SI Units and why I use them

[Besoeker]

Let me say at the outset, I'm not trying to sell them.

Pretty much all of engineering in UK is in SI units. I'd have to use them whether I like them or not. But I do think they have merit. I'll try to explain why with an example that I posted in another thread.

The first three lines are the known data.

The 156.5 is the conversion from revs per minute to radians per second.
This is probably the most complex part of the process.
There are 2*pi radians in a revolution and 60 seconds in a minute.
Thus we get 1494*2*pi/60 = 156.451. Which is usually referenced as ω
So not really very complex at all.

Next, power is speed times torque.
P = Tω (Power in Watts)
So
T = P/ω
Or 6630e3/156.451 = 42,377

Next, acceleration (α) is torque divided by inertia.
So
α = T/I
Or 42,377/378 = 112.1

And finally
Acceleration time is given by speed divided by accel rate.
156.5/112.1 = 1.4

Other than knowing how to convert RPM into rad/s using the universal constant pi, a widely known value and on most calculators, there are no conversion factors.
Being just multiplication and division of numbers, the arithmetic is simple.
And I like simple.

 

[John120/240]

Yes SI metric is easier to understand because of increments of 10 units. In the english system

16oz. means different things; Liquid vs weight. I think most American adults are too set in their

ways to change now. 120/208/240/277/480 the math to derive these systems works out

easily. Sqrt 3, 360 degrees in a circle, 60 hz, 60 seconds in a minute all can give one a

visual model that is easy to grasp

 

[Besoeker]

Yes SI metric is easier to understand because of increments of 10 units.

With respect, that is decimal.
It's a common misconception to see SI as just a decimal system.
For sure, units multiples and subunits are multiples of ten.

But it's the way the basic units hang together that is the merit of the system.
It's the simplicity.
A force of one Newton over one metre in one second is one Watt.
Mechanical and electrical units neatly related with no conversion factors involved.
Neat and uncomplicated.

 

[GeorgeB]

SI and ISO ... date/time

SI and ISO ... date/time

Along with SI units, let's standardize on ISO dates ... here, Gar timestamps all messages in ISO format.

Officially, as I understand it, there is no space or punctuation ... yyyymmddhhmmss ... but I tend when using for humans to read to put a space between the date and time, and to separate hours, minutes, and seconds with :. If there are decimal seconds, there will be a (technically country dependent, but I use the US period, not the more internationally common comma ... so right now it is approximately 20111218 10:13:30 GMT-5

 

[Besoeker]

Along with SI units, let's standardize on ISO dates ... here, Gar timestamps all messages in ISO format.

Officially, as I understand it, there is no space or punctuation ... yyyymmddhhmmss ... but I tend when using for humans to read to put a space between the date and time, and to separate hours, minutes, and seconds with :. If there are decimal seconds, there will be a (technically country dependent, but I use the US period, not the more internationally common comma ... so right now it is approximately 20111218 10:13:30 GMT-5

I haven't seen it in that format (my bold).

A related link here:

http://www.cl.cam.ac.uk/~mgk25/iso-time.html

But I agree with your other points. Sticking to international standards, particularly in engineering and science, isn't a bad idea in my opinion.

 

[G0049]

DTG

DTG

Along with SI units, let's standardize on ISO dates ... here, Gar timestamps all messages in ISO format.

Officially, as I understand it, there is no space or punctuation ... yyyymmddhhmmss ... but I tend when using for humans to read to put a space between the date and time, and to separate hours, minutes, and seconds with :. If there are decimal seconds, there will be a (technically country dependent, but I use the US period, not the more internationally common comma ... so right now it is approximately 20111218 10:13:30 GMT-5

In my Army days, that was known as the DTG (Date Time Group), corrected to GMT, which was known as ZULU time. I've never known why it was called ZULU. Any ideas?

 

[jim dungar]

I've never known why it was called ZULU. Any ideas?

Base time zone = a time adjustment of Zero.

"Radio" alphabet equivalent for Z = Zulu

????

 

[GeorgeB]

In my Army days, that was known as the DTG (Date Time Group), corrected to GMT, which was known as ZULU time. I've never known why it was called ZULU. Any ideas?

Courtesy of wwp.greenwichmeantime.com,

The Navy, as well as civil aviation, uses the letter "Z" (phonetically "Zulu") to refer to the time at the prime meridian.

I, in my ham radio days, was told that the Z was zero (as in baseline); Zulu as the phonetic I understood. (Oh, yes, that is wwp ... new to me.)

More than one software applications, which create file names, use that ISO DTG format as their default.

 

[mikeames]

I think most American adults are too set in their

Yes we are just like we use the slowest possible keyboard layout "QWERTY"

 

[Besoeker]

Yes we are just like we use the slowest possible keyboard layout "QWERTY"

I think the qwerty is pretty much international.

