Introduction to Steam and Humidity

What is Steam? In engineering, steam refers to vaporized water. It is a chemically pure, invisible gas (not a mist) which, at standard atmospheric pressure (1 atmosphere), has a temperature greater than 100 degrees Celsius. It occupies about ~1,600 or more volume than the same mass of liquid water. Steam can, of course, be at a much higher temperature than the boiling point of water at any pressure. Such steam is referred to as high-temperature or superheated steam. Steam is a capacious reservoir for energy because of water’s high heat of vaporization. Steam can form at any pressure, but the boiling point (called the phase change temperature) increases with pressure. Beyond (>647.096K and >22.064 MPa), steam is neither gas nor water but is found in the supercritical state, a particular state described further below.

Brief History of Steam Production Methods
Nature of Device Steam Temperatures Illustrative Picture Who developed it first? Remarks
Late 1700 –

Early 1800’s

Combustion fired steam-boilers. 121°C

Ancient Steam Generator

Ancient Steam Generator

These were the earliest boilers.

Source Wiki: Stirling Boiler Company, Ohio, USA. Later merged with Babcock and Wilcox.

19th Century

Saturated steam at low temperature.  

Late 1900’s Higher pressure. Combustion-fired and some electric boilers. 134°C- 162°C

1900's boiler

1900’s Boiler

Several manufacturers. Steam generation required high-pressure boilers and was bulky. 

20th Century

Advances were made post World War II. primarily for size and safety. Low-quality steam.

2015 onwards

MightySteam® Products

Electric high-temperature. Steam generators 850°C and higher

Instant Steam. Variable flow. Variable pressure. Variable temperature. Steam above the inversion temperature.

Modern Steam Generator

Compact Modern OAB® Steam Generators

OAB® Modern Steam Generator

MHI Inc.  USA.

Compact, Instant, Reliable.

Sample Video

21st Century

New methods with better materials and techniques. High-quality instant steam, high temperature, energy-efficient with modern controls. Variable flow rates, adjustable back pressure, and independent temperature settings.

Modern Steam Generators offer instant high-quality superheated steam that can overcome backpressure. They also offer variable steam rates.

Modern, high-quality superheated steam is employed for several critical applications. High-temperature steam is helpful for pharmaceutical, biopharmaceutical, comfort, utility, and chemical uses because of a lack of water droplets. Noncondensing superheated steam is the most energy-efficient method of steam use. Using dry superheated steam above the inversion temperature of steam leads to the best antimicrobial and most rapid drying steam applications. The wicking properties and oxygen control of this type of steam are attractive features of high-temperature steam. High-temperature steam availability leads to high-productivity applications. Steam generator products are classified into two types, namely:

MightySteam® Pure Steam Products (Industrial and R&D Use)

MigthySteam® Controlled Humidity Products  (Commercial Use)

Request OAB and VHGA Information

In the US, more than 90% of electric power is produced using steam as a working fluid, mainly by steam turbines. Condensation of steam to water occurs downstream, but such wet-steam conditions must be carefully controlled to avoid excessive blade erosion and preserve energy efficiency.

There is no moisture from the start-up in many modern steam generators. The HGA’s and OAB produce high-quality pure steam. The HGA-M is for applications requiring steam-gas(air) mixtures. The Mightysteam® and SaniZap® models are helpful for steam cleaning. The OAB® and GHGA models are used for industrial and R&D purposes. The high-temperature steam also tends to be useful for pharmaceutical, biopharmaceutical, comfort industry, utility, and chemical uses because of a lack of droplets and rapid antimicrobial action for several orders of log reductions in a short duration.

