Talk:Psychrometrics

Does anyone know the relationship between  and wetbulb?
I've looked around the net and there seems to be a lack of formulae. Looking at the graph the wet bulb temps appear to be a constant gradient intersecting the saturation curve but how is this derived? njh 10:16, 11 January 2006 (UTC)

See Arden_Buck_Equation and references therein. The relationship is not a matter of definition, but relates to the physics of water and air. So the curve involved has to be measured. ArthurOgawa 00:08, 23 September 2007 (UTC)

Dew Point Formulas
This calculation is trickier than it looks. The standard source is the ASHRAE Handbook: Fundamentals. Look in Chapter 6, Psychrometrics, under Numerical Calculations of Moist Air Properties, to find what you are looking for. Most libraries should have a copy in the reference section. Note that it's published in English and metric editions. Tex 16:15, 20 February 2006 (UTC)


 * Not in Australia :-(. I can probably deduce it from the Clausius-Clapeyron relation though. Thanksnjh 20:54, 20 February 2006 (UTC)


 * The relevant chapter can be downloaded from the ASHRAE website as a PDF file, for a fee which is greater than nominal but not outrageous. I think it is USD 50. Tex 23:39, 20 February 2006 (UTC)

Mollier diagram
This external link shows the relation between the Mollier diagram and the Psychrometric chart. 57.66.65.38 10:25, 11 April 2006 (UTC)


 * This is a little confusing because in fact several different thermodynamic diagrams are known as "Mollier diagrams". The most common is the enthalpy-entropy, or hs diagram. Probably the next most common is the pressure-enthalpy, or ph diagram. In fact I have never heard the enthalpy-composition diagram, as discussed here, called a Mollier diagram, but apparently it can be. Old Mollier must have really been working hard in those days! Tex 15:12, 11 April 2006 (UTC)


 * See Richard Mollier. Any diagram using enthalpy as one of its axes can be called a Mollier diagram, by decree of some physicists at a conference in 1923. ArthurOgawa 00:42, 23 September 2007 (UTC)

Inappropriate editorialization
Under the heading Psychrometric Chart, Relative Humidity, Rkinch added"The notion that air 'holds' moisture, or that moisture dissolves in dry air and saturates the solution at some proportion, is an erroneous (although widespread) concept."Frankly, I do not see Rkinch's point. In what way is the condensation of water out of, say, air not a manifestation of the saturation of a solution? I propose removing this language as a gratuitous editorialization (the claim is neither supported nor explained). Or, a reference to an explanation should be added. ArthurOgawa 00:42, 23 September 2007 (UTC)


 * I agree that the notion of air "holding" moisture is flawed. There is no affinity between water vapor and air.  Air is displaced by the presence of water vapor, in accordance with Dalton's Law of Partial Pressures.  When water vapor is liberated, either the air will go elsewhere or the local pressure will rise.  Unlike a solid dissolving (or not) in a liquid, there are are no immiscible gases.  Gases are either present in a mixture or not.  There is no "holding" of water by the air.  Warm air does not "hold" more moisture; rather, warm water vapor has a higher pressure and is able to displace a greater fraction of the air at a given local pressure. -Ac44ck (talk) 22:11, 15 June 2008 (UTC)

Sections on dry- and wet-bulb temperatures seem redundant
The text below the chart already discusses the concepts of dry- and wet-bulb temperatures. Wiki articles exist for both concepts, and the text below the chart links to those articles.

I tried to fix the wet-bulb temperature section some, but it became too much of a time sink to correct it and to coordinate with the information provided beneath the chart and in the existing wet-bulb temperature article.

The discussion here about a wet-bulb thermometer is somewhat misleading. Will a moist wick on the bulb of a thermometer always cause its reading to drop? No. And if does drop, it won't drop below a limiting value.

Why evaporation causes cooling isn't explained. And this article doesn't seem like the place to explain that. -Ac44ck (talk) 03:44, 25 March 2008 (UTC)

Question removed without being answered
This question was removed for reasons unknown:


 * All properties evaluated at the saturation temperature of the liquid?

I objected on the editor's talk page:Kilmer-san. They left a note on my talk page to discuss it here instead.

