Humidity
7000 GRAINS= 1 lb. in a cu ft of air.
Relative Humidity is the percentage of saturation
Air is like a sponge- it takes more humidity at higher temperatures.
10 degrees of temperature is 25% of the initial value.
Air at 32 degrees holds 2 grains of water = 100% relative humidity
Air at 70 degrees holds 8 grains of water = 100% relative humidity
Example:
At just about freezing, you bring in air at 100% relative humidity. You heat it to 70 degrees— therefore the maximum humidity in the air is 2/8 = 25% relative humidity. Anything above that is coming from another source such as people, furniture, carpets, etc.
On the other hand, on a hot rainy day the air is coming in at 100% relative humidity and passing through cooling coils on an air conditioner. This lowers the relative humidity of the air to 100% at 60 degrees or 50% relative humidity. It then mixes with the air in the room which will have a higher relative humidity.
To dehumidify, one of the standard methods is to lower the incoming air to its 100% level (dew point), and then raise the temperature back up (the old trick of putting a light bulb in a closet).
Dewpoint Chart
This chart shows the Dewpoint for temperatures from 70 to 85°F and for relative humidity from 50 to 90%.
| Temperature (°F) |
Relative Humidity (%) |
Dewpoint (°F) |
| 70 |
50 |
58.6 |
| 70 |
60 |
61 |
| 70 |
70 |
63.5 |
| 70 |
80 |
65.6 |
| 70 |
90 |
68 |
| 75 |
50 |
62.5 |
| 75 |
60 |
65.3 |
| 75 |
70 |
68 |
| 75 |
80 |
70.5 |
| 75 |
90 |
72.8 |
| 78 |
50 |
65 |
| 78 |
60 |
68 |
| 78 |
70 |
70.8 |
| 78 |
80 |
73.2 |
| 78 |
90 |
75.6 |
| 80 |
50 |
66.8 |
| 80 |
60 |
69.9 |
| 80 |
70 |
72.6 |
| 80 |
80 |
75 |
| 80 |
90 |
77.8 |
| 85 |
50 |
70.9 |
| 85 |
60 |
74 |
| 85 |
70 |
77.2 |
| 85 |
80 |
80 |
| 85 |
90 |
82.6 |
According to the U.S. Department of Energy, the temperature of an area may be reduced below 78°F to bring the Dewpoint to 65°F.
Hydrodynamics
Creating and Maintaining Humidities by Salt Solutions
Need for known Humidities Exists
The requirement for reliable humidity measurement or investigation into the effects of humidity is increasing. Many studies are incorrect or inconclusive due to limitations of the humidity measuring or control equipment used.
Unfortunately there are no devices available which directly measure the moisture content of gases with accuracy without involved and painstaking techniques. Practically all humidity instruments require calibration or operate on the assumption (frequently erroneous) that the principle employed indicates true thermodynamic properties. This is rarely the case and can be only if the basic design requirements could be met and perfect operational techniques assumed. Therefore, the demand exists for reliable humidity generators to produce known reference humidity conditions for calibration purposes.
Primary Humidity Generators
The basic or primary methods of producing a known constant humidity (usually expressed as relative humidity at a particular temperature) involve devices controlling the variables of the fundamental gas laws. Relative humidity is the ratio of the partial pressure of water vapor present to that which could be present at saturation, times 100. The saturation vapor pressure is determined only by the temperature of the (ideal) gas. Therefore, assuming behavior in accordance with the gas laws, known humidities can be created by manipulation of the temperature and pressure of saturated gas or by dilution of the saturated gas with an extremely dry gas, or by a combination of these techniques.
These basic methods of producing known humidities require expensive equipment and exacting techniques, and they assume complete saturation of the gas at a particular temperature and pressure. This last condition is a theoretical assumption and the degree of completeness can vary with the design of the equipment, external factors, such as temperature and barometric pressure, and the skill of the operator.
Secondary Humidity Generators
Known constant humidities can also be created by secondary methods, by materials whose affinity for water regulates the water vapor pressure in the atmosphere surrounding the material. Among the more readily controllable materials are salt solutions and acid solutions. Numerous lists of the humidities created by various salts and acid concentrations appear in the literature, inferring a certain ease of producing the specified humidity conditions. An ease of maintenance of these humidity conditions with high accuracy is also inferred since the temperature dependency is very small. The ease of handling and low cost of salts has promoted the widespread use of salt solutions as humidity generators.
Conflicting Humidity data from Published Tables
Saturated salt solutions can provide a stable humidity condition; however, the condition created is not necessarily the relative humidity cited in the literature. Particularly, since the various references disagree as to the humidity created. Tests at the U.S. National Bureau of Standards were compared with the published results of various investigations (1). In general the scattering of reported values were within a band of ±1.5% R.H. from the NBS results. Therefore, only a qualified expression of the humidity created should be made, with the particular salt and reference source cited.