What is superheated steam? Steam can be saturated or superheated. When at the boiling temperature (which depends on the pressure), the type of steam is called saturated steam. Saturated steam is often a mixture of water and gas. When above the boiling temperature, it is called superheated steam, especially when water droplets are no longer present (i.e. dry or high-quality steam). At sea level and one-atmosphere pressure (101KPa,  1 bar), steam boils at ~100°C (~212°F), which is the saturated steam temperature for this pressure. At 1-bar, above this ~100°C temperature. Above the corresponding boiling point at a lower or higher pressure than 1-bar, the steam is superheated. Steam generators make superheated steam. Modern steam generators use sophisticated electric and computer controls, making them versatile for different applications from chemical reactions, dry cleaning, and antimicrobial use to efficiently dry ores and wet materials, including food drying. Modern steam generators offer significant benefits in downstream efficiencies and dryness.

What is Supercritical Steam: Supercritical steam is used for generating power. This type of steam forms beyond the critical point (>647.096K and >22.064 MPa). At the critical point, about ~30% of free monomeric H2O molecules exist. The rest contain different types of bonds of H2. Supercritical water has very low surface tension with its gas or liquid phase; therefore, no interfaces exist that can delineate the liquid/gas interface. Above 647.096K, supercritical steam cannot be liquefied by increasing the pressure.   Another example of a supercritical material is Supercritical CO2 (above 304.13 K and 7.38 MPa). Co2 in this state is used to decaffeinate coffee. Its viscosity and diffusivity are like gas, penetrating the beans easily. However, its density is like that of a liquid. It also binds to caffeine (this property is much more important than its supercritical fluid properties). Supercritical CO2 is also used in dry cleaning.

Typical pictures of compact modern steam generators are shown below. Properties of superheated steam and types of steam kinetics are shown on the  Steam Calculator page.

Take me to the Airtorch® Models Page                      Take me to the Steam Generator Models Page.

The International Association for the Properties of Water and Steam (IAPWS) maintains international-standard correlations for the thermodynamic properties of steam, including IAPWS-IF97 (for use in industrial simulation and modeling) and IAPWS-95 (a general-purpose and scientific correlation).

A pound of water x 0.016 = cubic foot of water at 62.2 F.
A pound of water x 0.12 = gallons of water
1 gallon of H2O liquid = 8.33 lb. @ 62.2 degrees Fahrenheit (°F)
Boiling water temperature at one Bar pressure is 212 Fahrenheit (°F) = 100 Celsius (°C)
The freezing temperature of water at one Bar is 32 Fahrenheit (°F) = 0 Celsius (°C) (Very mild pressure dependence)

Take me to the Airtorch® Models Page                      Take me to the Steam Generator Models Page

  • Water Hardness Ratings:
    Water softness description in Grains/Gallon  or Parts/Million
    Less than 1.0 less than 17.1 soft
    1.0 to 3.5  or 17.2 to 60 slightly hard
    3.6 to 7.0  or 61 to 120 moderately hard
    7.1 to 10.5  or 121 to 180 hard
    10.6 & over 181 & over very hard

Typical water feed requirements for ultraclean steam

  • DI,  RO water, or purified water
  • Free of Amines, Chlorine, and chlorides
  • Silica less than one ppm
  • Total suspended solids – close to none
  • total hardness less than 1ppm
  • Conductivity less than five microOhm/cm

More (tallow) soap is required for washing in hard water than in soft water. A water softener may be needed if grains/gallon of water hardness exceeds 3. Steam generators require soft water and very low or negligible particles.

Does high pressure enable the outcome of steam or chemical reaction for H2O? The short answer is seldom, especially above 100°C. Long Answer.

More important is having an open-continuous dry high-quality steam system made by the OAB and HGA generators, where molecules of H2O can enable more of the desired reaction because of continuous production. This is a steam generator.

High-quality superheated steam has applications in drying, cooking, proper and complete bacterial inactivation, fracking, chemical processes engineering, comfort heating, chemical processes, fuel production, tablet making, mixing, and materials processing. High-temperature steam in one atmosphere packs the proper punch required for these applications while minimizing the dangers of using high-pressure boilers. This type of superheated steam is used in applications requiring a critical need to reduce the processing time. Superheated steam often offers a higher heat transfer coefficient and high enthalpy content, enabling many unique applications. When at a high temperature, significantly above the inversion temperature, such steam is often considered a non-toxic antimicrobial agent. Superheated steam at high temperatures also offers superior reactions, for example, in energy reactions such as bio-fuels, reforming, hydrogen production, ammonia production, and denaturing, all with rapid heat transfer kinetics.

