How we learned to measure, map, and control moist air
John C. · September 2026
Contact: John.carioto@gmail.com
Psychrometrics is the science of moist air, of dry air mixed with water vapor. It is the basis of all air conditioning design (a little bombastic, but not far off). It is how we know how to size a cooling coil, how a data-center CRAC can hold a dew point, and how a weather report can say "65 percent humidity" with accuracy. The field consists of two facets: the physical law(s) about how much vapor warm air can hold, and the instrument(s) that measures that vapor by watching water evaporate off a wet thermometer.
This note traces the history from early hygrometers through Willis Carrier's chart to the ASHRAE formulas still used in HVAC work today. With some of my own observations mixed in. I used my yellowed psychrometric chart that I got from Trane 20 years ago while I wrote this. There is a difference between being an engineer and understanding why this works, and just understanding how to use the chart.
What the word means
The name is from Greek: psychros (cold) and metron (measure). In the nineteenth century this was the wet-and-dry-bulb instrument, not the whole science of moist air. Engineers later kept the word and applied it to the properties, the equations, and the chart.
Two instruments were used initially, and together. A hygrometer measures humidity directly. Classically this was done by a human hair that lengthens as it takes up moisture (surprisingly good tool: entirely mechanical, and works between 20 and 100 percent RH. A bit slow for reactions though, and it required recalibration). A psychrometer measures it indirectly: one thermometer stays dry, the other is wrapped in a wet wick. I did this as an experiment in college. I personally feel like this should be taught in 5th grade science since it was so simple. Evaporation cools the wet bulb. The gap between the two readings, the wet-bulb depression, is a function of how dry the air is. Translate that gap with an equation or a table and you have vapor pressure, relative humidity, and dew point.
The physics underneath
Long before anyone drew a chart, the thermodynamics was already in place. John Dalton (1766–1844) is well known for quite a few remarkable works, including contributions to modern atomic theory. For the purposes of this paper, we're using his 1802 law of partial pressures, which treated water vapor as a gas that occupies the same volume as the dry air and contributes its own partial pressure. That is still how HVAC textbooks start: total barometric pressure equals dry-air pressure plus vapor pressure.
The amount of vapor the air can hold is set by saturation vapor pressure, which rises steeply with temperature. Benoît Clapeyron wrote the phase-change slope in 1834; Rudolf Clausius put it in modern form later in the century (Clausius–Clapeyron equation below). The useful rule of thumb that falls out of that relation is familiar: roughly seven percent more moisture capacity per degree Celsius of warming, if relative humidity stays about the same.
ln(P2/P1) = -(ΔHvap / R) × (1/T2 - 1/T1)
This relationship is why a psychrometric chart is not a flat grid. Saturation is a curve. Heating at constant moisture is a horizontal line. Humidifying at constant dry-bulb is a vertical line. Adiabatic saturation (the wet-bulb process) cuts diagonally across the chart. All of that geometry is Clausius–Clapeyron plus Dalton plus an energy balance on evaporating water.
Measuring moisture before air conditioning
Hair, whalebone, and dew
Horace-Bénédict de Saussure's hair hygrometer (1783) made humidity a laboratory quantity. Organic fibers swell with moisture; the expansion can be geared to a needle. The instrument is simple and still used in some mechanical humidistats, but it drifts, lags, and depends on the particular hair.
A second family of instruments watched condensation. Cool a surface until dew forms and you have dew-point temperature, which of course is the temperature at which the actual vapor pressure becomes the saturation pressure. That idea is still the gold standard for humidity calibration.
The wet bulb
In 1792 the Scottish geologist James Hutton reported that a thermometer wetted with water reads lower when air moves across it. John Leslie built an early condensation hygrometer from wet- and dry-bulb thermometers around 1801. The name "psychrometer" was popularized by Ernst Ferdinand August in the 1820s to distinguish the evaporative instrument from hair hygrometers.
James Ivory published a usable psychrometric equation in 1822. August and James Apjohn refined it. The physical picture is a heat balance at the wet wick: heat arriving from the air equals the latent heat leaving with evaporated water. Once you accept that, wet-bulb temperature is not a curiosity. It is close to the temperature of adiabatic saturation, defined as the temperature air reaches if water evaporates into it with no external heat.
Henri Victor Regnault's mid-nineteenth-century vapor-pressure measurements gave the tables a real foundation. In the United States, William Ferrel fitted an empirical formula (1886) to Weather Bureau data. Charles F. Marvin's 1900 tables, which gave vapor pressure, relative humidity, and dew point from wet- and dry-bulb readings, were the working handbook of American meteorology. Willis Carrier (yes, that Carrier) mentioned Charles Marvin in his paper "Rational Psychrometric Formulae."
