Talk:Vienna Standard Mean Ocean Water

Temperature formating and other writing conventions
The name “kelvin” is not capitalized unless it begins a sentence. With the exception of the kelvin, all the scale symbols are immediately preceded by a ° (degree) symbol. Note also that a space is inserted between the value and the degree symbol (or in the case of the kelvin, before the “K”. Only angular measurements such as 180° have the degree symbol placed immediately after the value.  Also, the scale's symbol is the always capitalized; thusly, we have…

The freezing point of water (expressed as kelvins) is 273.15 K The freezing point of water (expressed as degrees Celsius) is 0 °C The freezing point of water (expressed as degrees Fahrenheit) is 32 °F and The freezing point of water (expressed as degrees Rankine) is 459.67 °R

Remember that it is important to capitalize the kelvin symbol (K) because the lowercase version (k) is the SI prefix for 1 x 103

Various reasons can be found for why the name “kelvin” is lowercase and its symbol, “K,” is not preceded with a ° (degree) symbol. For instance, some sources say that kelvin has no degree symbol because it has the distinction of being an absolute scale. However, the definition of “degree” is simply “a unit division or step” so it doesn’t matter where a temperature scale’s null point is. Furthermore, the symbol for Rankine (also an absolute scale) is properly preceded by the degree symbol. There are two simple reasons for why the unit name kelvin and its symbol are written as they are: Notwithstanding the naming convention historically used for the other temperature scales, SI unit names are never capitalized. Also, the 13th General Conference on Weights and Measures (CGPM) in 1967 declared that the kelvin symbol (K) would no longer be preceded by a degree symbol because kelvin would no longer refer to the entire scale; it would thereafter be recognized as referring to an individual unit comprising the scale. Thus, the numeric value preceding the unit symbol would simply specify “how many kelvins there are” (just like 15 km or 4 µg). This is why the kelvin is the only unit measure of temperature that may be written in its plural form (e.g. “…is 12 kelvins greater than…”) in the same fashion as with all the other SI units such as mass (e.g. “…is 12 grams greater than…”). Thus, since kelvin has not referred to the entire scale since 1967, it is not appropriate to refer to a degree-related position on it, nor is it proper to precede the “K” with a ° (degree) symbol.

For more information on how the full word “kelvin” is not capitalized, see this article by the NIST as well as Wikipedia's kelvin article.

For more information on conventions used in technical writing, see the informative SI Unit rules and style conventions by the NIST.

Still more information on SI (International System of Units) conventions from the NIST can be found here.

There is one notable exception to SI writing conventions in this article: the use of “PPM”. For scientific papers, PPM is discouraged by SI and the NIST. However, PPM is commonly used in engineering, is unambiguous for the intended audience, and the merits of its usage offsets the alternatives suggested by the NIST (which would seem cryptic and cumbersome for many readers).

Why do you see the mixed use of the kelvin and Celsius scales?
In science and engineering, the Celsius and kelvin scales are often used simultaneously in the same article. This is because Celsius is used to express a specific temperature (e.g. “The melting point of gallium is 29.7646 °C.”)

The kelvin scale, on the other hand, can be used two ways. It is uniquely used to express differences between temperatures and their uncertainties (e.g. “The error is 10 mK,” and “Agar exhibited a melting point hysteresis of 25 K.”) And of course, the kelvin may also be used to express specific temperatures along its absolute scale (e.g. “The triple point of hydrogen is 13.8033 K.”)

Celsius is not used to express differences between temperatures or their uncertainties. In part, this prohibition eliminates ambiguity. In a chart, the value “25 °C” would be confusing if it was actually intended to represent a difference between two temperatures.

Typography hints
In any document that has variable-width pages and where word-wrap can occur at almost any point on a line, try using a hard-space (option-space on a Mac) between a temperature value and its degree or scale symbol. Thusly, you will obtain: “A temperature of 99.987 °C at the end of a line instead of 99.987 °C, which is fragmented and is harder to read.”

“Typographers quotes” should be used rather than simple "typewriter quotes."