Request OAB and VHGA Information

 Given below are the standard steam diagrams for process design. What is Humidity?

There are several ways to describe Humiditydity of the air (a term generally used only below 100C, at 1-atmosphere conditions). At high temperatures, it is more accurate to talk about Specific Humidity (also called the humidity ratio), which is the mass of water vapor present in a unit mass of dry air; that is, it is the ratio of the group of water vapor to the mass of dry air. Below the saturation temperature (e.g., @1 Bar, 99.6C), one often uses Relative Humidity when steam is mixed with air. The relatHumiditydity is a ratio that compares the amount of water vapor in the air with the amount of water vapor present at saturation. The relatHumiditydity is given as a percentage: the amount of water vapor is expressed as a percent of saturation. For example, at 25oC and one atmospheric pressure, the corresponding specific maxiHumiditydity is about 3%, although the relatHumiditydity is at 100%. Endotoxins, microbes, and bacteria are known to be inactivated by heat and H2O. All MHI superheated steam productions produce high-temperature steam that, during production, encounters temperatures over 500C. The steam output temperature and humidity condition are controllable, as discussed below for several models.

Vapor pressure measures the air’s water vapor content using the partial pressure of the water vapor in the air. (Pressure may be expressed using a variety of units: pascals, millibars, pounds per square inch (psi), among others). The gases in the atmosphere exert a certain amount of pressure (about 1013 millibars at sea level). Since water vapor is one of the gases in the air, it contributes to the total air pressure. The contribution by water vapor is relatively small since water vapor only makes up a few percent of the total mass of a parcel of air. The vapor pressure of the water in the air at sea level, at a temperature of 20oC, is ~24 mbar at saturation (about 3% by volume). Does the saturation pressure of H2O in the air depend on the total pressure? Yes.

As the total pressure of a system decreases, the relatHumiditydity will decrease because the pressure will decrease, and the saturation pressure does not change much at the same temperature.   Likewise, as the total pressure of a system increases, the relatHumiditydity will increase until saturation is reached.

RelatHumiditydity: we can compare how much water vapor is present in the air to how much water vapor would be in the air if the air were saturated. For this, we use relatHumiditydity. The relatHumiditydity is a ratio that compares the amount of water vapor in the air with the amount of water vapor present at saturation. The relatHumiditydity is given as a percentage: the amount of water vapor is expressed as a percent of saturation. If 10 grams of water vapor were present in each kilogram of dry air, and should the air would be saturated with 30 grams of water vapor per kilogram of dry air, the relatHumiditydity would be 10/30=33.3%. For example, a parcel of air at sea level, at a temperature of 25oC, would be completely saturated if there were 20 grams of water vapor in every kilogram of dry air. We use a mixing ratio: grams of water vapor per kilogram of fully saturated air. If this air contained 20 grams of water vapor per kilogram of dry air, we would say that the relatHumiditydity is 100%.    If the parcel of air (at sea level and 25oC) had 10 grams of water vapor per kilogram of dry air, what is its relatHumiditydity?   Answer:  The relatHumiditydity would be 50%. Ten grams of water vapor/kg dry air compared to the maximum possible 20 grams of water vapor/kg dry air is 10/20=50%. If a parcel of air (at sea level at 25oC) had 18 grams of water vapor per kilogram of dry air, what is its relatHumiditydity? RelatHumiditydity would be 18/20=90%. In the examples above temperature was taken as about 25°C. Water vapor content/water vapor cap city is Relative Humidity (RH). Another more technical term is the ratio of the actual vapor pressure to the saturation vapor pressure. You will note below that the HGA-M produces a lot of steam with very high speciHumiditydity because above 100C at one-atmosphere air and steam mix well as any other “ideal gasses could” Below 100C, the RH is an important limitation on how much steam can mix with air. For one atmosphere condition, Above 100C, one should use the term specific humidity, which is the mass of water vapor (i.e., steam) ratio when mixed with a unit mass of dry air.