So here I am. If it isn't a question, then replacing the question mark with a period seems appropriate. If the properties are to be evaluated at some other temperature, then why not list it? If it is an open question, then why not leave it as a question? No guidance at all is given to users of the formula by simply removing the question. Users may not think to ask the question if it isn't mentioned and no guidance is given. There are lots of "temperatures" for moist air: dry-bulb, wet-bulb (of various kinds), etc. Why not identify the temperature to use with formula if we know it? Why remove the question if we don't? Perhaps the phrase "at a wetted surface" above the formula is adequate to identify the appropriate temperature. Perhaps it is not. -Ac44ck (talk) 22:38, 15 June 2008 (UTC)


 * It is not appopriate to question the validity of a statement in the article. If you know that the properties are evaluated at the saturation temperature of the liquid because you have a source, than state that in the article. If, lacking a cited source, you are not sure if the properties are evaluated at the saturation temperature of the liquid, and as a consequence you do not believe the equation provides sufficiently useful and certain information to be included in the article, then you should state that as your concern or your intention to remove the item in question. Kilmer-san (talk) 19:39, 19 June 2008 (UTC)


 * The validity of statements are questioned in Wikipedia all the time. That's what a "cite" tag does -- without necessarily doing any legwork before raising the question.


 * I added the formula in the edit at 23:08, 8 April 2008. I included two references with the formula.  It wasn't clear to me what temperature the sources cited had in mind.


 * I don't think that the formula is in question. I had a question about how to use it.  That doesn't invalidate the formula, so I don't think that removing the formula would be appropriate.  Perhaps it is clear to others how to use the formula.  The question mark was an invitation to confirm or correct my interpretation of how the formula is to be used.


 * I don't see that readers of Wikipedia are served by burying ambiguity, if it exists. -Ac44ck (talk) 06:58, 22 June 2008 (UTC)

Category tag removed
A recent edit removed the "Category:Thermodynamics" tag.

Please explain why this article does not belong in that category. -Ac44ck (talk) 19:43, 12 January 2009 (UTC)


 * The page linked in the edit summary suggests that the tag was appropriate:
 * Categorization_and_subcategories:
 * Topic articles
 * the fact that the article is a member of this subcategory is not a reason for it to be excluded from the parent category.
 * -Ac44ck (talk) 20:20, 16 January 2009 (UTC)

Wikiproject: Chemical and bio engineering tag removed
I replaced this tag with the more general engineering tag. It really doesn't seem to apply to either chemical or bio engineering. If this doesn't reflect consensus feel free to revert, though I'd be fine with a comment here, too. Jminthorne (talk) 01:03, 24 August 2009 (UTC)

Psychrometric Chart
There needs to be a psychrometric chart. This is what HVAC engineering and psychrometrics is based off of. The chart on there does not show up correctly. There preferably needs to be one from ASHRAE on there with a compass in the upper left corner for the sensible heat ratio and enthalpy/(humidity ratio) ratio. I have one available in either U.S. or SI units if no one else does. —Preceding unsigned comment added by 192.31.106.34 (talk) 17:52, 10 November 2009 (UTC)
 * There was a chart until it was replace with an SVG version on November 1. Maybe other browsers render the SVG version; mine doesn't. - Ac44ck (talk) 05:00, 11 November 2009 (UTC)

Mixtures of other components
Psychrometrics studies mixtures of condensing vapors in non-condensing gases. Neither water vapor nor air are required to be part of the mix:
 * http://www.ingentaconnect.com/content/els/03645916/1996/00000020/00000003/art00030
 * Psychrometric charts for hydrocarbon vapours in nitrogen

One might argue that no gas is non-condensing if the temperature is low enough. Perhaps there is a better way to describe the mixture.-Ac44ck (talk) 14:26, 31 May 2010 (UTC)

The sources I have specify water and air. "The study of systems involving dry air and water is known as psychrometrics." (Fundamentals of Engineering Thermodynamics, Moran & Shapiro). "Psychrometry is the study of the measurement of the moisture content of atmospheric air." (Heating and Cooling of Buildings, Kreider, Curtiss & Rabi). Djd sd (talk) 09:23, 1 June 2010 (UTC)
 * That's how I think of it, too. But it seems to be a more general term. The structure of your first cite is not exclusive. It does not say the study of other systems cannot be known as psychrometrics. The second cite is from an industry-specific reference, where other mixtures are not relevant and would get no ink. - Ac44ck (talk) 03:58, 2 June 2010 (UTC)

