A PRIMER ON HUMIDITY
by William C Schreiner
(See also luthier David Schramm's page on humidity for more information on this topic.)
1. WHAT IS HUMIDITY?
2. HOW DO YOU MEASURE HUMIDITY?
3. SOME DEFINITIONS
4. WHAT CAN TOO MUCH OR TOO LITTLE HUMIDITY DO TO YOUR GUITAR?
5. HOW DO YOU CONTROL HUMIDITY?
6. A PSYCHROMETRIC CHART PROGRAM IS AVAILABLE FOR THE PC
1. WHAT IS HUMIDITY?
Relative humidity (RH) is the ratio of the density of the water vapor in the air at any given temperature to the density that would exist if the air were saturated with water vapor at that temperature. As an example, if the air at a temperature of 21 C. contains .00777 lbs. water per lb. air, and the air is CAPABLE of holding.01574 lbs. air, the relative humidity is: 100 X .00777/.01574 or 50%. A RH of 20% would be considered dry, 40-60% average, and 80% or more is likely to be uncomfortable. These are general numbers because comfort is also dependent on temperature, speed of air movement, and how active you are.
The water vapor is a variable constituent in air mixtures. In 1,000 cubic feet (28.3 cu. m.) of air at sub-zero temperatures (F.) as little as .005 lb. (2.3 g.) of water vapor can be present. Comparatively, air in the 100 deg. F. (38 C.) range can hold as much as 3 pounds (1.4 Kg.) of water vapor.
Dalton's Law of Partial Pressures -- I hate to get too technical here but I feel this is important. The law states that: When a mixture of gases is confined in an area, each gas exerts a partial pressure equal to the pressure it exerts if occupying the space alone. So, if you have dry air with a partial pressure of 14.5 lbs. per sq. in. and you add water vapor with a partial pressure of .2 lbs./sq. in. the combined air mixture will have a pressure of: 14.5 + .2 = 14.7 lbs./sq. in.
Consider that the outside temperature could be 92 F (33.3 C) and 80% RH while the inside of your house is 78F (26.5 C)and 60% RH. The difference in vapor partial pressures is .22 pounds/sq. in. (11.22 mm. Hg.). A vapor partial pressure difference of .22 psi is enough to get water vapor into or out of a guitar case. It is enough to pass through a house through cracks or openings, or to get between the walls on its way in or out of the house and cause frosting, mildew, rotting of wood. IT CAN BE VERY DIFFICULT TO STOP WATER VAPOR FROM GETTING WHEREVER IT WANTS TO GO.
To understand relative humidity a little better, let's pretend our entire practice room is wrapped with an impermeable vapor barrier. Nothing can get through it.
1. If we heat the room, the humidity will decrease The reason is that the amount of water vapor is the same but now the air is CAPABLE of holding more water. Your guitar may start to dry out.
2. If we cool the room, the humidity will increase.
3. If we somehow could move out the walls to create a slight vacuum, the humidity will decrease.
4. If we somehow could move in the walls to create a slight pressure, the humidity will increase. As an example, air compressors have to have a bleed valve on the storage tank because when the air is compressed, the humidity rises so much that water condenses on the bottom of the tank.
Now, let's do another experiment. We'll fill a plastic bag, an impermeable one, with air at 70 deg. F. (21 C.) and 50% humidity. Now let's seal the bag. Will you agree with me that the pounds of air and the pounds of water in the bag are constant? Okay, now let's take this bag in an airplane and climb to an altitude of 20,000 feet (6 Km.). The air in the plane is heated to 70 F. and you better have an oxygen mask or you'll die because this plane is not pressurized. I hope you remembered to put a humidity gauge in the bag and if you look at it now, you'll find that the humidity is 23%!
That's too low a humidity for a guitar so let's put some water vapor in the bag until we get the humidity back up to 50%. When we land our plane at sea level and a temperature of 70 F. (21 C.) we will find that our humidity in the bag is now 100% and the inside of the bag is soaking wet! Aren't you glad you didn't have your guitar in there? The reason is, as we decreased altitude, the pressure and humidity increased until the air simply could not hold any more vapor. The vapor then condensed into liquid, because we had reached the dew point. If this happened on the leading edge of the wing of the plane you would get icing and the increased weight could cause the plane to crash.
