Engineering units

[Besoeker]

Some posts in a couple of threads prompted me to post on this.
In UK, and Europe as far as I know, metric (SI) units are widely used for engineering calculations. As it happens, I?m old enough to have been taught both Imperial and metric and I can work in either.
But metric is so much simpler for many calculations.

Take power, for example.
In Imperial, mechanical and electrical power use different units.
One HP is 550 ft-lbf/s.
Or 33,000 ft-lbf/minute.
And, if you want to do electrical calculations, you can convert it to Watts by multiplying by 746.
SI gives 1Watt = 1 Nm/s
And, for DC, 1 Watt is = 1V* 1A
No conversion factors required.

For motor torque?
Power is torque * rotational speed.
HP is 2*pi()*n*T/33000
n in rpm, T in ft-lb.
SI gives P=Tw
Again, no conversion factors required.

A couple of other observations in no particular order.
There is no equivalent Imperial measure for the Amp or the Volt.
You can buy an electric light bulb rated W but not in HP.

Doesn?t it make sense to rationalise power to one system of units?

 

[charlie b]

There is no equivalent Imperial measure for the Amp or the Volt.

Sure there are. They are called, respectively, the "amp" and the "volt." :wink:

You can buy an electric light bulb rated W but not in HP.

If you felt the need, as I do not, you could perform a unit conversion.

Doesn?t it make sense to rationalize power to one system of units?

Sure. Right after they standardize the world's languages, the world's monetary system, and (Heaven help us) the terms used by mechanical and electrical engineers. After all, the mechanical engineers are still foundering under the nonsensical notion that if you close something, it should stop flow! :grin:

 

[WDeanN]

There is no equivalent Imperial measure for the Amp or the Volt.

Ahhh, but you are backwards. Of course the Amp is Imperial, after all,
1 amp = coulomb per second = 6.25 E 18 electrons per second
If it was metric, 1 Amp would equal 1 electron per second, and we'd all have to speak of MegaKilo Amps just for a house service, and would have to get a 1.2 Gigawatt lightbulb! :wink:

You can buy an electric light bulb rated W but not in HP.

That's because over here, we don't use our lightbulbs to do work, although a lot of dim bulbs do work for the government...

 

[Besoeker]

Sure there are. They are called, respectively, the "amp" and the "volt." :wink:]

One ampere is defined to be the constant current which will produce an attractive force of 2?10?7 newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum.
SI Units, of course.
:wink:

 

[charlie b]

One ampere is defined to be the constant current which will . . . SI Units, of course.

That is how it is defined. But it's unit of measure is "one coulomb per second." A coulomb is the same in both SI and Imperial. So to is a second. Therefore, the unit of measure "amp" is both SI and Imperial.

The unit of measure "volt" is based on SI fundamental units (kilogram-meter squared per coulomb-second squared). But its value is the same in SI and Imperial.

 

[Besoeker]

That is how it is defined. But it's unit of measure is "one coulomb per second." A coulomb is the same in both SI and Imperial. So to is a second. Therefore, the unit of measure "amp" is both SI and Imperial.

The unit of measure "volt" is based on SI fundamental units (kilogram-meter squared per coulomb-second squared). But its value is the same in SI and Imperial.

Given that a Coulomb is defined in terms of Amperes and time, both fundamental units, you can't define an Ampere in terms of Coulombs.
The Ampere is the fundamental unit. And defined in SI units.

 

[drbond24]

Ahhh, but you are backwards. Of course the Amp is Imperial

The ampere is one of the seven fundamental SI units. It is metric.

length = meters
mass = kilograms
time = seconds
electric current = amperes
temperature = kelvin
amount of substance = moles
luminous intensity = candelas

I do not agree that the Amp is both imperial and metric. There isn't an imperial equivalent, we just use the metric units. That doesn't mean it is both.

 

[winnie]

And the second is an imperial unit that the SI system steals? Or does the second predate imperial units.

The best feature of SI is its attempt to be a self consistent system of units. That self consistency could have been achieved with _any_ system of base units, and if you look at the history of the 'metric system' you will find several competing systems that were finally resolved into SI.

There is nothing sacred about the 'meter' or the 'kilogram'. You could come up with a self consistent system using any set of base units. I've sometimes thought that the distance that light travels is 1/1,000,000,000 second would be a far better unit of length than the meter, which is the distance that light travels in 1/299,792,458 of a second. (Note: that is the actual current definition of the meter, not some randon number that I happened to derive from the length of the meter.)

-Jon

 

[drbond24]

And the second is an imperial unit that the SI system steals?

Nope. That one is the same in imperial and metric.

I was just stating my opinion about the ampere. It doesn't really matter, so long as we know what it means when we say 'ampere'. :smile:

The best feature of SI is its attempt to be a self consistent system of units. That self consistency could have been achieved with _any_ system of base units, and if you look at the history of the 'metric system' you will find several competing systems that were finally resolved into SI.

Yep. We (America) need to jump on board that ship as well. It is confusing to have both Imperial and Metric. We needs to just go metric and forget about feet and pounds.

 

[Besoeker]

There is nothing sacred about the 'meter' or the 'kilogram'. You could come up with a self consistent system using any set of base units.

You probably could, but then SI already exists.
My original point was that it often makes engineering calculations less complex.
That said, there is fairly strong body of opinion in UK opposed to metric units for general use and I have some sympathy with it. It?s seen by many as yet another erosion of UK sovereignty at the hands of the European Union bureaucrats.
There was a high profile case some years ago where a market trader was convicted allegedly for selling bananas by the pound. The press had a field day and the phrase ?metric martyr? was coined and has stuck in similar cases since.
Actually, what the trader, Steve Thorburn, did was to use an uncalibrated weighing machine.
If a seller and a buyer agree on how a product is to be traded, that?s a contract between them. State imposition of unfamiliar units seems to me to be inordinately heavy-handed.