Specific volume
Using your reference -- ASHRAE Fundamentals Handbook 6.6 eq. 9, 1989): This is the equation for "density of a moist air mixture." It is not the equation for specific volume. One of the inputs to eq. 9 is 'v' -- "the moist air volume ... as defined by equation 25." The text before equation 25 defines 'v' as the total volume of the mixture divided by the volume of dry air. To say 'v' on a psych chart
 * indicates the space occupied by the "dry" air with no consideration given for any potential water vapor.

is an erroneous statement. The water vapor is part of the mixture. The basis is per unit mass of dry air, but that is not saying the water vapor is ignored in the quotient. Please revert. -Ac44ck (talk) 05:07, 23 November 2010 (UTC)
 * From the most recent edit history:
 * 01:55, 28 November 2010 66.189.203.152 (talk) (14,947 bytes) (Undid revision 399072357 by Ac44ck (talk) See Discussion)
 * What 'discussion' supports this most recent edit to the article? -Ac44ck (talk) 09:08, 28 November 2010 (UTC)

I suppose my statement is erroneous if you are implying that I am referring to the volume, however I am referring to the mass. You state that eq. 9 from ASHRAE is not the equation for specific volume, precisely my point. Someone, not I, has made the unsubstantiated claim that the inverse of the specific volume is also the density of the air regarding psychrometric charts. Which is rather odd that the chart itself would not make this claim, if true. Equation 9 gives the true density of moist air as (1/v)(1+W) which obviously would give a different result than (1/v). As a check, if one inverts v from the i.p. psychrometric chart you would have units of lbm (dry air) per cubic foot, hardly units of lbm (moist air) per cubic foot. Rather the “specific volume” found on most charts is merely the volume required by the mixture at that point in order to contain one lbm of dry air. There are practical reasons why it is now presented this way on modern psychrometric charts, however, this was not always the case (see http://www.ashrae.org/members/doc/Gatley_8090903.pdf, notice the two different specific volume curves and the addition of the density curves). As I previously said, this is a common error throughout psychrometry, including academia, that typically is not catastrophic being that at standard conditions the error would be on the order of one or two percent. However, at more elevated temperatures it can be quite significant since the error grows nonlinearly. I will not attempt to expound the physics here but a short treatise on the subject can be found at: http://www.cibsejournal.com/cpd/2009-08/ see “The final volume of the psychrometric story”? Regardless, I believe the correct protocol would be to revert to my now trice sourced edit (or something similar) from peer reviewed sources until the unsubstantiated claim that the specific volume is “also called the inverse density” can be referenced (and not merely parroted from other unaccredited sources). Maybe it is not appropriate in an encyclopedic work but a more complete discussion of the issue would probably go a long way to help individuals since Wikipedia often tends to be a first initial source for information. Kind regards… —Preceding unsigned comment added by 198.232.211.130 (talk) 20:12, 29 November 2010 (UTC)
 * Agreed that "inverse density" is a misnomer in this context.
 * I would suggest the following
 * rather it is merely  the volume   required by the mixture at a particular point in order to   contain  one unit of mass of “dry air”.
 * But I think it is technically accurate now. Cheers. -Ac44ck (talk) 05:18, 30 November 2010 (UTC)

Feel free to edit it as you wish...regards... —Preceding unsigned comment added by 198.232.211.130 (talk) 19:07, 30 November 2010 (UTC)
 * Two points concerning the most recent edit:
 * Is there a cite for the assertion that specific volume is usually confused as the inverse density of the mixture?
 * Whether the symbol v is used on a psychrometric chart seems unimportant. The article is about the topic psychrometrics, where the symbol v is used for specific volume. The previous paragraph in the same section gives the symbol h for specific enthalpy. This symbol is not an essential feature of a psychrometric chart, either; but it doesn't need to be expunged from this section any more than the symbol v does.
 * -Ac44ck (talk) 03:08, 3 December 2010 (UTC)