Why does the house get so dry in the Winter? Let's take our bag outside where the temperature is 32 F. (0 C.) and the relative humidity is 50%. This would create air that had .00189 lbs. water per lb. of dry air. We'll put this air in the bag, take it inside and let the bag warm to 72 F. It will still have .00189 lbs. water/lb. air won't it? Nothing got in or out of the sealed bag. Yet when we check the relative humidity in the bag, we will find it is 11.4%. When outside air infiltrates into your home in the winter, it has to be humidified to a comfortable level.
Also, do you remember Dalton's Law of Partial Pressures? In winter the partial pressure of the vapor inside the house is higher than that vapor pressure outside. The vapor is literally blowing out of the house and being replaced with dry air.
2. HOW DO YOU MEASURE HUMIDITY?
A sling psychrometer consists of two matched thermometers mounted on a common frame which swivels from the end of a handle. The bulb of one thermometer is covered with a wick wetted with distilled water and the unit is whirled through the air (hopefully, at the required standard of 1000 feet per minute or 304 m./min.). The moving air will pull heat out of the wetted wick causing the temperature at this thermometer (wet bulb) to read lower than the dry bulb thermometer. Using these two temperatures, it is possible to read the RH, and other information, from a psychrometric chart. Assuming the thermometers have an accuracy of + or - 1 degree F. and you are swinging the sling at exactly 1000 feet per minute, the accuracy is about + or - 4% around the 50% range of humidity. The chart is accurate only at standard atmospheric pressure. Also the error may be higher if you are using a small, hard-to-read chart. My brother-in-law, who has his Masters Degree in meteorology (weather forecasting), said he usually assumes a 5% error when using a sling psychrometer.
DIGITAL ELECTRONIC HUMIDITY METER
I recently purchased two models of these from a well-known ubiquitous shopping center electronics store (Oh all right, it was Radio Shack!) on sale for U.S. $19.95. The Radio Shack Digital Humidifiers can be set up to read Fahrenheit or Celsius scales. They were tested with a calibrated thermometer and the temperature accuracies were + or - 1 F on unit A and + 2 F on unit B. The temperatures were then used to determine the accuracy of the humidity portion using wet bulb and dry bulb readings in the range of 80% and 45% RH. Unit A was + or - 3% and unit B was + or - 2%. I used a computerized psychrometric chart which is accurate to + or - .1 F. Both units, when moved from, for example, an air-conditioned automobile to a humid room would register the new humidity to within 3% in 10 minutes. Unit A was an indoor/outdoor unit. The indoor portion read to the nearest degree and the outdoor portion read to the nearest tenth of a degree. The two thermometers were always within .5 of a degree of each other. Actually, they couldn't get any closer, could they?
Gauge hygrometers measure relative humidity, often by using the change in dimensions of a hygroscopic material such as human hair, wood, or paper. A hygroscopic material will get longer or shorter depending on the amount of water it absorbs. This material is connected to a dial to read humidity in percent and although they are cheap to produce, they aren't very accurate. The scale on these meters should just read "dry," "okay," and "wet" rather than humidity from zero to 100%. They are also slow to react and the reading should not be depended upon unless the gauge has been in the room for a couple of hours. Of the gauges I tested, unit one read 60% rather than 53%, unit two read 64% rather than 53%, unit three read 55% rather than 49%, unit four was accurate and read 70% rather than 71%, and unit five was found to be broken and always read 60%. ALL OF THESE GAUGES HAD BEEN CALIBRATED FOUR YEARS AGO!
I would recommend that everyone get a digital electronic humidity meter because of its reaction speed, ease of reading, and accuracy.
3. SOME DEFINITIONS
Specific volume: cubic feet of air per pound of dry air (cu. m./Kg. air)
Specific humidity: pounds water per pound of dry air. (Kgw/Kga) It is also called the humidity ratio and is abbreviated as W. If you had your guitar sealed in an impermeable plastic bag, the specific humidity would remain constant no matter what you did.
Enthalpy: BTU per pound of air. (KJ/Kga) This is mainly used by HVAC (heating, ventilating and air conditioning) engineers to determine how much energy it will take to extract water from, for example, the air in an office building.