Water saturation pressure at a temperature

(total pressure 1 atmosphere)

(This chart is helpful for comfort heating)

Temperature, °C Bar Psi
2 0.007 0.1024
4 0.008 0.1180
10 0.012 0.1781
14 0.016 0.2319
20 0.023 0.3393
25 0.031 0.4598
30 0.042 0.616
34 0.053 0.7723
40 0.073 1.0711
50 0.12 1.7915
60 0.20 2.8929
70 0.31 4.5253
80 0.47 6.8768
90 0.693 10.179
96 0.87 12.730
100 1.00 14.710

The enthalpy of vaporization and the molar volume change fall with increasing pressure from 1 Bar to 10 Bar.

Molar Volume Change with Pressure

Enthaphy and Molar Volume as a function of pressure for water

Power Steam Jet

SaniZap 1

A must for cleaning (Mobile Platform)

Discover Steam Tunnels for Continuous use

MHI

Sometimes you need a steam gas mixture instead of pure steam. The HGA-M or HGA-S-01-CGP1100 can enable this.

You are calculating your parameters.

Refer to the HGA-M manual to see how your HGA-M is configured/rated. The mass fraction of steam in the final flow is about 18% for a valve setting which gives, for example, 200ml/15.5 minutes (i.e., speciHumiditydity is about 21%). We are assuming that the water vapor is ideal and that the enthalpy of the water vapor in the air can be taken to be the enthalpy of saturated vapor at the same temperature  (~ 2501.3+ 1.82 T (kJ/kg)). Temperature, T, is in units of degrees centigrade.    Of course, after a specific temperature and pressure ~374C and 22.06 MPa called the critical temperature Tc and critical pressure Pc, respectively, no amount of pressure can cause condensation. The superheated steam generator can produce a steam-air temperature over this temperature, but the output is not at critical conditions because the pressure is lower!

The HGA and OAB operate at close to 100% power efficiency from which the steam temperature can be calculated. n the equation below for the efficiency of HGA-M, h is enthalpy, and T, the temperature is given in Kelvin (Kelvin=273+Centigrade):

h (enthalpy per kg) of steam is obtained from the figures above at 1-atmosphere pressure, steam tables, or the Mollier diagram. Or, you can measure the temperature from the exit thermocouple.

Request OAB and VHGA Information

GENERAL INSTRUCTIONS FOR HGA-M

Instructions on how to use and protect the HGA-M are enclosed with the product. It’s a unique, simply designed device with ease of use. Turn on the air and monitor with a flow meter so that the SCFM does not fall below 1.4. Higher airflows give lower temperatures, or you may control power with a separately obtained controller.   Then turn on the heater and finally open the water metering valve.   Steam-air will be the product.   Remember to read the manual for the shutdown procedure.

  An HGA stand and exit thermocouple are available as an option.

  Q:  When do you dry with steam instead of drying with hot air:  Answer there are many benefits to using steam. Still, the main one is the availability of a gas with a lot of stored enthalpy at a lower temperature than corresponding dry air with the same enthalpy. So if you were interested in drying paper with an ignition temperature of say 450F, then the use of superheated steam at a much lower temperature may produce the same drying efficiency as hot air at a high temperature which could be more than the paper ignition temperature.

Follow all safety procedures. NOTE STEAM IS A ODORLESS GAS AT VERY HIGH TEMPERATURES. STEAM, LIKE OTHER HOT GASSES, WILL BURN YOU. STEAM PACKS A LARGE AMOUNT OF ENTHALPY SO THAT THE BURN COULD BE SEVERE. DO NOT ALLOW THE STEAM TO FALL ON THE SKIN.   WEAR GOGGLES AND GLOVES And Protection for your clothes,   ALWAYS.