For further information on writing style and typography issues, see Wikipedia’s article Manual of style.

VSMOW "water"?
Saying "VSMOW water" throughout the article seems to be a bit redundant, as it says, in effect, "Vienna Standard Mean Ocean Water water." Should we eliminate the "water" after VSMOW? --Carl (talk 04:05, 14 September 2006 (UTC)
 * A valid observation. It is redundant.  But this is the way VSMOW water is referred to.  There are other examples of accronyms usage that results in a techncial redundancy (I can't think of an example at the moment), but they do exist.   Greg L 21:37, 17 September 2006 (UTC)


 * For some reason we tend to say "LED light" which, again, is redundant. [Greg W] 23 April 2008.

Freezing Point
Braindrain0000: I like the revisions you made to the second–to–last paragraph of the VSMOW in temperature measurement section. They make for more fluid reading.

There were several technicalities that weren’t correct or were misleading when the last paragraph of VSMOW in temperature measurement said “The differences between water’s classical melting and boiling points and those of calibrated VSMOW water have little practical meaning in real life, as environmental factors may alter these values by a greater amount. For example, an altitude change of only 28 cm (11 inches) causes water’s boiling point to change by a millikelvin.” (Go to current paragraph.)
 * 1: Calibrated VSMOW water is an unusual term. VSMOW water automatically refers to water of certain isotopic composition.,
 * 2: “Environment factors” is plural. However, there is only one environmental factor that influences the boiling point of water: pressure.
 * 3: The temperature coefficient of water’s ice point looks like a brick wall when the phase/temperature graph is scaled to visualize the boiling point. Huge pressure changes are required in order to observe pressure’s effect on the freezing point.  Accordingly, the paragraph was revised to focus correctly on the only technical issue at play: pressure's effect on water's boiling point.
 * 4: This revision also helps to avoid implying that the 16.1 mK difference is due to standardizing on a particular isotopic composition. VSMOW water is simply a definition intended to produce an “average” ocean water.  Even if someone used water made from melted snow to delineate the triple point in a particular experiment, the error would only be a fraction of one millikelvin.  The entire 16.1 mK difference is due to the current values chosen to define absolute zero and the triple point for the Kelvin and Celsius scales.

In order to make 0 °C and 100 °C once again be the melting and boiling points of water (VSMOW water to be precise), absolute zero would have to be re-defined as -273.1940 °C, and the triple point would have to be defined as 0.0104 °C (as opposed to the current values of –2373.15 and 0.01). Greg L 21:56, 17 September 2006 (UTC)


 * While I know that pressure is the only technical factor on water, I was referring more to the layman's definition of "environmental factors," which would include the physical conditions that influence pressure. --Carl (talk 23:39, 17 September 2006 (UTC)

Precision of the definition of the kelvin
I have two points:

1. The kelvin being defined by way of the triple point of VSMOW, it seems that VSMOW should be perfectly well defined itself. But the definition is given with uncertainty margins. There is a contradiction here.

2. Since water in equilibrium with its vapor (or with ice) will have a different isotopic composition than that vapor (or ice), it is not enough to say that the triple point used to define the kelvin will be that of VSMOW. Will it be the liquid phase, the vapor phase, or the ice phase, that will have the isotopic composition specified as VSMOW? Perhaps in practice the specification is interpreted as referring to the total composition of all the water in the vessel used to determine the triple point; but then the results will vary depending on how much of this water is in each of the three phases at the moment of determination.

David Olivier 22:20, 15 October 2006. (UTC).

No Salt, Right?
I'd imagine, given the use in high accuracy measurements, that VSMOW is (supposed to be) pure H2O, without any dissolved materials in it. I bring this up, because the incorporation of "ocean water" in the name. The first thought that comes into my mind when people are making an ocean water/non-ocean water distinction is the presence of salt. It took me a while to come to the understanding that the "ocean water" distinction is referring to the fact that isotope ratios are different in ocean water that's been purified, versus rainwater that's been purified. -- 19:22, 1 December 2006 (UTC)

What does "(10)" at the end of the values for the difference between the triple point of VSMOW and the melting point of ice, and the actual melting point of ice mean?
Excerpt from actual article:

However, current measurements show that the triple and melting points of VSMOW water are only 0.009 911(10) °C apart. Thus, the actual melting point of ice is +0.000 089(10) °C.

