units of measurement on altimeters

In misc.metric-system Jukka K. Korpela wrote:
"Julian Scarfe" wrote:

How can you possibly suggest that it would be more "practical" to
use kPa?

Because one digit less is needed, and conversions are easier when
powers of 1,000 are used as normally.


No, you need the same number of digits, and a decimal point in
addition. A pressure difference of 1 hPa corresponds to an altitude
difference of 8 m at sea level. That is just enough precision, but 80
m (corresponding to 1 kPa) would be intolerable. Pilots are usually
required to keep an assigned altitude to within +- 15 m (50 feet).

--
Klaus Wacker
Experimentelle Physik V http://www.physik.uni-dortmund.de/~wacker
Universitaet Dortmund Tel.: +49 231 755 3587
D-44221 Dortmund Fax: +49 231 755 4547

"Julian Scarfe" wrote:

How can you possibly suggest that it would be more "practical" to
use kPa?

"Jukka K. Korpela" wrote in message
. ..

Because one digit less is needed, and conversions are easier when
powers of 1,000 are used as normally.

I forgot to mention in my response, BTW, that the same number of digits *is*
required. Aviation applications require a precision of 100 Pa in measured
pressures. Your choice is between 1013 hPa or 101.3 kPa. By adding the
"daycimal", you simply make it more difficult for pilots to say.

Julian Scarfe

"Julian Scarfe" wrote:

I forgot to mention in my response, BTW, that the same number of
digits *is* required.

It depends on the quantities. I was referring to the most common
quantities that people see expressed. When tagging isobars in weather
maps, the trailing zero is just a nuisance. And when more accuracy is
needed, it is natural to accept that fractions might be needed.

Your choice is between 1013 hPa or
101.3 kPa.

You just gave one more reason to favor kPa. The numeric value 1013 is
not in the recommended range, and it raises the question of a thousands
separator, which is language dependent, so that some cultures would use
1 013 (and would need a no-break space to prevent undesired line
breaks, and an en space to avoid too wide a gap, and cannot get both)
while some would use 1'013 or 1.013 or 1,013. Situations where the
quantity will be taken as a thousand times too small would be quite
rare, but the damage could be serious, so why take the risk.

--
Yucca, http://www.cs.tut.fi/~jkorpela/

On Sun, 7 Mar 2004 17:42:25 -0000, "Julian Scarfe"
wrote:

"Jukka K. Korpela" wrote in message
...

In practice, using hPa means being just _nominally_ metric, i.e. using
actually millibars but under a different name. The odd thing is that
the correct kPa would be more practical.

How can you possibly suggest that it would be more "practical" to use kPa?
There is an installed base of tens of thousands of altimeters in aircraft
out there that are calibrated in mbar. Describing them as hPa makes it
clear what the unit is for someone familiar with the SI, without risking
accidents through unit confusion.

The preference to use powers of 1000 is just a preference because
practicality and pragmatism is sometimes more important than an arbitrary
recommendation. This is a perfect example of where pragmatism should (and
does) win.

Oh, good grief.

Does anybody use feet for altitude?

Do all of the analog instruments show all the digits in those feet?


Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/

"Gene Nygaard" wrote in message
...
On Sun, 7 Mar 2004 17:42:25 -0000, "Julian Scarfe"
wrote:

"Jukka K. Korpela" wrote in message
...

In practice, using hPa means being just _nominally_ metric, i.e. using
actually millibars but under a different name. The odd thing is that
the correct kPa would be more practical.

How can you possibly suggest that it would be more "practical" to use
kPa?
There is an installed base of tens of thousands of altimeters in aircraft
out there that are calibrated in mbar. Describing them as hPa makes it
clear what the unit is for someone familiar with the SI, without risking
accidents through unit confusion.

The preference to use powers of 1000 is just a preference because
practicality and pragmatism is sometimes more important than an arbitrary
recommendation. This is a perfect example of where pragmatism should
(and
does) win.

Oh, good grief.

Does anybody use feet for altitude?

Do all of the analog instruments show all the digits in those feet?


An just to add to the confusion our aircraft uses pieze for manifold
pressure

Does anybody use feet for altitude?

Everyone except the Former USSR and China, as near as I can figure.

all the best -- Dan Ford
email: (requires authentication)

see the Warbird's Forum at www.warbirdforum.com
and the Piper Cub Forum at www.pipercubforum.com

On Sun, 07 Mar 2004 17:33:47 -0500, Cub Driver
wrote:


Does anybody use feet for altitude?

Everyone except the Former USSR and China, as near as I can figure.

bravo--you were able to answer the rhetorical question part.

What about the rest of it?

Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/

"Gene Nygaard" wrote in message
...

The preference to use powers of 1000 is just a preference because
practicality and pragmatism is sometimes more important than an arbitrary
recommendation. This is a perfect example of where pragmatism should
(and
does) win.

Oh, good grief.

Does anybody use feet for altitude?

Do all of the analog instruments show all the digits in those feet?

Pilots have become accustomed to using feet for altitude. Can you imagine
the confusion that would arise if the unit suddenly shifted by a factor of
10?

"Descend and maintain 300 decafeet"

Any room for confusion there?

We already have trouble enough with pilots screwing up inHg to mbar
conversions. What does an altimeter setting of "nine ninety two" mean?

As a physicist and a pilot, I'd rather live with the inconvenience of using
a hecto prefix for stuff that comes across my desk than risking confusion in
the cockpit. So would ICAO.

Julian Scarfe

"Julian Scarfe" wrote:

We already have trouble enough with pilots screwing up inHg to mbar
conversions.

So while you take the trouble, wouldn't it be best to move to something
that lasts, due to being part of a system that is meant to be applied
in all areas of life when expressing physical quantities? It's better
to move directly to the SI system as recommended in general, rather
than first moving, say, to a partly decimal-based variant of the Anglo-
Saxon system, or - to take an example about different quantities -
first move from the use of different gallons to a Unified Gallon, then
to hectoliters, later to what the SI system really recommends.

--
Yucca, http://www.cs.tut.fi/~jkorpela/

In misc.metric-system Julian Scarfe wrote:
Pilots have become accustomed to using feet for altitude. Can you imagine

As a european pilot I would prefer meters.
But how to rearrange separation standards, any ideas?

--
--Peter Hermann(49)0711-685-3611 fax3758
--Pfaffenwaldring 27 Raum 114, D-70569 Stuttgart Uni Computeranwendungen
--http://www.csv.ica.uni-stuttgart.de/homes/ph/
--Team Ada: "C'mon people let the world begin" (Paul McCartney)