I have already provided references to this position, if you are requiring a reference to the specific word "usually" then I think that would call into question say 95% of this article. As for the use of "v", it simply is not found on any psych. chart I have ever seen, and interestingly the words "specific volume" as well. As I implied, I do not have a problem with it but it should be explained don't you think? Else, someone will look for the little "v" and when it isn't found they will probably be a little confused and upset. Keep in mind this topic is specific to Psychrometric Charts not psychrometrics in general. So, if the editor wanted to explain it then I would have no problem with it, in fact in my opinion it would be most helpful. I believe that many things could be added to improve the usefulness of the "specific volume" topic. The little "h" is oftern found on psych. charts but unfortunately not on these, however, my comments were intended soley for the specific volume section. Kind regards.... —Preceding unsigned comment added by 198.232.211.130 (talk) 19:32, 3 December 2010 (UTC)
 * The word 'usually' is exactly what I am questioning. You are not confused by it. I am not confused by it. Anyone conversant in psych calculations is not confused by it. Novices may stumble over it temporarily; but then they aren't confused by it, either. I don't believe that it is true that the term is usually confused with inverse density.
 * In other contexts, the term specific volume is used for "inverse density." But those are not this context. In those contexts, one working with a pure substance actually wants "inverse density" and they don't care how the term is used in the specialty psychrometrics.
 * Where else is enthalpy expressed as total heat of mixture per unit mass of dry air? Is this use on a psych chart usually a source of confusion, too?
 * Some terms here are jargon. Learning the jargon is part of the cost of admission. What less-confusing term would you prefer for the concept "volume of mixture per unit pass of dry air?"
 * The topic of the section where the definitions are given is about psychrometric charts. But the topic of this article is psychrometrics. I see no reason to get exercised about whether a symbol appears in a psych chart. I don't see the letters DBT in psych charts, either. But I don't think we should remove the notation from the section for that reason.
 * There are several examples of charts containing the words "specific volume" here:
 * http://www.google.com/images?q=psychrometric+chart+specific+volume
 * -Ac44ck (talk) 06:03, 4 December 2010 (UTC)

To the contrary, I am quite sure that you were confused on the subject as our prior discussions show. And I know that I struggled greatly with the issue at one time myself. And the original editor obviously got it wrong as well. Not only would the novice have a difficulty but actually I feel that the “experts” are even worse; based on all the calculations (often from licensed Professional Engineers) that I review I have never had one engineer get it right. Whereas, the novice has not yet been infected with the erroneous convention. The words that the article I referenced used is: “widespread uncertainty”. I used “usually” instead but feel free to substitute “widespread uncertainty” if you wish. As for DBT, enthalpy, what is jargon, cost of admission, etc. as previously stated I take no position, I solely wished to edit the erroneous information I found regarding Specific Volume. I am not aware that I have had comment regarding the term “volume of mixture per unit pass of dry air?”, please explain. Your references to the words “specific volume” on various charts appear to be annotations to charts and not part of the chart originally. Regardless, I was just pointing out problems with trying to understand the chart in regards to specific volume, I wasn’t being critical of anything in the edit. As I have stated before I think “v” (if that is the correct term) should be in the article but an explanation or reference should be included. I would do it myself but I don’t have the time to produce an edit that would then be dissected by you to the point of uselessness. So I suggest you give it a crack…regards… —Preceding unsigned comment added by 198.232.211.130 (talk) 17:26, 6 December 2010 (UTC)


 * I have not been confused. I reverted an edit that said this:


 * Specific volume ... the space occupied by the "dry" air with no consideration given for any potential water vapor.


 * That was clearly incorrect. I reverted it to what existed before, as one does when reverting. I didn't read what was there before carefully. Nor might I have taken the time to fix it then if I had read it carefully.


 * The oxymoron of a psychrometric calculation that gave "no consideration ... for any potential water vapor" simply raised a red flag that was too big to ignore.


 * The same wording was used in two later edits. I reverted them for the same reason.


 * The fourth edit seemed technically correct (though wordy) to me, and I said so. After an invitation to edit it, I did so. While I was at it, I edited other items in the same section.


 * Then came the digging-in-of heels about using the word "usually" and objecting to the use of the 'v' label.


