Metric vs Imperial

[DoMePlease]

Are the Americans going to adopt the metric system anytime soon. Just asking cause I thought it would be a safer world if everyone talked the same numbers. Some blunders have happends... most notably the crash of a satellite because of miscommunication between imperial and metric engineers.

Any chance the whole world will change to the imperial system? Thank you.

[OzExpat]

Any chance the whole world will change to the imperial system?

Considering the speed with which some countries out-manoeuvred the rest of the world, thru ICAO, to bring us the dreaded metric measurement system, I don't think I'd bet on any wholesale move back to imperial measurements. And, at this stage, I suspect THAT sort of change would be more likely to create greater confusion than the way the system is right now.

[criticalmass]

At least the imperial unit of the nautical mile has a natural relationship to the way we measure distances and positions on the globe (1 minute of angle in latitude is equal to 1 nautical mile, 60 nautical miles equals 1 degree of latitude etc.)

I understand the metric system is based on a (now known to be incorrect) measurement of the distance from Paris to the North Pole. With remarkable hubris, the French (and some collaborateurs) decided this would be the system of measurement for the whole world.

As far as I am concerned, the only decent thing about the metric system is it is all base ten so you don't have to take your shoes and socks off when counting! It's convenient but it has no natural relationship to the geometry of the earth. The continental system of dividing a circle into four right-angles each containing 100 "grads" is just about extinct...or should be :D

I am perfectly comfortable with either system. For cutting a piece of wood to length I'll use metric. For navigation (and aviation) I'll use the old system. If it comes to that, I'll even trust a sextant and compass in preference to a GPS receiver. The US DoD doesn't own the stars or the sun and has no control over the earth's magnetic field or magnetic variation.

[ORAC]

Criticalmass, you understand wrongly.

The definition of the metre was selected as 1/10 000 000 of the distance between the north pole and the equator.

One of the scientists appointed to survey the distance did fudge his figures, but only in such a way that the overall accuracy was maintained. This was caught and the other surveyor, in secrecy, reviewed and corrected the data and found the final data to be correct.

When they did the survey, the Earth was known to be an oblate spheroid, but the ellipticity was not known with certainty. A meridian distance for an ellipticity of 1/334 was selected. Satellite measures now put the ellipticity at 1/298·4, which means that the length of the metre is within 0·2 millimetres of the intended length. Not bad for the time.

Radians work well in calculus and formulas since no conversion factors are required.

[Squawk7777]

(This reply is from someone who was raised in the Metric system)

One problem I had with the "uneven" system, as I called it some time ago, was to recall e.g. how many feet were in a mile. Then there were to kinds of miles: the nautical mile and the statue mile. So 5280 ft and 6080 respectively. This never made sense to me. I'd rather prefer 1km equals 1000 meters. 1 meter = 100 cm etc. To make things worse I discover the yard, and then the gallon which also exists in two different versions. So for a metric guy, this can be quite a mental challange at the beginning.

...but thanks to the CAA I was given numerous conversion challanges

Speaking of the statue mile... I heard that this mile was "invented" or "requested" by Queen Victoria. Any truth in it?

7 7 7 7

DMP, I forgot to mention that some parts of Arizona, USA use the metric system. Drive on a road through the desert and between cacti you suddenly see signs like "Nogales 76km". The speed limits are still in miles though ...

[PaperTiger]

The statute mile predates Queen Victoria by several centuries. It derives from the Roman milia which is actually 'metric', being 1000 steps (passi). Bluddy big steps though at 5.28ft each, although the accepted value of a passus is 4.85ft. Don't know where the discrepancy comes from.

Victoria probably decreed that since the Empire encompassed most of the 'known world' ;) the British mile should be the universal value. Why they are called Imperial Measures I imagine.

The nautical mile we use is an arbitrary value too, being only accurate along the equator, viz. "the length of an arc of meridian that subtends an angle of one minute at the centre of curvature of the place". In other words one minute of latitude, measured at your latitude on the Earth's surface, equals one Nautical mile and will be less than 6080ft except at 0deg long.