Thanks Confusedreader (talk) 17:13, 22 January 2008 (UTC)


 * It is a shorthand for 0.009 911 ± 0.000 010 °C. See Uncertainty. --62.224.94.221 (talk) 02:36, 20 March 2008 (UTC)

In the section where the development of SMOW is described, it may be misleading to describe the material as resulting from distillation of ocean water, since ordinary distillation would change the isotopic ratio. To make SMOW, one must distill all the water in a sample of ocean water, including the very last bit--not the typical procedure for a distillation. Special procedures are required.63.226.222.247 (talk) 05:58, 29 June 2011 (UTC)

Why Triple Point?
What was the reason that the triple point was chosen to define the Kelvin rather than freezing point at standard pressure? Is it that the triple point is easier to define or what? XinaNicole (talk) 01:38, 19 October 2012 (UTC)
 * It is because the triple point is a propery of water only.  The freezing point requires an arbitrary standard pressure. Palter5 (talk) 18:01, 21 January 2015 (UTC)
 * That is not quite true. Other materials also exhibit a triple point.  For example hydrogen has a triple point of 13.8033 K.  Using the freezing point is problematic because the freezing point of water varies with applied presure (recall that the pressure applied by the weight of a skater on ice is enough to lower the freezing point of the ice immediately under the skate).  The triple point is a fixed point for any given material.  For a material to exist as a solid, liquid and gas at the same time occurs only at one specific temperature and one specific pressure.  For water this occurs at a temperature of 273.16 K (by definition) and a partial pressure of 611.73 pascals.  The term 'partial pressure' means that the gas part of the water may contain other gases and the resultant pressure be correspondingly higher.  It is only the component part of the pressure exerted by the gaseous water that counts.  86.149.139.90 (talk) 15:10, 6 September 2015 (UTC)

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It would be helpful to add some information about how VSMOW differers from rainwater, in respect to distilation
As a non expert, I found it very confusing when the creation of VSMOW was described as "mixed distilled ocean waters collected from different spots around the globe". Wouldn't the distillation produce the same effect on the composition of water as would be found in rain? Is this procedure to avoid airborne pollution such as dust, pollen, etc, which would be found in rainwater? Or does water's evaporation and condensation in the water cycle have different effects than distillation has? If so, why?

I suspect it is the latter, on this page at The International Association for the Properties of Water and Steam, it mentions "When water vaporizes, the vapor is slightly depleted in the heavier isotopes. The opposite occurs when water condenses from the atmosphere; the rain or snow has more of the heavy isotopes, leaving lighter water vapor in the atmosphere." I suspect that the distillation process used to create VSMOW captures this 'lighter water vapor' as well. It seems '63.226.222.247' brought this up at the end of talk section 6, but didn't give it a new heading, so it may have been missed. I would also note that the technical document describing the creation of VSMOW2 (PDF) doesn't mention distillation, only mixing, as far as I can see.

Then, for the layperson, would be helpful to have a discussion of how 'SMOW' differs from 'seawater' (salt? as section 5 on the talk page brings up) as well as from 'rainwater'.

Akda5id (talk) 22:19, 18 January 2019 (UTC)
 * I'm not sure how to add this to the article, since I have no explicit reference. The heavier molecules evaporate lightly more slowly than the lighter ones under typical ocean surface conditions (i.e., well below the boiling point of water.) It's possible to take advantage of this on purpose, using fractional distillation, which is pretty much the opposite of what we want here. Simple distillation boils the water, which "evaporates" all of the molecules, thus eliminating the difference in evaporation rate. -Arch dude (talk) 17:39, 20 August 2019 (UTC)
 * Thank you, that does clarify why (complete) distillation would be different from rainwater.