A selection of steam tables is given below. Best to use a standard text or International Association for the Properties of Water and Steam (IAPWS)

If you are looking for superheated, high-quality steam, you should now search for the HGA or OAB models that address your specific need. The information below refers to the HGA-M models only where an air or gas-steam mixture is required.   You may encounHumiditydity as a design term or property when using air or gas mixtures with steam.

IT IS EASY TO BE CONFUSED BETWEEN RELATIVE AND SPECIFIC HUMIDITY. RELATIVE HUMIDITY DEPENDS ON TEMPERATURE. SPECIFIC HUMIDITY IS RELATED TO THE MASS FRACTION ONLY.

See below from the public site http://www.pals.iastate.edu/mteor/mt206/lectures/feb7/tsld020.htm

Potential uses:  For layering, epoxy drying, and other film use, superheated steam is required at one atmospheric pressure. Ideal for steam drying or steam oxidation. Attempt use also for precipitating crystals of several small sizes, including nanocrystals from solutions. Precipitation droplet sizes may be controlled by controlling the cooling rate, impingement conditions, and surface type. 

The steam temperature depends on the water valve setting and air inflow setting.

HGA-M (typical settings at Full Power):

  • Air 1.45 CFM (inlet at ~30C) and water 330ml in 45 mins (inlet ~30C) yield steam-air temperature of about 350°C.
  • Air 1.4CFM (inlet at ~30C) and water 200ml in 20 mins (inlet ~30C) yield steam-air temperature of about 250°C.
  • Air 1.8CFM (inlet at ~30C) and water 200ml in 20 mins (inlet ~30C) yield steam-air temperature of about 150°C.

The graph below gives a fair idea of adjusting the HGA750-1 for different specific humidity levels. Note as the specific humidity increase; there is a corresponding decrease in overall temperature as total energy is conserved. For the graph below the steam, the gas thermocouple is correct as the exit. If you try and reduce the steam gas temperature too much, you may not be able to get superheated steam, and instead, a heated mist may be the output product. The red line graph required that a power controller is in use.

HGA

Your results may vary. The values above should be considered approximate Because of the placement of the thermocouple, restrictions on flows, and other random errors usually present in multivariate measurements. The user must optimize all valve settings for the best result for specific applications.

OUPE RH EATED STEAM IS AN ODORLESS GAS (not to be confused with mist). Output: constant steam-air (superheated steam). Safety precautions must be taken when dealing with hot gasses.

DO NOT USE WITH COMBUSTIBLE LIQUIDS. THE DANGER OF SUPERHEATED STEAM SHOULD BE WELL UNDERSTOOD.   PLEASE WEAR GLOVES, GLASSES AND A HARD HAT. PROTECTIVE CLOTHING IS REQUIRED. STEAM CAN PENETRATE CLOTHES.

Product has uses in surface technologies, cleaning technologies, drying technologies, curing technologies, and nanotechnologies.

Patents issued applied and are pending for the HGA.

A control thermocouple for the hot air generator part is included. Steam output temperature thermocouple and bracket are sold separately, or user may provide their own on their part.   An electrical 110-120V 50/60Hz supply is required for this unit. 1KW system requires compressed air input.

MHI

IT IS EASY TO BE CONFUSED BETWEEN RELATIVE AND SPECIFIC HUMIDITY. RELATIVE HUMIDITY DEPENDS ON TEMPERATURE. SPECIFIC HUMIDITY IS RELATED TO THE MASS ONLY. See below from http://www.pals.iastate.edu/mteor/mt206/lectures/feb7/tsld020.htm

Equations for the physical properties of moist air

Water vapor pressure

In a closed container partly filled with water, there will be some water vapor in the space above the water. The concentration of water vapor depends only on the temperature. It is not dependent on the amount of water and is only very slightly influenced by the pressure of air in the container.  The water vapor exerts pressure on the walls of the container. The empirical equations given below give a good approximation to the saturation water vapor pressure at temperatures within the limits of the earth’s climate.