 * Your anecdotal evidence that "I have never had one engineer get it right" seems like an overstatement to me. If not, one hopes it is only a local problem.


 * The CIBSE article says, "An element ... that causes widespread uncertainty is the specific volume and more particularly its relationship with density. Many texts simply relate one as the inverse of the other - however there is a convention in the HVAC&R world that, for humid air, this is not the case."


 * What kinds of texts? I would hope that all thermodynamics texts "relate one as the inverse of the other." As they should -- unless it is a chapter on psychrometrics; then they shouldn't. The quote above notes that it the term is used differently "in the HVAC&R world."


 * That is evidence of the same term being used for different concepts. It happens all the time in standard language. It is unfortunate that technical language isn't always unambiguous, but sometimes the correct context is needed for the correct interpretation.


 * There is not "widespread uncertainty" simply because this one author claims there is. Nor is it evidence that people are usually confused.


 * I think it is enough to say what the psychrometric specific volume is . We can also say what it is not. I don't feel it is necessary to pronounce someone who made an error by typo or otherwise as "confused." If applicable, they can come to that conclusion themselves when presented with the facts.


 * My request for what may be a more palatable term for “volume of mixture per unit pass of dry air” contained a typo. As we are discussing specific density in the context of psychrometrics, I intended to write “volume of mixture per unit mass of dry air.”
 * - Ac44ck (talk) 04:31, 7 December 2010 (UTC)

My friend…so you reverted my edit of “the space occupied by the "dry" air with no consideration given for any potential water vapor.” to what? Yes, back to something that is completely incorrect “is the volume per unit mass of the air sample.” (volume of what per unit mass of what?)  But you weren’t confused, right? And I already have discussed with you that you made an incorrect assumption that I was referring to the volume of the potential water vapor when the implication is the mass, which would make the statement a correct statement. It is per unit mass of dry air right, not the water vapor, right? Regardless I thought we had crossed that bridge and accepted how it currently is (and I think it is good), why are you bringing the issue up again now? What I don’t understand is that your discussion of Dec. 4 was about the use of the word “usually” and confusion from it. Was I supposed to read your mind that it was actually regarding my statement of “the space occupied by the "dry" air with no consideration given for any potential water vapor”? Are you being sincere in your arguments? My friend if there is any digging-in-of-the-heels regarding “usually” it is from you not me. I again offer you to suggest something else. I have proposed an alternate of “widespread uncertainty” after all that is verifiable to a peer reviewed technical publication, please let me know. Myself, I found “usually” to be less “wordy”. Your mischaracterization that I object to “v” is again untrue, but it should be explained or referenced, should it not? The “anecdotal” evidence is a nice disparaging remark for something that is not anecdotal in that it is my own personal experience, what do you need an affidavit from me? Are you saying I’m lying? I suppose it is possible that I could have forgotten the odd occurrence when someone did get it right but I would think I would remember that. Regardless, it is not as if I am trying to enter my own personal experience into the edit, it is not as if I am trying to say that specific volume is “always” confused. What is your personal experience? Do you and the people you deal with “usually” get it right? Curious to know your experience with the subject. You seem to knock HVAC&R, but again we are dealing strictly with “specific volume” in regards to psychrometric charts, were they not developed and disseminated from that organization and its predecessors from the beginning? To what organization would you prefer we turn to for guidance? Notice I am not editing the specific volume under the “Common applications” heading. I don’t agree with that entry either, by the way. Certainly “specific volume” from a pysh. chart is its own beast and is different than the more general application. This begs the question as to why we are linking to it in this particular edit. I would eliminate that as well but I do not need another battle from you. But I don’t agree with you, we know exactly what those lines on the psych. chart represent and it is possible to “unambiguously” define it. There is no requirement that the definition need match anything else. And I again disagree with you that it is enough to define what something is, Wikipedia is replete with instances where the additional step is taken to show what something is not (please don’t needlessly require me to provide you examples). And I fail to understand why you would not want to provide the service of helping people avoid a common pitfall. Which of course it really the sole purpose that I attempted to edit this at all. I was not implying you are confused by a typo, you are confused for all the other reasons I have stated. Your typo did not interest me at all, my comment was that I did not understand what you were talking about by saying I had a problem with “volume of mixture per unit mass of dry air?” I wasn’t aware that I was having an issue with this. I wasn’t aware that it was currently an issue. So I asked you to explain. My comment had nothing to do with the typo. But with your further explanation I think I now understand. My problem with it is that when you say “air” this includes any possible water vapor and specific volume (regarding psych. charts) is given per unit mass “dry air”, potentially a big difference. Which you yourself seem to now realize. Although not necessary to be technically accurate do you find it unreasonable to define the very “mixture” (by including “dry air plus the water vapor “) that the specific volume lines are giving information about? I find it helpful, but I do not claim to be a wordsmith so maybe you want to make it “prettier”. Regarding this one author, he does provide additional references and there is even a reference from this Wikipedia entry (see http://www.uigi.com/technology.html, Moist Air Properties and Relationships) that appear to give emphasis as if it is a problem. The same could be said of the industry standard text for HVAC, by McQuiston and Parker; in my book the authors italicize “enthalpy, specific volume, and humidity ration scales are all based on a unit mass of dry air and not a unit mass of the moist air”  One wonders why the go to the effort to emphasize this point. Lastly you state that we are discussing “specific density” in context of psychrometrics. That my friend is an oxymoron (specific density) and again I am only trying to edit “specific volume” as it regards common psych, charts, which as you know now is a different beast than in other applications of the term….regards…. —Preceding unsigned comment added by 198.232.211.130 (talk) 19:52, 7 December 2010 (UTC)