Requirements for Saturated Salt Solutions
While interpretation of the humidity values created by the various salt solutions is questionable, the humidity created can be reproducible and stable. Attainment of stable humidity values is governed by the purity of the salt, by the purity of the water, by the preparation of the salt solution, by the water vapor equilibrium rate between the liquid and vapor, by the temperature equilibrium between the liquid and vapor and by the presence of hygroscopic materials within the vapor space.
Chemical Purity Essential
Obviously the purity of the salt and the purity of the water will determine the equilibrium humidity condition. Small amounts of containmants can, in some cases, seriously affect the end results. Chemically pure salts and distilled water must be used.
The introduction of high speed processes and waste control programs has developed the realization that many products do not always benefit from so called air conditioning which is no more than temperature control. Many materials, particularly those of organic nature, are more sensitive to humidity variations than to temperature fluctuations. Engineers and scientists responsible for quality control, new product development and better production who are specialists in their own fields frequently have the popular misconceptions toward humidity measurement and control Therefore, mechanical hygrometers, principally the hair type, are still quite popular since their limitations are not generally understood or appreciated.
The information contained herein has been collected, from reliable and authoritative sources to help explain unresolved problems encountered by present users of hygrometers, and to assist prospective users of hygrometers in properly evaluating these devices.
Hair Hygrometry
First Reported Humidity Instrument
The hair hygrometer is the first recorded instrument for measurement of the amount of moisture in the air. About the year 1500 Leonardo da Vinci described a device whereby the quantity of moisture in the air was noted from the change in weight of a ball of wool or hair.
Physical Changes provide mechanical Movement
Hair and other mechanical hygrometers operate on the principle that absorption and desorption of water from the air by an organic material causes a physical change in the material: change in weight, change in length, change in volume or a rotational effect. These physical motions have been translated through various techniques into calibrations in terms of relative humidity.
Only Minor changes in 175 years
In 1783 the hair hygrometer was developed into its present style, measurement of change in length, by De Saussure. Since that time the hair hygrometer has been probably the most widespread instrument used for humidity measurement and control. Numerous innovations have been tried, but, after over 175 years, the hair hygrometer is essentially unchanged; more is known about it technically, but attempts to improve it have been ineffective.
Hair still best Mechanical Hygrometer
Prior to the introduction of precision hygrometry, the hair hygrometer was the standard for general design and use. The basic limitations, appreciated during the early years of hair hygrometry, of limited strength and very small change in length has led to considerable investigation into other types of humidity sensitive materials. Materials studied included paper, wood, bone, textiles, animal tissues and in recent years plastics such as nylon. To date no material has been found that is as satisfactory as human hair. For example tissue is stronger, but also more unstable, and nylon has a greater change in length, but also a greater temperature coefficient.
Hair Hygrometer Adequate in its day
Hair Hygrometry has been a worthwhile contribution to the science of measurement and control. Prior to precision electric hygrometry in the past 20 to 30 years the hair sensors provided acceptable devices, suitable for the existing technology. Materials were not prepared to too exacting tolerances, high speed mass production was unknown, hand tailoring was commonplace, storage of perishables and even hard goods were relatively unknown, etc. In effect, the advantages of humidity knowledge were not appreciated because the necessity was not apparent. The hair hygrometer is frequently misapplied today. The mechanical devices serve a purpose, because of low initial cost, in non-commercial, domestic, comfort control. For commercial and industrial applications where humidity is important, the qualifications of the hair devices must be considered against other requirements.
Present Requirements Require Re-evaluation
Today, atomic technology delves into the innermost reaches of science, yet the same scientists do not understand or even consider the adaptability of one of the basic tools- the hygrometer- to their requirements. Improper humidity determinations or control can materially affect the success or failure in such diverse areas as space studies and bread baking. To properly evaluate the effect of humidity on a material, process or function, the quality of the measuring or control instrument must be evaluated.
Hygrometer Prerequisites
Evaluation of the hair hygrometer (or any other humidity device) requires consideration of the principle, the design requirements, measuring range (both humidity and temperature), accuracy, sensitivity, speed of response, stability and adaptability as well as basic psychrometric requirements such as calibration.
Humidity and Temperature Dependency
Hair and nearly all organic materials absorb and desorb moisture from the air as a function of the temperature of the material and the water vapor pressure of the surrounding atmosphere. This change in moisture condition of hair results in a change in length which only approximates a function of relative humidity.