Saturation vapor pressure, ps, in pascals:
ps = 610.78 *exp( t / ( t + 238.3 ) *17.2694 )
where is the temperature in degrees Celsius

The svp below freezing can be corrected after using the equation above, thus:
ps ice = -4.86 + 0.855*ps + 0.000244*ps2

The next formula gives a direct result for the saturation vapor pressure over ice:
ps ice = exp( -6140.4 / ( 273 + t ) + 28.916 )

The pascal is the SI unit of pressure = newtons / m2. Atmospheric pressure is about 100,000 Pa (standard atmospheric pressure is defined as 101,300 Pa).


Water vapor concentration

The relationship between vapor pressure and concentration is defined for any gas by the equation:
p = nRT/V
p is the pressure in Pa, V is the volume in cubic meters, T is the temperature (degrees Celsius + 273.16), n is the quantity of gas expressed in molar mass ( 0.018 kg in the case of water ), R is the gas constant: 8.31 Joules/mol.K

To convert the water vapor pressure to concentration in kg/m3: ( Kg / 0.018 ) / V = p / RT

kg/m3 = 0.002166 *p / ( T+ 273.16 )   where p is the actual vapor pressure


Relative Humidity

The Relative Humidity (RH) is the ratio of the actual water vapor pressure to the saturation water vapor pressure at the prevailing temperature.

RH = p/ps

RH is usually expressed as a percentage rather than as a fraction.

The RH is a ratio. It does not define the water content of the air unless the temperature is given. The reason RH is so much used in conservation is that most organic materials have an equilibrium water content that is mainly determined by the RH and is only slightly influenced by temperature.

Notice that air is not involved in the definition of RH. Airless space can have an RH. Air is the transporter of water vapor in the atmosphere and air conditioning systems, so the phrase “RH of the air” is commonly used, and only occasionally misleading. The independence of RH from atmospheric pressure is not important on the ground, but it does have some relevance to calculations concerning air transport of works of art and conservation by freeze-drying.


The Dew Point

The water vapor content of air is often quoted as a dew point. This is the temperature to which the air must be cooled before dew condenses from it. At this temperature, the actual water vapor content of the air is equal to the saturation water vapor pressure. The dew point is usually calculated from the RH. The first one calculates ps, the saturation vapor pressure at the ambient temperature. The actual water vapor pressure, pa, is:

pa= ps * RH% / 100

The next step is to calculate the temperature at which pa would be the saturation vapor pressure. This means running backward the equation given above for deriving saturation vapor pressure from temperature:

Let w = ln ( pa/ 610.78 )
Dew point = w *238.3 / ( 17.294 – w )

This calculation is often used to judge the probability of condensation on windows and within walls and roofs of humidified buildings.

The dew point can also be measured directly by cooling a mirror until it fogs. The RH is then given by the ratio

RH = 100 * ps (dewpoint)/ps (ambient)


The concentration of water vapor in the air

It is sometimes convenient to quote water vapor concentration as kg/kg of dry air. This is used in air conditioning calculations and is quoted on psychrometric charts. The following calculations for water vapor concentration in air apply at ground level.

Dry air has a molar mass of 0.029 kg. It is denser than water vapor, which has a molar mass of 0.018 kg. Therefore, humid air is lighter than dry air. If the total atmospheric pressure is P and the water vapor pressure is p, the partial pressure of the dry air component is P – p. The weight ratio of the two components, water vapor, and dry air is:

kg water vapor / kg dry air = 0.018 *p / ( 0.029 *(P – p ) )
= 0.62 *p / (P – p )

At room temperature P – p is nearly equal to P, which at ground level is close to 100,000 Pa, so, approximately:

kg water vapor / kg dry air = 0.62 *10-5 *p


Thermal properties of damp air

The heat content, usually called the enthalpy, of air, rises with increasing water content. This hidden heat, called latent heat by air conditioning engineers, has to be supplied or removed to change the relative humidity of the air, even at a constant temperature. This is relevant to conservators. The transfer of heat from an air stream to a wet surface, which releases water vapor to the airstream at the same time as it cools it, is the basis for psychrometry and many other microclimatic phenomena. Control of heat transfer can be used to control the drying and wetting of materials during conservation treatment.

