Well speaking of 'wheels', the 'rims' are designed and sized based on Imperial/English untis while the 'tires' are Metric.
And every mass produced automotive sparkplug ever manufactured was Metric, even 60 years ago when every other nut and bolt on an American-built car or truck was Imperial/English units (why do you think sets of SAE wrenches always came with special sockets designated for 'Sparkplugs').
But on the other hand, fire-hydrants and the standard for firehose connections are based on Imperial/English units. Sometimes absolute assurance of interchangeability is more important than complying with the local units of measure.
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
SI unifies electrical, mechanical and thermal analysis with no nasty conversion factors. So easy to simulate whole systems using it. You have to cope with rad/s for shaft speeds, but that's not a difficult leap to make.
Ask me what I weigh in kg though, and I'd need to think about it.
I need to do my calcs in SI to avoid forgetting a g sub c somewhere. What the )(*$ kind of unit system doesn't even allow you to calculate F = m*a without using a conversion factor?
I then convert the results to a unit my commonsense is calibrated in.
We work in THREE systems here: SI and derived metric units, US customary units, and Imperial units.
We were taught that F was proportional to m*a in physics classes at the age of about 13 and did loads of experiments to demonstrate it. A big light switched on in my brain when I finally realised that in SI units, not only was F proportional to m*a, it was actually equal to it.
Pat,
When pressure is reported in kg/m^2, you need a metric version of g(c). I see more gagues with that silly unit these days than kPa or bar. Welcome to my world.
It doesn't, hence the use of terms like a litre of milk but a pint of beer.
I still like to use psi or bar for pressure. I don't even know what the tables are that David refers to. Boiling point vs pressure maybe? I like psi because I can envisage it graphically in my mind and I am accustomed to it's effect on tyres. I like bar because it is a simple multiplier of atmospheric pressure for forced induction engines.
If we used deg K instead of C I guess temperature changes in gasses from pressurisation would become a bit easier.
This could go all year though.
Ultimately we use what we have at hand and learn the conversions if really necessary.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
Pat,
I'm talking about classroom materials. Things like the units that an empirical equation needs in fps in one column and the units in SI in another, or a tablulation of a physiscal parameter like saturation quantity of water in natural gas vs. pressure for a constant temperature.
Your bar example works in a lot of the world, but I always find myself in places where local atmophperic pressure is 0.7 bar(a) or less and using bar doesn't help the math much.
If it's a question of what you like to use, it's what you are comfortable with. If it's a question of what's a better system then lets look at some comparisons.
Temperature:
C - 0 is the freezing point of water, 100 is the boiling point. Relatable and useful in many scientific and industrial settings.
F - 0 means...ahh...the coldest temperature they could achieve at the time. 100 means....ahh...the internal temperature of the human body...wait, we got that wrong.
Length:
m - Base 10. Enough said.
ft - 3'7-1/2" + 4'3-5/8" + 6'5-1/4" + 8'7" = need to convert into either inches or feet before I plug into my calculator.
Force:
N - Not as relatable as lbf unless you use them.
lbf - Not to be confused with lbm...or are they interchangable...and what about that slug thing? Do I need to multiply or divide by gravity to get to it?
Volume:
m^3 - yes, L are commonly used for liquid but you just need to move the decimal point around.
Oz - fluid volume (8 oz per cup), dry volume(wait...oz don't exists for dry volume). And don't forget weight (16 oz per pound). Don't like oz, ok gallons...wait US or Imperial. Don't like gallons, ok ft^3....now what's the conversion.
Scalability:
Metric - change the prefix, move the decimal.
Imperial - use a completely different unit of measure, find what wacky conversion is required and calculate.
Really, the ONLY advantage Imperial has over Metric, is that people are more familiar with it. If it was the other way around and Metric was here first and someone was trying to promote a switch to Imperial with all its odd conversions we wouldn't be having this conversation.
I am fairly young and I can't easily visualize metric (I need to do the math to give my weight/height in kg/m)...I do my calc's in metric and convert to imperial to get a feel for the size and forces involved in a problem...hopefully I will eventually develop a better feel for metric and forget about imperial.
I am in a new building here in Canada. The architects prepared their drawings in metric. The actual construction drawings were converted to inches and feet.
A lot of stuff is nominally sized in Imperial unts, but does not actually conform. Lumber and pipe comes to mind. You could just re-label these things in conforming metric units. For example, 1/2" [ ]pipe (Ø.840") can be called 14mm pipe.
