When does the world go metric...

[lamont]

1 kelvin is defined as 1/273.16 of the triple point of water. one could define it as 1/whatever the triple point of any other element is and get the same thing to measure absolute 0, but the temperature scale (how much energy is in one "degree") would be different.

http://physics.nist.gov/cuu/Units/kelvin.html

Any measurement we make is "relative" to something. Hell, even the kilogram is still defined as "the weight of the standard," which is the platinum-iridium cylinder at the standards office.

Also consider the fact that we define the speed of light "per second," which is defined as:

The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Relative to the cesium atom is an arbitrary choice based on its stability / availability / whatever. What is an absolute scale?

And if the speed of light is not actually a universal constant and has changed over time and is currently varying that would change the EM spectra of cesium-133 slightly and change the frequency with which it transitions between those two levels, so that may not be absolute. Even without considering that, though, cesium clocks at different depths in a gravitational well or different space-time curvature will tick differently and disagree (which leads to the General Relativity corrections to GPS time since satellite atomic time drifts from atomic time at the Earth's surface).

Everything we measure needs to be with respect to some reference standard, and that reference standard is just an experiment which can be done by any scientists which agrees to a very high degree universally. So everything is relative to the characteristics of some physical phenominon like the triple point of water or the transition frequency of the hyperfine levels of cesium-133. Those standards both benefit from being precise and repeatable to a very high degree. The historical Farenheit scale is less precise and less repeatable because it was developed at a much earlier time and now it is defined and derived from the triple point of water instead of the experiments which initially defined it in order to give it the same precision.

 

[Blackwood]

1 kelvin is defined as 1/273.16 of the triple point of water. one could define it as 1/whatever the triple point of any other element is and get the same thing to measure absolute 0, but the temperature scale (how much energy is in one "degree") would be different.

What is an absolute scale?

You are relating K to C. 1K happens to be 1/273.16 triple point, but 0K is defined as the temperature at which something with no heat is. 0K = absolute zero, which happens to be -273 and change in C.

 

[lamont]

Really? To me, the only temperature scales that make logical sense are absolute (kelvin, Rankine, etc.).

It seems that basing the scale on properties of water is as arbitrary as basing it on the body (which I believe is how Fahrenheit devised his).

Oh, also, the only measurements which really make sense to me are in units of the planck distance, planck time, planck mass and planck temperature.

At a plank temperature of 1 is roughly where grand unification is and all broken symmetries in nature are restore -- it is rougly 1.41679 × 10^32 K

I'm not sure what 10^-33 is in SI units, but a 1.0 yocto-Thetas would still be 141,679,000 deg K.

Please convert all your temperature readings over to units of Planck Temperature since those are the only units which make any kind of universal cosmological sense to use.

 

[cfelliot]

I went to the Luxfer site to see how they spec tanks:

http://www.luxfercylinders.com/products/scuba/specifications/us_imperial.shtml

You can go between US, US-Metric, EU and Asia specifications.

 

[lamont]

You are relating K to C. 1K happens to be 1/273.16 triple point, but 0K is defined as the temperature at which something with no heat is. 0K = absolute zero, which happens to be -273 and change in C.

No, he's actually not. You've only defined the zero point, not the size of the unit. The actual size of the Kelvin unit is defined as 1/273.16-th of the temperature difference between 0 K and the triple point of water. Or the Kelvin scale is defined such that 0 K is absolute zero, while the triple point of water is defined to be exactly 273.16 K, and all other measurements are in reference to that.

 

[Chuffy]

I'm glad this thread stayed on topic.

Personally if find the idea of using al80, psi & feet quite bizarre. Everything diving related is so much easier in metric. Depth in meters makes ATM easier along with calculating SACs etc.

One thing that I would like to know is what hand signals do you use when discussing tank pressure underwater? I only know bar and the 'timeout' T sign for 100bar and then one finger for each bar.

It did get confusing when working as a DM in Thailand. Despite briefing hand signals some divers were obviosuly used to PSI and different hand signals. (All kit in Thailand was bar). I used to get some strange sign language that was completely alien to me!!!

 

[lamont]

I'm glad this thread stayed on topic.

Personally if find the idea of using al80, psi & feet quite bizarre. Everything diving related is so much easier in metric. Depth in meters makes ATM easier along with calculating SACs etc.

counting in threes to do your deco stops is hella annoying. one night after resetting the battery in my depth gauge i noticed that it was telling me i was at something like 327 feet (and me with only one single Al40 deco bottle and no bottom stages) until i noticed it was reading 32.7 meters... then on the way back my brain wasn't doing threes-times-tables very well right off the bat, so i did the stops at 21m and 18m by peeking at my buddies gauge that was in feet (70 feet and 60 feet) until I got on track at 15m, 12m, 9m, 6m, and 3m...

ATM from feet is also not a huge problem. and typically i just memorize things like at 100 fsw i have a typical SAC of 300 psi / 10 mins with my doubles... if you do all the stupid math up front, 100 psi / 10 min / ata is my usually planning SAC rate which keeps the numbers easy to get a conservative gas plan (300 / 10 min @ 4 ata is what i use to see if I'm on track with a halfway decent SAC...).

One thing that I would like to know is what hand signals do you use when discussing tank pressure underwater? I only know bar and the 'timeout' T sign for 100bar and then one finger for each bar.

just use hundreds of psi. so 25 is 2500 psi. some divers will use 2, 5, 0, 0 to signal 2500 but the gauges are barely accurate to 100s of psi so the extra two zeros usually get dropped.

 

[Blackwood]

No, he's actually not. You've only defined the zero point, not the size of the unit. The actual size of the Kelvin unit is defined as 1/273.16-th of the temperature difference between 0 K and the triple point of water.

And it is defined like that so that it neatly works with the centigrade scale.


Re: your other post: I'm not saying that I'm necessarily right, or that kelvin is necessarily easier to use that celcius or fahrenheit. I was speaking on a personal level. To me, it doesn't seem logical to call something 0 simply because it is the triple point of water, when an actual minimum temperature exists (however theoretical it may be).

 

[bluesbro1982]

I'm glad this thread stayed on topic.

Sorry bout the hijack. Maybe the feet/bar thing is a question of abstract vs. concrete. I can see / judge how far away something is, but pressure isn't something I can really see so its easier for me to abstract and switch units.

I'll bow out of this thread now and be content to

 

[lamont]

And it is defined like that so that it neatly works with the centigrade scale.


Re: your other post: I'm not saying that I'm necessarily right, or that kelvin is necessarily easier to use that celcius or fahrenheit. I was speaking on a personal level. To me, it doesn't seem logical to call something 0 simply because it is the triple point of water, when an actual minimum temperature exists (however theoretical it may be).

technically, the triple point of water is 0.1C (and defined to be exactly that).

i agree that kelvin is a better scale when you're dealing with thermal energy, but its all relative to what the problem is. farenheit is actually very nicely suited to human use because the range of temperature outside tends to swing from about 0 to 100F. celcius also makes good sense because water freezes at zero and boils at 100C which are relatively common things to do in the kitchen. in normal human use the distance of the temperature from absolute zero is totally irrelevant, so what utility would it give to force everyone to adopt that as the zero point of temperature? its about as useful as forcing everyone to adopt planck units of temperature.

until most people are having to add 273.15 all the time to convert to kelvin anyway it doesn't make a whole lot of sense to change the temperature scale. probably 99% of the population wouldn't recognize the number 273.15 as being special at all if you asked them, which indicates there's really no problem to solve here...