Absolute humidity
The definitions of "absolute humidity" by the World Meteorological Organization (WMO), the American Meteorological Society (AMS) and the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) are in agreement. Nevertheless some text authors and others give absolute humidity the same definition as "mixing ratio", a.k.a. "humidity ratio". This is redundant and confusing. Definitions of the Wikipedia psychrometrics page should in this contributor's opinion conform to those of the WMO, AMS, and ASHRAE.

K289g (talk) 16:04, 18 September 2011 (UTC)


 * There is EN 12792 which defines absolute humidity as moisture per mass of dry air. The standard is used for HVAC. --Ikiwaner (talk) 07:34, 8 December 2023 (UTC)

Specific Humidity
The October 6 revision deleted the definition of specific humidity from the discussion of the psychrometric chart because the psychrometric chart does not display the property specific humidity. The deleted definition then replaced the specific humidity definition in the "Psychrometric properties section". Please note that the October 4 revision had two different definitions of specific humidity. The definition in the October 6 revision agrees (with minor rewording) with the American Meteorological Society and current ASHRAE definitions. See 20 September 2011 for references and links. K289g (talk) 22:59, 6 October 2011 (UTC)

Not defined
''The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.''

You haven't defined wet-bulb temperature. You haven't defined wet-bulb thermometer. You haven't said whether wet-bulb temperature and thermodynamic wet bulb temperature are the same. You have made it clear to the discerning reader that there are a number of fine distinctions here and by implication, that the wet-bulb temperature is not really the temperature recorded by a wet bulb. 84.227.241.146 (talk) 06:11, 13 September 2014 (UTC)

Reference 3
The link is no longer active. A possible replacement is: Canadian Wood Council. http://cwc.ca/wp-content/uploads/FAQ-durability-Mould.pdf, accessed 20160711 // k289g — Preceding unsigned comment added by K289g (talk • contribs) 13:09, 11 July 2016 (UTC)

Psychrometric chart
The article text states: ''Dry bulb temperature: These lines are drawn straight, not always parallel to each other, and slightly inclined from the vertical position. This is the $$t$$–axis, the abscissa (horizontal) axis.''

However the corresponding image has vertical and parallel dry bulb temperature lines.

On the other hand I think if lines are not straight and/or parallel the corresponding curves are not an axis but rather a group of curves labelled along an axis.

Question remains: If the dry bulb curves in a true psychrometric chart are not parallel then the psychrometric chart is identical to a Mollier diagram rotated 90° CW and flipped along the horizontal axis. If they would be straigtht then it would be something different. But then the h curves would not be parallel. Which is true? --Ikiwaner (talk) 07:47, 8 December 2023 (UTC)