OT eye candy

Yes, pretty. One might wonder what's wrong with m/s^2 that they have to resort to using yet another 'funny' unit.

Jeroen (SI everywhere, always) Belleman

If you google, moon gravity map, you can find the nasa webpage for this. It's pretty cool physics. Two satellites linked by RF interferometry.

I missed the units part. I often find the meter to be too big. I?ve been doing all these thermal conductivity estimates and millimeters are a nice size. (My brain doesn?t know how big 6e-4 m is, but 0.6mm is fine.) I then rewrite the thermal conductivity as mW/mm-K.

George H.

Using a prefix that's some exponent of 10^3 is within SI units. The centimeter isn't exactly SI, but at least cm/s^2 is fairly easy to get into SI units in one's head. It's certainly better than using a Pisan cloth yard/(lunar month)^2.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

mGal are the customary units for terrestrial gravity surveys.

SI has been invented with the express purpose to stamp out customary units, with the objective to have all of science, commerce and everything in between speak the same language when it came to measuring things. It's sort of sad that after two centuries, it's still impossible to reach an agreement on that, even within the physical sciences.

OK, at least it's derived from metric units.

Jeroen (Stamp out ergs, Gals, AUs, sols, horsepower, etc.) Belleman

The customary gravity gradient unit is the Eotvos (1 E = 1E-6 mGal per cm). ;-) or 1E-9 s^-2 in SI units.

Does anyone actually use Pascals for everyday pressure units? I think we use everything else but.. PSI/PSIA, Torr, bar, mm Hg etc.

most meteorology seems to be in hectopascal

I generally use bar, then theres those who work with refrigeration they use bar gauge which to me makes no sense since what they work with is generally hermetically sealed so atmospheric pressure doesn't make any difference

-Lasse

Of course, I also use customary units in everyday communication. No one would understand me if I didn't and anyway, I wouldn't be able to apply all conversions on the fly in a casual conversation.

But using a consistent set of units helps in comprehending nature. It exposes the links between phenomena and avoids silly errors. For example, expressing rocket motor performance in seconds is wrong because force and mass do not cancel. Messing this up gets expensive really quickly.

In writing, I'll always use SI.

Jeroen (kB = 13.8 yJ/K, 1 Torr = 133 Pa) Belleman

Yup, quite useful. Magnetics engineers, that aren't stuck in the dark ages with "lines of force" and the like, ;-) usually use 1T == 1 uWb mm^-2, A_e in mm^2, l_e in mm, mu_0 ~= 1.256nH/mm, etc.

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com

My calipers are in inches! (I just divide by 25)

George H.

text -

Yeah.. I use pascals. 1 atm = 10^5 Pa. It makes calculating mean free paths lengths in gasses easier.

George H.

tyre inflators read kPa and PSI

meteorologists use hPa ( same size as mBar, the Cheats) and occasionally kPa

I had a paint sprayer that had a gauge that read in MPa

--
?? 100% natural 

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

The metric system was a typical French Revolution idea, like so many others--very logical except when it becomes a monomania, as it inevitably does, being French. (Lobster Thermidor, anyone?)

It's very useful, but balance is necessary. It really isn't too important for astronomers to be able to communicate their results to microbiologists--it's far more important to have those within a field be able to communicate with each other, and spot errors easily. How many petametres is it to Alpha Centauri? What parallax would you measure? Those are a lot easier to communicate in AUs and parsecs. If you tried putting petametres in an astronomy talk, you'd get laughed off the stage.

Cheers

Phil "microns forever" Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Of course, I know. I also use electron volts rather than Joules and particles/bunch rather than Coulombs/bunch. But it's mostly just tradition and inertia. One could get used to the former as easily as to the latter. All these 'convenience' units harm understanding. I still vividly remember how much time it took me to see my way through Oersteds, Gauss, Teslas, Webers and whatnot, and the various constants associated with magnetics.

And astronomers are artists of obscurity. E.g., what had they been smoking when they chose km/s/MParsec for H0? Two different units of length in a single constant! How many would notice, off hand, that

1/H0 = age of the universe? And they would laugh at me if I would use meters to express distances? Sheesh!

Jeroen (Alpha Centauri is 41 Pm from here) Belleman

And lose over 15 mils each inch?

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

Be careful using the Spice Chan model. It uses meters and square meters.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

As damn well it should! It's perverse to have multiplier prefixes in both numerator and denominator. Seems to be a typical west- pondian malady.

Jeroen Belleman

I still use Gaussian units for electromagnetics, because the equations are simpler, and doing the unit conversions at the end is easy. A centimetre (1 cm =1.12 pF) is a much more intuitive unit for capacitance than a farad, because it's closer to normal experience. (Of course I do a lot more front ends than I do big beefy power supplies. Also inductance is a bit less wieldy in Gaussian units.)

My distaste for People Who Know Best extends past politics and into professional societies and standards bodies, e.g. the photometrists again, who like to redefine pre-existing terms to mean something totally different, and then scold folks who hold to the original and more common usage. ("Intensity" is one of the victims.) I'm old-fashioned enough to think that communication and not perfect consistency is what matters.

You aren't the target audience, after all. The idea is that with H0 in km/s/Mpc, you know what the mean recessional velocity goes up by about

70 km/s for each megaparsec of distance, which makes sanity-checking easy. Anyway, 1/H0 isn't the true age of the universe--it's a linear extrapolation.

Having units of a convenient size that are connected simply with things we work with all the time is a win. It's good if astronomers all over the world use a consistent set of units, but it's much less important that that be the same set as used by machinists and surveyors.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 USA 
+1 845 480 2058 

hobbs at electrooptical dot net 
http://electrooptical.net

ALL standardized Spice models use meters as the reference unit for length. It's been years, and only in HSpice crap, where I ran into microns. ...Jim Thompson

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