Air at zero degrees Celsius is defined to have zero enthalpy. The enthalpy, h, in kJ/kg, at any temperature, T (K),  between 0 and 60°C is approximately:

h = 1.007T – 0.026,   below zero: h = 1.005T

The enthalpy of liquid water is also sometimes defined to be zero at zero degrees Celsius. To turn liquid water to vapor at the same temperature requires a very considerable amount of heat energy: ~2501 kJ/kg at 0C.

At a temperaturethe heat content of water vapor is:

hw = 2501 + 1.84

Notice that water vapor, once generated, also requires more heat than dry air to raise its temperature further: 1.84 kJ/kg.K against about 1 kJ/kg.K for dry air.

The enthalpy of moist air, in kJ/kg, is therefore:

h = (1.007*T – 0.026) + g*(2501 + 1.84*T)
g is the water content (specific humidity) in kg/kg of dry air



The Psychrometer

The final formula in this collection is the psychrometric equation. The psychrometer is the nearest to an absolute method of measuring RH that the conservator ever needs. It is more reliable than electronic devices because it depends on the calibration of thermometers or temperature sensors, which are much more reliable than electrical RH sensors. The only limitation to the psychrometer is that it is difficult to use in confined spaces (not because it needs to be whirled around but because it releases water vapor).

The psychrometer, or wet and dry bulb thermometer, responds to the RH of the air in this way:

Unsaturated air evaporates water from the wet wick. The heat required to evaporate the water into the air stream is taken from the air stream, which cools in contact with the wet surface, thus cooling the thermometer beneath it. An equilibrium wet surface temperature is reached which is very roughly halfway between ambient temperature and dew point temperature.

The air’s potential to absorb water is proportional to the difference between the mole fraction, ma, of water vapor in the ambient air and the mole fraction, mw, of water vapor in the saturated air at the wet surface. It is this capacity to carry away water vapor that drives the temperature down to, Tw, the wet thermometer temperature, from the ambient temperature ta :
(mw – ma) = B(Ta- Tw)
B is a constant, whose numerical value can be derived theoretically by some rather complicated physics.

The water vapor concentration is expressed here as a mole fraction in air, rather than as vapor pressure. Air is involved in the psychrometric equation because it brings the heat required to evaporate water from the wet surface. The constant B is therefore dependent on total air pressure, P. However the mole fraction, m, is simply the ratio of vapor pressure p to total pressure P:  p/P. The air pressure is the same for both ambient air and air in contact with the wet surface, so the constant B can be modified to a new value, A, which incorporates the pressure, allowing the molar fractions to be replaced by the corresponding vapor pressures:

pw – pa= A* ( Ta- Tw)

The relative humidity (as already defined) is the ratio of pa, the actual water vapor pressure of the air, tops, and the saturation water vapor pressure at ambient temperature.

RH% = 100 *pa/ ps = 100 *( pw – ( Ta- Tw) * 63) / ps
When the wet thermometer is frozen the constant changes to 56

The psychrometric constant is taken from: R.G.Wylie & T. Lalas, “Accurate psychrometer coefficients for wet and ice-covered cylinders in laminar transverse air streams”, in Moisture and Humidity 1985, published by the Instrument Society of America, pp 37 – 56. These values are slightly lower than those in general use. There are tables and slide rules for calculating RH from the psychrometer but a programmable calculator is very handy for this job. Alternatively, click on the calculator.