09:32 AM
 hibernicus Charter Member Join Date: Oct 2000 Location: Dublin, Ireland Posts: 1,855
Quote:
 Originally Posted by Dr. Strangelove Also, quick: how many kilowatt-hours is 100 megajoules?
Funnily enough, that one's easy for me, because my work involves lots of converting between kWh, MJ and therms.
#50
04-26-2012, 09:38 AM
 Quercus Guest Join Date: Dec 2000
Hmm.. I think the last few calculations I did (in the U.S.) used mg/l, inches and acres and convert back to pounds. Which perhaps reflects the mix of scientific, civil engineering, and legal definitions I deal with.

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