And we won't mention the German mile which equalled 4 equatorial nm.

[canberra]

ive no problem with either metric or imperial in fact i prefer imperial, the problem in aviation is that we use both systems! we give height and altitude in feet distances in nautical miles except for the old berlin control zone which had a radius of 20 statute miles. the reason for the berlin zones difference was due to field marshal montgomery. but to continue we give the met vis in metres and kilometres wind speed in degrees and knots and the temperaure in degrees celsius. aand the army use mils to direct their artillery, no wonder people get confused!

[Squawk7777]

hm, one more thing....

Where and what's the UK gallon (=4.54609 litres for the ones who care) used for? As far as I remember the petrol stations in the UK use litres. I guess this is done to make the fuel look cheap ... :o

7 7 7 7

[Straight Up]

I tend to use a mixture of both systems depending on what I'm measuring.

Though what confuses me is the different types of nautical miles.
Theres a UK Nm, International Nm and a NATO Nm. :confused:

I UK Nm = 1.00064 International Nm = 1.01333 NATO Nm

This caused a fair bit of confusion when checking over a colleagues calcs that converted from ft/sec to knots as there three different types of Knots to go with the nautical miles.

I spent all day reviewing a very large spreadheet only to discover it was the wrong bleeding conversion factor. :mad:

[bookworm]

The nautical mile is a minute of arc along a meridian (i.e. a minute of latitude). If the earth were a perfect sphere, all would be well, but unfortunately it's not, so the length of a minute of arc depends on where you measure it.

At about 45 deg N, a minute of arc is almost exactly 6080 ft, so that was the Admiralty (UK) definition. The ICAO definition actually uses a whole number of metres (1852) instead as a definition, which comes out about 4 ft shorter.

I've never come across a NATO nm. Do you have any further info or references on the definition?

[Four Seven Eleven]

Criticalmass

The US DoD .......has no control over the earth's magnetic field or magnetic variation.

I guess you haven't heard about the top secret work being done by the US DoD Earth Sciences Division in Massachusetts. They have been working on a device.....

Hang on, someone's at the door, I'll be right ba............................
............................................................ ..

[Genghis the Engineer]

The organisation I work for prefers to do aircraft W&CG calcs in kg.inches.

Apart from that, I fly in knots and feet, do calculations in m, N, and m/s.

To quote the immortal Darrol Stinton "Be ambidextrous in units".


G

[Flash2001]

Gengis

kg*inches?? BAD MOVE!

Any organization that mixes its systems like that ought to be drowned slowly in its own excrement. That's right in there with the EPA with the grams per mile emission requirements.

The United States passed a metric conversion act sometime around 1900 but it didn't get off the ground.

The nice thing about the SI subset of the metric system is that most constants drop out except for the horribly inconvenient electrical and magnetic characteristics of free space.

Canberra

Yeahbut...

The mil is a natural unit of measurement (milliradian) and is real handy for directing gunfire as for small angles the angle is equal to its own sine if radian measure is used. Thus at a range of 1000 whatevers, traversing the gun 10 mils moves the point of impact 10 whatevers. The radian and its subdivisions, being dimensionless, leap back and forth among systems of measurement gleafully removing the dreaded 2*pi from many equations involving rotation.

[A Very Civil Pilot]

Sq 7777

A Gallon is 1/9 of a firkin.

A firkin is a small beer cask, holding nine Imperial gallons (72 pints).

Two firkins make a kilderkin

Two kilderkins make a barrel

One-and-a-half barrels make a hogshead of 48 gallons (384 pints).


I hope this clears it up.

(I copied this from another site, and looking at it the maths doesn't add up! Not sure if a hogshead is 48 or 54 gallons)

[Captain Stable]

Personal preferences are one thing (I have great difficulty thinking of my petrol consumption in the car in anything but miles per gallon).