Small Non-uniform mechanical Movement
Human hair will increase in length approximately 2½% as the relative humidity is raised from 0 to 100% (1). This means that a hair 4.0 inches long at 0% R.H. will be 4.1 inches long at 100% RH. Unfortunately, this change in non-linear and is logarithmic over most of the humidity range with sensitivities as follows: (values are given for a 4 inch hair).
| RH Level |
Sensitivity |
| |
(Change in length for 1% change in R.H) |
| 15% |
0.020 inch |
| 50% |
0.007 inch |
| 80% |
0.00035 inch |
In order to use these very small and non-uniform changes in length, mechanical amplification must be used. Also, linearization, which introduces errors due to minute amounts of friction, backlash, etc. must be considered. To offset these effects a number of hairs are usually multiple mounted to provide greater strength. This requires adequate spacing for free moisture ventilation and uniform tension in all hairs.
Many air born contaminants are water soluble and may be of a mineral or hygroscopic nature. Since these contaminants may cause a shift in the equilibrium humidity condition, reliability requires that stale solutions be replaced with fresh solution. In addition to this shift, some salts react chemically with the container or air borne contaminants, altering the composition of the solution and thus the equilibrium humidity.
"Solution" Not a Solution, But a Slush
The preparation of the salt solution is a very important but frequently overlooked consideration. The solution should be a slush consisting of a solution with excess undissolved crystals (2). Too much water, as in a true solution, even with undissolved crystals at the bottom, can result in a layer of less-than-saturated solution at the surface which will produce a higher humidity than anticipated. Conversely, solid crystals protruding above the surface of the liquid can reduce the humidity. A good solution can usually be made by adding distilled water slowly, with constant stirring, until about half the salt crystals are dissolved.
Large Solution Surface Area and Small Vapor Space Desirable
The diffusion rate of moisture from a saturated salt solution is exponential, with a quick approach to the near equilibrium value, but a very gradual final approach to the actual end point. The diffusion rate is governed by the difference in water vapor pressure of the solution and of the vapor and by the attraction of the salt for the water. With some salts, the attainment of complete equilibrium requires days. In stagnant air even the approach can take considerable time. For faster stabilization (to the near equilibrium condition) it is recommended that the surface area of the solution be as large as practical, that the vapor space be as small as practical and that the air be circulated over the solution.
Temperature of Prime Importance
At first glance, the temperature is not a significant factor. For example, sodium chloride provides 75.5% R.H. at 68° F, 75.8% R.H. at 77°F and 75.6% R.H. at 86°F (1), or a maximum change of only 0.3% R.H. for a change of 9°F in this temperature range. However, temperature is extremely important from a psychrometric view point. The literature values for saturated salt solutions are based on temperature equilibrium between the solution and the vapor above it.
A saturated salt solution has a definite water vapor pressure at a given solution temperature. The relative humidity created in the vapor space is determined by the water vapor pressure and the saturation water vapor pressure at the temperature of the vapor. For example, a saturated solution of sodium chloride at 77°F generates a vapor pressure equal to 75.8% R.H. if the vapor is also at 77°F. If the vapor is at 75°F, the solution still generates a vapor pressure equivalent to 75.8% R.H. at 77°F but the relative humidity is now 83.1% R.H. at the vapor temperature of 75°F. At these particular conditions a temperature difference of only 2°F between solvent and vapor causes a non-apparent error of approximately 5% R.H.
Allow Fresh Solution to Cool
The effect of temperature differences between solution and vapor can be compensated for if reasonably constant. Unfortunately this effect is frequently overlooked upon initial preparation of solutions when the heat of reaction of the salt and water can raise the temperature of the water very significantly. Fresh solutions should be made up well in advance of actual use and sufficient time allowed for them to cool off.
Unseen External Factors
The function of a saturated salt solution is to create a specific humidity condition by giving off or absorbing moisture from the vapor space above it. Anything hygroscopic, or organic in nature will also absorb or give off moisture; therefore, wood, rubber, etc., are not usually recommended for constructional materials in humidity chambers. However, one of the prime functions fo a humidity chamber is to test or precondition hygroscopic materials. Thus, introduction of a large source of unseen humidity balance by the item being tested is often ignored with detrimental effects.
Materials such as paper products and textiles are very hygroscopic and their rate of absorption or desorption of moisture is considerably greater than that of the saturated salt solution. The approach to the literature cited values under some comon circumstances could take weeks and even months. Experiments with paper products have shown differences of as much as 10% R.H. between actual and literature (predicted) values even after a week's exposure period in small chambers with relatively small paper samples (3).
Actual Humidity Created Must be Determined
All of the foregoing indicates the uncertainty as to the actual humidities created by the saturated salt solutions and the necessity for measuring them by an independent means. A suitable device for measuring the humidity would only be one that does not add or subtract a significant amount of moisture from the vapor nor introduce a temperature unbalance (unless measuring absolute humidity). These requirements preclude the general use of the wet-dry bulb psychrometer for salt solution chambers as the water from the wet bulb wick probably would drastically alter the humidity and the frictional heat of the fan required for the wet bulb would cause adverse temperature effects.