Carrier, Cramer, and the chart
Brooklyn, 1902
The practical problem was a color printing plant in Brooklyn. Paper stretched and inks misregistered when humidity swung. Willis Haviland Carrier, just out of Cornell and working for Buffalo Forge, designed a system that pulled air over coils fed with cold water so the air would drop below its dew point. That is dehumidification by condensation, which is still the standard cooling-coil process used throughout HVAC engineering today.
Carrier then pushed the envelope again. If you spray water through an airstream and control the water temperature, you can drive the air to a chosen dew point. Saturate the air, set its temperature at saturation, and the moisture content is fixed. He later recalled the insight arriving on a foggy night on a railroad platform: cool air holds less vapor; a machine-made fog with a controlled temperature is a humidity machine. The spray washer was working to his satisfaction by 1904. The 1906 patent (U.S. 808,897) covered apparatus for humidifying and dehumidifying air.
Stuart W. Cramer, a North Carolina textile engineer, coined the phrase "air conditioning" in 1906. He was talking about conditioning cotton fiber, not human comfort, but the name stuck. Carrier later said Cramer independently found several of the same moist-air relations.
From Marvin's tables to a picture
In 1904 Carrier graphed Marvin's tables so he would not have to interpolate them for every job. Dry-bulb temperature ran along the bottom. Moisture content ran up the side. Percent-saturation curves and wet-bulb lines filled the field. The 1906 Buffalo Forge catalog printed a "hygrometric chart." The shape is still the chart we use today.
Between 1908 and 1911 the vertical axis settled on humidity ratio, which is the coordinate you actually want for energy and mass balances. William Mason Grosvenor Jr., a chemical engineer, published a humidity chart in 1908 for dryer design that used that same ordinate and added lines for specific volume and latent heat. Most later charts followed Grosvenor's coordinate choice even when they kept Carrier's overall look.
The 1911 paper
On 3 December 1911 Carrier presented ASME paper 1340, "Rational Psychrometric Formulae." It is still called the Magna Carta of the field. He replaced Ferrel's empirical wet-bulb formula with a thermodynamic one based on adiabatic saturation, stated the relationship between wet-bulb temperature and dew point, and put evaporative moisture determination on the same footing as steam tables.
The era of depending on empirical formulae (spelling used because of Carrier's paper) deduced merely from simultaneous readings of dry bulb, wet bulb, and dew point was over. The wet bulb, properly used, was a thermodynamic instrument.
Rival charts and a long argument about axes
The chart was never the only picture of moist air. Richard Mollier's 1923 h–x diagram plots enthalpy against humidity ratio. In much of Europe the Mollier diagram is the working sheet; the ASHRAE-style chart is the American cousin. They contain the same information. They just swap which property gets the straight line. And it may be due to exposure, but I always found the Carrier style layout more intuitive.
In 1926 Claud A. Bulkeley offered ASHVE a different layout. The society adopted it from 1926 to 1938, seconded by Carrier himself, then went back. In 1961, on the fiftieth anniversary of the 1911 paper, ASHRAE issued a new official chart. The plotting coordinates were enthalpy and humidity ratio, in the spirit of Mollier, but the printed sheet was arranged to look like Carrier's original so that heating, cooling, and humidifying processes still read the way American engineers expected.
That 1961 sheet, revised as property data improved, is the ancestor of the charts in the ASHRAE Handbook — Fundamentals. Sea-level charts plus altitude versions (750 m, 1,500 m, 2,250 m) cover the pressures that actually show up in building work.
From handbook tables to real-gas models
Everyday HVAC calculations treat moist air as two ideal gases sharing a volume. That is accurate enough from about −58°F to 122°F at ordinary atmospheric pressure — errors in humidity ratio, enthalpy, and specific volume stay under about one percent. The saturation pressure that feeds those calculations is not ideal-gas guesswork. It is a fit to laboratory data.
John A. Goff and S. Gratch produced a widely used saturation-pressure formulation in 1945. Richard Hyland and Arnold Wexler's 1983 equations, still printed in the Handbook and used for psychrometric calculations today, cover ice from −100 °C to 0 °C and liquid water from 0 °C to 200 °C. They are logarithmic polynomials in absolute temperature. They look empirical but, surprisingly, they are curve fits whose shape is dictated by Clausius–Clapeyron.