In aviation, I can see excellent reasons for using knots and nautical miles. I see little reason why we should not convert to altitude in metres and windspeed in metres/sec.

I dislike fuel gauges in lbs. You tend to run into problems with ordering fuel in pounds, getting it on the bowser driver's sheet in litres, accounting for it on the trim sheet in kilos, and then being back in pounds for consumption purposes. Remind me - where IS Gimli? ;)

Kilos (or tonnes) and litres for fuel makes total sense. Weight (kgs) = Volume (litres) x S.G.

[ORAC]

I think this should solve any of your conversion problems:

Convertor (http://www.convertit.com/Go/GovCon/Measurement/Converter.ASP)

[Genghis the Engineer]

Why pray tell Flash is there a problem multiplying any convenient force unit by any convenient distance units.

It happens that, for small aircraft with a datum somewhere around the middle, if you use kgf", you never need more than 4 digits and can ignore anything past the decimal point - it happens to work rather well. Confuses a few people I'll grant, but technically perfectly sound.

Personally I save my own ire for people who fly aircraft in mph, or the Swedes where they have combined military:civil airports, but civil traffic work in feet, and military in metres. Now that is seriously confusing.

G

[5milesbaby]

No-one has mentioned how different ATC's worldwide use either feet or metres for vertical separation, and I think we all know how much danger can ensue from this confusion with conversion.

[Flash2001]

Gengis

Yeah, I used to do energy calculations in stone-barlycorns just to wind the physics professor up. However I stop at using kgf, let alone mixing systems. There's a perfectly servicable unit called the newton. If you only ever do one sort of calculation I s'pose you can use any units you want that are dimensionally correct. If you do a lot of calculation, best to stay within 1 system. I don't much care which one, I am equally uncomfortable with SI, BES and the various colours and flavours of imperial. Little problems with measurement systems can cause incorrect separation between spacecraft and planets, aircraft and Winnipeg etc.

[bluskis]

Anyone who has been involved with science or technology for more than 30 years may have noticed that so called metric units change with monotonous and confusing regularity.

At least so called imperial units remained more or less fixed, except for the errors the Americans introduced into the system that is.

Decimalisation of imperial units within the system worked fine, thous of an inch for precision, or tenths if really precise, or decimals of a foot for road buildingetc etc.

Ever heard the confusion with metric users when confronted with dc/cl/ml etc etc, and aren't millibars now some derevision of hectopascals, or are they the same thing?

With another half dozen or so nations being introduced into the EC, I expect each will require a unit to be named after their favourite scientist. Stand by for yet more post rationalisation additions to our unit vocabulary.

Just because Napoleon was a lousy mathematician, we have a metre which is not a metre.

Make mine a pint, and back to the Romans.

[411A]

Genghis the Engineer,

So then, by you're reasoning, the aeroplanes that were originally certificated in MPH are therefore ah...void?
The list includes,
DC-3
DC-4
DC-6
DC-7
Lockheed L-49 series
B-377
most light aeroplanes pre 1970 (give or take),

Well, the list goes on...and on.

Are these aeroplane flight manuals now to be ignored, just because they are in (shock, horror) MPH?:rolleyes:

[compressor stall]

Wasn't the metre defined as 9 million something wavelengths of light reflected off the atom ceasium to create a standard?:confused:
..............

Nope, I was wrong, that;s for the second. Links to these (admittedly Americanised?) standardisations are here (http://www.unc.edu/~rowlett/units/dictM.html#meter)

And the three nautical miles are dicussed here (http://www.unc.edu/~rowlett/units/dictN.html#nautical_mile)

[OzExpat]

I have the same problem faced by Capt S and many, many others... fuel flow in PPH, loadsheet in KGS and fuel from the tanker/bowser etc., in litres. I find that it pays to keep fresh batteries available for me calculator! :eek:

[pulse1]

As bluskis says, those of us who have been involved in science and technology for more than 30 years, before calculators were readily available, are probably less impressed by the loss of the convenience of some Imperial measures just for the lazy advantage of the metric system.