Dew Point: the temperature at which dew starts to form because the air simply cannot hold all of it as a vapor. Put it this way: If you had a glass full of ice cubes and water, the dew point would be the temperature at which water droplets would form on the outside of the glass. It is also the temperature at which dew would start to form on grass as the temperature cools in the evening. In other words if your dry bulb and wet bulb temperatures are the same, you are also at 100% humidity and the dew point temperature, and condensation will take place. If the dew point temperature is below freezing you will get frost instead of dew.
4. WHAT CAN TOO MUCH OR TOO LITTLE HUMIDITY DO TO YOUR GUITAR?
The term hygroscopic refers to the characteristic of a material such as wood to absorb and retain moisture from the atmosphere. The amount of moisture it retains depends upon ambient environmental conditions.
A cubic foot (.028 cubic meters) of wood weighing 30 pounds (13.6 Kg) can hold 3 pints of water at 60% relative humidity (RH). Therefore a 3-pound guitar could hold 4.8 oz.(142 ml.) water at 60% RH If the humidity is lowered to 10%, which is common in unhumidified winter environments, the moisture in your guitar can drop to less than 1.6 oz (47 ml) water. The dimensional changes due to drastic shifts in moisture content of your guitar can result in shrinking, expanding, warping, checking and splitting. Dryness can actually pull glued joints of furniture apart. One factor that isn't covered in the engineering manuals is: what does this do to the tone of a guitar? Just the difference in weight of the wood will change the resonant frequency noticeably.
Expansion of wood due to moisture varies with the direction of the grain. Imagine the stress on a piece of wood that is glued to a stiffener that has the grain running in a completely different direction.
Painting or other forms of moisture proofing wood will protect it from moisture. This has been done with the outside of your guitar but the inside is bare, untreated wood.
Your guitar will be happiest when it lives in the same humidity conditions as it experienced when it was being built. Most luthiers seem to keep their workshops in the range of 50% RH.
Extreme dryness can also cause problems because of static charges when you are recording or amplifying your guitar. Static build-up occurs easily when the air is dry. Everything wants to act as a capacitor and store up a static charge. The result may be an excess of popping noises, sparks when you touch the microphone or other objects, static build-up on recording tape, etc. Many of these noises will actually be recorded. Adequate humidity allows static charges to bleed off before they can cause problems. I have solved stray voltage (static) problems in a computer lab by increasing humidity and pouring a few buckets of water on the earth surrounding the grounding rod for the electrical system.
5. HOW DO YOU CONTROL HUMIDITY?
An electric dehumidifier is like an air conditioner but the hot (condenser) coils and cold (evaporator) coils are both in the room and in the same cabinet. A fan blows air over the cold coils where it is cooled below the dew point at which time droplets of moisture will form on the evaporator coil and drop into a collection bucket. Then the air is passed through the condenser coils and reheated. The overall result is a slight increase in temperature, due to the heat input of the motor which drives the system. The air will be at a lower humidity, however.
An air conditioner has the compressor and condenser outside of the conditioned room so there is no heat gain. Just the evaporator and a circulating fan are in the conditioned room. The evaporator coil collects drops of moisture from the air, just as the dehumidifier does, but the remaining cold air flows into the room and cools it. The amount of dehumidification depends upon the temperature of the condenser coil and the speed at which the air flows through it. If it flows slowly, you will get maximum dehumidification and poor efficiency. Flow the air too slowly and the extracted moisture might freeze. A high flow will produce cool but damp air. It may feel uncomfortable due to the high moisture content but it is energy efficient. Some of today's high EER (energy efficiency ratio) air conditioners in the U.S. sacrifice good dehumidification in order to get low operating cost (but damp and "clammy") cooling.
If dry air is the only requirement, consider desiccant dehumidification. This is a material that has a high affinity for moisture. If put in a plastic bag with your guitar and a humidity gauge (for monitoring) it will remove moisture from the surrounding air and guitar. Some common desiccants include silica gel, activated alumina and molecular sieves.
After a desiccant approaches its adsorption capacity it becomes inefficient but some desiccants can be removed and subjected to heat in order to allow the desiccant to reject the moisture it contains. They can then be reused.
There are two kinds of desiccants: adsorbent and absorbent. The adsorbent desiccants change physically or chemically or both during a dehumidifying cycle. Absorbent desiccants do not change. A sponge would be a good example of an absorbent desiccant as it picks up water and it remains in the sponge as water.