When the job leaves the comfort range for extreme conditions such as compressed-air storage, high-pressure process air, or very low temperature, the ideal-gas shortcut thins out. ASHRAE Research Project 1485 (2009), by Sebastian Herrmann, Hans-Joachim Kretzschmar, and Donald P. Gatley, rebuilt moist air as a real-gas mixture using virial coefficients and current IAPWS water properties. (Virial coefficients are used in the virial expansion of the pressure in a many-particle system. This is far beyond the scope of this small history to explore, but fascinating reading.) That model produced the Handbook property tables from 2009 onward and the LibHuAirProp libraries used in Excel and engineering solvers. For a rooftop unit in New Jersey, Hyland–Wexler plus the perfect-gas relations is still the right tool. For the edges of the map, RP-1485 is the better one.
What the chart is for
The reason the history matters is that the chart is not decoration. Each line on it is a process an HVAC system actually runs:
- Sensible heating or cooling — horizontal, constant humidity ratio.
- Humidification or dehumidification at constant dry-bulb — vertical.
- Cooling coil with dehumidification — down and to the left, toward the saturation curve.
- Evaporative cooling / cooling tower / air washer — along a wet-bulb line.
- Mixing two airstreams — a straight line between the two state points.
Software has not retired the chart. It has just moved the same state point into a psychrometric library. The wet-bulb thermometer is now often a polymer or chilled-mirror sensor. The underlying relations — Dalton, Clausius–Clapeyron, adiabatic saturation — have not changed.
Timeline
| Date | Who | What happened |
|---|---|---|
| c. 1450 BCE | Egypt | Reed mats and evaporative cooling used to temper indoor air. |
| c. 1593 | Galileo | Thermoscope — the ancestor of the thermometer. |
| 1643 | Torricelli | Barometer; atmospheric pressure becomes measurable. |
| 1783 | de Saussure | Hair hygrometer — humidity by organic expansion. |
| 1792 | James Hutton | Notes that a wet thermometer reads colder in moving air. |
| 1802 | John Dalton | Partial pressures; water vapor treated as an independent gas. |
| 1822–1837 | Ivory, August, Apjohn | First psychrometric equations from wet- and dry-bulb readings. |
| 1834 / 1850 | Clapeyron, then Clausius | Phase-change relation that later explains saturation vapor pressure. |
| 1845 / 1886 | Regnault; Ferrel | Precision vapor-pressure work; Ferrel formula used by Weather Bureau. |
| 1900 | C. F. Marvin | U.S. Weather Bureau psychrometric tables. |
| 1902–1906 | Willis Carrier | Brooklyn printing plant; spray washer; first hygrometric chart. |
| 1906 | Stuart Cramer | Coins the term "air conditioning." |
| 1908 | W. M. Grosvenor | Humidity chart using humidity ratio as the ordinate. |
| 1911 | Carrier | "Rational Psychrometric Formulae" presented to ASME. |
| 1923 | Richard Mollier | h–x diagram; still the European form of the chart. |
| 1926–1938 | C. A. Bulkeley | ASHVE briefly adopts the Bulkeley chart. |
| 1945 / 1983 | Goff–Gratch; Hyland–Wexler | High-accuracy saturation-pressure formulations. |
| 1961 | ASHRAE | New official chart on the 50th anniversary of Carrier's paper. |
| 2009 | ASHRAE RP-1485 | Real-gas moist-air model (Herrmann, Kretzschmar, Gatley). |
Sources
- Carrier, W. H. "Rational Psychrometric Formulae." Transactions of the ASME 33 (1911).
- Gatley, Donald P. Understanding Psychrometrics. 3rd ed. ASHRAE, 2013. The best single narrative of charts, pioneers, and property models.
- Gatley, Donald P. "Psychrometric Chart Celebrates 100th Anniversary." ASHRAE Journal (2004); "Grosvenor Humidity Chart." ASHRAE Journal (2008).
- Nagengast, Bernard. "100 Years of Air Conditioning." ASHRAE Journal (June 2002).
- Herrmann, S., H.-J. Kretzschmar, and D. P. Gatley. ASHRAE RP-1485, 2009.
- Hyland, R. W., and A. Wexler. "Formulations for the Thermodynamic Properties of the Saturated Phases of H2O." ASHRAE Transactions 89 (1983).
- ASHRAE Handbook — Fundamentals, Chapter 1, Psychrometrics (current edition).
- Royal Society Notes and Records: "James Hutton and the measurement of atmospheric moisture" (2024), on the wet-bulb line from Hutton through Ivory and August.
Note
Written for personal reference. Dates and attributions follow the ASHRAE historical literature, especially Gatley and the 1911 Carrier paper. Where sources disagree on a first-publication year for the earliest chart (1904 blueprint versus 1906 catalog), both are noted.