 

[mikeames]

Maybe but some individuals have switched. The point is many times people do things because we are just use to it regardless if it makes sense or not.

 

[Besoeker]

Maybe but some individuals have switched. The point is many times people do things because we are just use to it regardless if it makes sense or not.

OK.
Do you think SI makes sense?

 

[mikeames]

Sure do, that's without question.

 

[Besoeker]

Sure do, that's without question.

Thanks for that.
I wasn't quite sure from your previous posts.
As I noted at the start, the SI system is pretty much universally used here for engineering.
It's sometimes informally referred to metric and in that guise it has been imposed on the general public whether they liked it or not. And a good many didn't.
I didn't like it being forced on people for no good reason that could or can see. If a shopper wanted to buy 5 pounds of potatoes and the seller was willing to sell potatoes by the pound, then why not? People were used to it. They knew how much food they needed to buy to feed their family. They new what a 2 lb bag of sugar looked like.
But, all changed. And for what?

 

[charlie b]

I grew up with the FPS system (i.e., Foot, Pound, Second), also called (in my neck of the woods) the "English Engineering System," as did my parents and the generations preceding them. I had to learn the MKS system (i.e., Meter, Kilogram, Second), also called the "Metric System" or the "SI System," in high school and college. My kids grew up with both systems, as will their kids. But when enough generations have been using both, I believe evolution will put the SI system on top, and the whole notion of feet, miles, pounds, and ounces will disappear completely. I have no objection to that happening.

By the way, Besoeker, speaking as a resident of the UK, does it bother you that the nomenclature of "SI" has its origins in the French language? Does it bother anyone else that the nomenclature of "SI System" is redundant? :happyyes:

 

[Besoeker]

I grew up with the FPS system, also called (in my neck of the woods) the "English Engineering System,"

As did I. But we called it Imperial, not English. It is not specifically an English system. I'm still pretty conversant with it and know most of the conversion factors between FPS and SI.
My father could wok out bushels per acre in his head and set his machinery accordingly.

I had to learn the MKS system..

Did that also but not until college. At school, it was CGS.
MKS was used in electrical engineering but on the mandatory mechanical engineering subjects it was still Imperial units.

My kids grew up with both systems, as will their kids.

For mine it was SI.

But when enough generations have been using both, I believe evolution will put the SI system on top, and the whole notion of feet, miles, pounds, and ounces will disappear completely. I have no objection to that happening.

For our profession and those like it, I have no objection. As I have pointed out here and in a few other threads, it has the merit of making calculations simpler.
But changing things like road signs from miles to km and speed limits from mph to km/h would seem to me to be a pointless, costly waste of money.
And in pubs here, we have retained the Imperial pint. And jolly good that it has been spared from the stupid bureaucratic nonsense that comes from the EU.

By the way, Besoeker, speaking as a resident of the UK, does it bother you that the nomenclature of "SI" has its origins in the French language?

Pas du tout!

Does it bother anyone else that the nomenclature of "SI System" is redundant? :happyyes:

My bad. Really sloppy on my part. I should say SI units.
Apologies for that.

 

[charlie b]

My bad. Really sloppy on my part. I should say SI units.

I am in no position to complain, since I used "SI System" also. :happyyes:

 

[Besoeker]

I am in no position to complain, since I used "SI System" also. :happyyes:

Still sloppy on my part.
I'm old enough to have known better.
I won't beat myself up over it.

And I think you are right in that we will gravitate internationally and globally to SI for science and engineering.
But I still don't want things like km imposed on us for applications where it provides no benefits whatsoever.

 

[mikeames]

I am just starting to get comfortable with the SI system and the only reason why I was not, was because of perspective. In my head I could picture the difference between a gallon and a quart but had no reference between a liter and a mili-liter. working with it more I have more of a perspective and so feel much more comfortable. The math perspective is a no brainier.

 

[Besoeker]

I am just starting to get comfortable with the SI system and the only reason why I was not, was because of perspective. In my head I could picture the difference between a gallon and a quart but had no reference between a liter and a mili-liter. working with it more I have more of a perspective and so feel much more comfortable.

Yes, I suppose that might be an issue for many/most new to SI units.
Mrs B is from GA and loves to cook but many of her recipes are in in Imperial measure so I do the conversions, either way round, as required.

Some of the conversions, if not too critical, can be done mentally.
For example, to get from kg to pounds, double it and add 10%. That's pretty close.
Litres to pints, double it and knock off 10%. Again pretty close for the UK pint.
An inch is about 25mm.
A foot about 300mm.
A metre about a yard plus 10%.

These, of course, are not precisely accurate but the errors are of the order of about a percent, so close enough everyday use.

The math perspective is a no brainier.

Kinda what I was trying to get at in posts #1 and #3.