units of measurement on altimeters

"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.

There is an installed base of tens of thousands of
altimeters in aircraft out there that are calibrated in mbar.

This is not about calibration, this is about expressing physical
quantities. Besides, if the installed base of equipment were decisive,
each of us would still use one's country's inch, pound, or whatever
local measures were in use long ago.

Describing them as hPa makes it clear what the unit is for someone
familiar with the SI, without risking accidents through unit
confusion.

Would it be clearer to use a non-recommended prefix than a recommended
prefix? Besides, your argument indicates a fundamental confusion. There
is only one SI unit of pressure, the pascal (Pa). That's part of the
beauty and practicality of the system. All the rest that is used to
express pressures relates just the way of expressing the numerical
value. For convenience, we can use multiplier prefixes of _the_ unit if
we like, or a multiplier of the number, consisting of a power of ten.

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.

The reason for preferring powers of 1,000, explicitly expressed in
several recommendations and standards, is its practicality, based on
the use of the system as a whole. If you take arbitrary special
aspects, you can always find arguments in favor of using non-SI units
or non-recommended SI expressions - but then you lose all the benefits
of a unified system. Using hPA is a half-hearted "solution" that
combines the trouble of transition (after all, it needs to be
introduced to people who didn't know it, and they need to be reminded,
and some people will inevitably misunderstand or forget) with the
effect of gaining almost nothing. (We _can_ convert millibars to
pascals too.)

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

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

Describing them as hPa makes it clear what the unit is for someone
familiar with the SI, without risking accidents through unit
confusion.

Would it be clearer to use a non-recommended prefix than a recommended
prefix? Besides, your argument indicates a fundamental confusion. There
is only one SI unit of pressure, the pascal (Pa). That's part of the
beauty and practicality of the system. All the rest that is used to
express pressures relates just the way of expressing the numerical
value. For convenience, we can use multiplier prefixes of _the_ unit if
we like, or a multiplier of the number, consisting of a power of ten.

I'm not sure where you believe the "confusion" lies. Describing the unit as
hPa rather than mbar makes it clear that the unit is Pa and the prefix,
which is a standard SI prefix, gives the multiplier.

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.

The reason for preferring powers of 1,000, explicitly expressed in
several recommendations and standards, is its practicality, based on
the use of the system as a whole. If you take arbitrary special
aspects, you can always find arguments in favor of using non-SI units
or non-recommended SI expressions - but then you lose all the benefits
of a unified system.

Do you really believe that you lose *all* the benefits of a unified system
by using a prefix described (without deprecation, BTW) in the SI Brochure?

Using hPA is a half-hearted "solution" that
combines the trouble of transition

One man's half-hearted solution is another's essential compromise. :-)

Julian Scarfe

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/

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.


I think this depends on the context of usage. In aviation, I think the
thousands separator would be omitted most of the time, it's primarily a
convenience when you have lots of digits, and four isn't "lots". As for units,
it depends on what you are integrating with. In aviation, you are integrating
with nothing, so you could measure in quattloos for all it matters. It is in
engineering, where many conversions and calculations are taking place, that the
units need to fit into a system and kPa would be preferred.

I live with meters, millimeters, and centimeters just fine. And
(interestingly) in aviation, I live with hundreds of feet and thousands of feet
just fine too. (I flight plan in thousands, such as 4.5K for 4500 feet, but
weather comes in hundreds, as in 45 for a cloud layer at the same alititude. I
kinda wish it were more consistant, but only kinda. Each system has its place.

Jose

--
(for Email, make the obvious changes in my address)

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

But I think you forget where you came into this, Jukka. The thread is
entitled "units of measurement on altimeters". The quantities that need to
be expressed are in the approximate range of 970 to 1040 hPa, with a
precision of 1 hPa. The hPa is the right unit for that job.

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.

In context, the need for a thousands separator is not great, is it?

Julian

"Julian Scarfe" wrote:

But I think you forget where you came into this, Jukka. The thread
is entitled "units of measurement on altimeters".

It's part of the very idea of the SI system that a single unit is used
for each physical quantity, in a unified manner, not varying the system
by application, country, or phase of the moon. It is clear that the
system is not always optimal when judged from a narrow perspective of a
specialized field, but if we go that way, we'll end up with expressing
quantities in incompatible ways - there's _always_ at least some reason
to deviate from a system.

The pascal is a very small unit in many areas of everyday life,
technology, and science. This is handled, as usual in the SI system,
using a systematic set of multipliers that correspond to powers of
1000, so that the numeric values can be scaled to a reasonable range,
[0.1, 1000). In some situations it might be, at least due to historical
reasons, marginally more convenient to use 100 or 42 as a multiplier.
But that's not a good approach. (It is true that some additional
multipliers exist in the SI system. But this is due to historical
reasons and discouraged in many standards, and tends to create
confusion because prefixes like h or da are not widely known outside
some specific areas of application, like the hectare.)

The quantities
that need to be expressed are in the approximate range of 970 to
1040 hPa, with a precision of 1 hPa.

It's against the principles of the SI system to select units according
to the range and precision that you have in some special situation.
We don't invent new units every time we encounter a new situation.
That was the old way.

Quantities in the range 97 kPa to 104 kPa can easily be expressed to
any precision you need or the current technology permits. Surely people
who work with such things can be expected to be able to work with
numbers with a decimal part.

(If it becomes relevant to work with a precision of 50 Pa, would you
insist on inventing a unit that equals 50 Pa, so that you can keep
using integers only? What about 42 Pa?)

The hPa is the right unit for that job.

No, the hPa is not a unit in the SI system, any more than 100 Pa is.

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

It's part of the very idea of the SI system that a single unit is used
for each physical quantity, in a unified manner


This is fine and well while you're sitting in an armchair. But in the real
world there are sometimes compelling reasons to do something different from
the way a machine might handle things. In the case where

1: Not much interfacing with other units is involved
2: Rapid and accurate organic processing of the numbers is essential,
sometimes in adverse conditions.
3: Communications is suboptimal
4: A narrow range of values is involved

I'd say that it makes sense to use whatever units are most convenient in that
case, not whatever would make some world standards body twinkle its toes.
Altimeter settings are such a case.

Jose



--
(for Email, make the obvious changes in my address)