Years ago, converting ounces/gallon (US or Imperial) to g/l, thou's or mils to microns, farenheit to centigrade etc, one became so adept that we would use whichever measure was most convenient for the task in hand. The exercise encouraged us to mentally check our results, ie. does the answer make sense? For precise measurement the metric system is probably the best but, when it comes to estimations, a task regularly required by pilots who should always challenge any information coming from their calculator/computer, imagination plays an important part.

As the Imperial system developed from useage, all measurements have, at some time or other, had some meaning which makes imagination easier. OK, many of the useages have changed and we no longer need some of them (e.g. poles & perches).

As an example, I can easily imagine a pressure of 14.7 lb/sq.in. - 15 bags of sugar on a surface area measuring 1 in. x 1 in. For the life of me I cannot begin to imagine what 1013 hectopascals would feel like. When I am setting my altimeter, imagination is not important, but when I am checking my tyre or oil pressures, I prefer to think about 28 p.s.i. than 1.9 bar.

[john_tullamarine]

I must have missed something here ... doesn't everyone work in slugs, poundals, and ergs ?

While the historical development of such things, in itself, is interesting, the multiplicity of units systems is the stuff of errors and disasters, especially for the new chums with their fancy GIGO calculators and computers and no mental checks as to reasonableness of the answer ... resurrect the slide rule ought to be the phrase of the day ....

Then again, if it had not been thus, I might have had to do something else as an undergraduate other than master converting from one set of units to four hundred and seventy three other sets .... a skill at which all engineers tended to become quite adept ....... ah, those were the days ...

[shaky]

All these new measurement systems are a mystery to me. When my aged Land Agent does the annual survey of the Shaky Towers landholdings in order to calculate the tithes he does so using the only sensible system which, of course, is the English one:

1 Rod (or Pole or Lugg) = 5.5 Yards
1 Chain (or 100Links) = 22 Yards
10 Chains = 1 Furlong
8 Furlongs = 1 Statute Mile

It is then a simple task to convert these linear measurements into areas:

1 Furlong X 1 Rod = 1 Rood and
4 Roods = 1 Acre

Simple and completely logical.

Regards

[Flash2001]

OBTW

The one place where the metric system is not, repeat not, simple is when screw threads are cut on a lathe. Metric threads are different not only in measurment, but in dimension. Even the Whitworth system is dimensioned in units length^-1. Metric threads are dimensioned in units length. Thus the threading indicator which is a simple pinion gear driving an 8 point dial on an inch lathe turns into something like a beehive on a metric machine. You get that setting wrong at your peril. The tool breaks or the workpiece ejects.

The scientific and engineering communities and the arbiters of the S.I. discourage the use of prefixes that don't describe a increment of 10^3 one way or another in the multiplier. In other words no deci, centi, deka etc. Thus it was perfectly consistent that avaition authorities adopted the hectopascal. Go figure!

[bluskis]

The metric solution of sealing a thread is to use a sometimes effective face seal, assuming they manage to use an appropriate seal polimer with a suitable machining finish and a closing torque..

The imperial/yank version of dryseal threads manages to achieve the same goal by precision engineering of well designed shapes and tolerances.

[Genghis the Engineer]

For that matter the Spitfire was certified in mph too; but I don't have to like the units. To my mind mph are one of the most dangerous units going, because they are so similar to knots (which so far as I know, most pilots are tought to navigate in) - but 15% error when, say, crossing the Atlantic, could be mildly inconvenient.

Bottom line is, always state clearly what units you are using. Personally I do:-

Moments in Nm, except aircraft weight and balance, which is kgf.in.
Height in ft, except when doing calcs, when I use metres.
Rotational inertia in kg.m², except propellers, when I use kg.cm²

etc. etc.

There is no real standardisation, and probably never will be - witness the teaching in primary schools of the very non-SI centimetre. What we need is an acceptance of that fact, and teach people to know what unit they're using, and switch when needed or appropriate.

G