The following values were selected from the literature referenced with some values adjusted by interpolation to the temperatures listed.
Equilibrium Relative Humidities for Saturated Salt Solutions
| Saturated Salt Solution |
Formula |
Percent Relative Humidity at Stated Temperatures |
| 68°F (20° C) |
77°F(25°C) |
86°F(30°C) |
| Lithium Chloride |
LiCl • H2O |
12.4 |
12.0 |
11.8 |
| Potassium Acetate |
KC2H3O2 |
23.3 |
22.7 |
22.0 |
| Magnesium Chloride |
MgCl2 • 6H2O |
33.6 |
33.2 |
32.8 |
| Potassium Carbonate |
K2CO3 • 2H2O |
44.0 |
43.8 |
43.5 |
| Potassium Nitrite |
KNO2 |
49.0 |
48.1 |
47.2 |
| Magnesium Nitrate |
Mg (NO3)2 • 6H2O |
54.9 |
53.4 |
52.0 |
| Sodium Nitrite |
NaNO2 |
65.3 |
64.3 |
63.3 |
| Sodium Chloride |
NaCl |
75.5 |
75.8 |
75.6 |
| Ammonium Sulfate |
(NH4)2SO4 |
80.6 |
80.3 |
80.0 |
| Potassium Nitrate |
KNO3 |
93.2 |
92.0 |
90.7 |
| Potassium Sulfate |
K2SO4 |
97.2 |
96.9 |
96.6 |
Conclusion
Saturated salt solution generation of humidity is only a secondary method of obtaining an approximate humidity condition. Disagreement among references as to the humidities created indicates a need to qualify this method by independent measurements and to note the salt and reference source used.
To obtain stable humidity values it is essential that the following conditions be met:
- Chemically pure salt
- Distilled Water
- Large solution surface area and small vapor space
- Adequate air circulation
- Elimination of hygroscopic construction materials
- Solution and vapor at same temperature or in balance.
- Time allowed for vapor equilibrium.
Most of the above requirements (1 thru 5) can be met inexpensively and without elaborate techniques except for the temperature and diffusion rate requirements (6 and 7).
While a difference in the temperatures of the solution and vapor will produce a humidity other than the literature value, a stable humidity condition can be created and maintained without the expense of precise temperature regulation. As long as the temperature relationship between solvent and vapor is kept constant and the resultant humidity is also constant. Normally, in a properly designed small chamber the temperatures will be within relatively close agreement so that a stable humidity, within several percent relative humidity of the literature value, will be maintained. However, it is recommended that an independent humidity measuring device, such as Hygrodynamics' Hygrosensor, be used to determine the actual humidity condition created and to monitor its stability (4).
The introduction of the hygroscopic material to be tested or conditioned can greatly affect the time required to approach the true equilibrium relative humidity. The mass and hygroscopic nature of the material may be such that fairly stable humidity conditions consdiderably different from the literature values are created. Therefore, when conditioning or testing a hygroscopic material it is essential that a humidity measuring instrument be used to verify the existing humidity condition.
Saturated salt solution generation of humidity is not as simple as generally believed, but it need not be complex. As noted, the properly designed humidity chamber has adequate air circulation, it minimizes use of hygroscopic materials and temperature unbalance and uses a reliable independent humidity measuring device. For information on chambers meeting these requirements, or if only the humidity measuring equipment is desired, contact Hygrodynamics, Inc.
References
- A. Wexler and S. Hasegawa, "Relative humidity-temperature relationship of some saturated salt solutions in the temperature range 0° to 50° C". J. Research NBS 53, 19 (1954).
- A. Wexler and W.G. Brombacher, "Methods of measuring humidity and testing hygrometers", NBS Circular 512 (1951).
- S. Martin, "The control of conditioning atmospheres in small sealed chambers", J.Sci. Instr. 39, 370 (1962).
- C.P. Hedlin and F.N. Trofimenkoff (Division of Building Research National Research Council, Saskatoon, Sask., Canada), "Relative humidities over saturated solutions of nine salts in the temperature range from 0 to 90°F, Paper presented at 1963 International Symposium on Humidity and Moisture.
Additional Salt-Humidity Tables and References
Wilmer A. Wink (Institute of Paper Chemistry), "Salt Solution, Equilibrium Relative Humidity", Industrial and Engineering Chemistry, April 1946, page 251.
F.E.M. O'Brien, "The control of humidity by saturated salt solutions— a compilation of data", J.Sci. Instr. 25, 73 (1948)
International Critical Tables 1, 68 (McGraw-Hill Book Co. New York, N.Y.)
Handbook of Chemistry and Physics (Chemical Rubber Publ. Co., Cleveland, Ohio)