Consider the following factors when selecting a desiccant that will be placed in the guitar case or near the guitar:
Sufficient moisture retention capability
Physical and chemical stability. Make sure it doesn't eat up your guitar
Resistant to contamination and deterioration
Non-toxic, odorless, inflammable and non-corrosive
Capable of regeneration with practical and available methods and at readily attainable temperatures
Commercially available at reasonable cost
I wouldn't suggest that you touch this stuff with your hands. I have seen some that will painfully take the moisture right out of your skin. Read the directions carefully.
I think everyone has some experience with humidification of your guitar in the carrying case using everything from wet rags to quality humidification devices purchased from music stores or elsewhere. Some basics to remember are:
1. Don't let the water touch your guitar as it could mar the finish or cause too much local water absorption if it gets inside the sound hole.
2. Experiment to find the right amount of humidification. Do you need 4 square cm. of a wet rag? More? Less?
3. Your guitar case should be reasonably impermeable, i.e., a good seal between case and lid, good locks, and a hard shell case.
4. You may want to experiment with a plastic bag in which to put your guitar before you put it in the case. The humidifier or dehumidifier and humidity gauge can also be placed in the bag. I use a garbage bag and tie tape for my guitar for short term storage, when traveling, and when the weather forecast calls for drastic humidity changes.
5. I'm not connected with this company in any way, but I learned from other guitarists that GSI sells Credo humidifiers and hygrometers for instrument cases. The unit consists of a permeable case containing a sponge with a solution absorbed in it. It acts like magic but it isn't and it works like this:
Remember that if you put a wet rag in a plastic bag, the humidity will go to 100%. If, instead, you put the Credo unit in the bag, the humidity will stay at 50%. If you put in more moisture, the Credo unit will take it out and store it in the sponge. If you take moisture out of the air, the Credo unit will take moisture out of the sponge and put it into the air. Something, I don't know what, is mixed with the water to trick it into thinking it has a vapor pressure that is one half that of pure water, therefore, you get 50% humidity.
ROOM OR HOME HUMIDIFICATION OR THE CASE OF THE LEAKY ROOF
I'm not going to cover room humidification as it is reasonably well known by everyone. I do want to dwell on overhumidification. In colder climes it is almost impossible to humidify to 50% relative humidity or more without problems unless you have a good vapor barrier in the outside walls and triple-glazed windows. The reason is that the high vapor pressure virtually squirts the moisture out of every crack and opening in your house and this can cause rotting, mold, and fungus in walls. Condensation on windows not only looks bad but can cause rotting of the sills.
Excessive humidification can also cause BLOCKS OF ICE in the attic. I once did an energy audit for a lady who complained that her roof was leaking, and it wasn't even raining. I discovered a small block of ice in the corner of the attic above a bedroom and it was melting due to the warm spring day. She was overhumidifying (with about 50 potted houseplants) and some of the vapor was exiting through the ceiling in the corner of the bedroom. The top, outside corner of a house is the hardest to insulate. Once it got through the insulation (which did not have a vapor barrier installed) it reached the cold attic air where it froze.
I recommend cutting back on winter humidification when excess moisture starts to condense on the windows. If you need the humidity, try lowering the room temperature somewhat.
To those of you living in warmer climates, remember that excessive air conditioning can also cause problems in your house due to outside moisture migrating through the walls and condensing in the wall cavity on the cool inside walls. This can cause rotting etc. in the wall cavity just as humidification does in winter climates. A vapor barrier will prevent this.
6. A PSYCHROMETRIC CHART PROGRAM IS AVAILABLE FOR THE PC
To more accurately determine humidity I use a psychrometric chart on the computer. No interpolation between lines is necessary, nor are adjustments necessary because you live at 2000 feet above sea level. The chart takes 34 Kb of space and is free to anyone who asks me for it. English or metric units can be used.
If you enter any two of the following, the chart will automatically give you the others:
Dry bulb Wet bulb
Dry bulb Relative Humidity
Dry bulb Specific Humidity
Dry bulb Enthalpy
Dry bulb Dew Point
Specific Humidity Wet Bulb
Specific Humidity Relative Humidity
Specific Humidity Enthalpy
In addition, the printout will give you specific volume.
Just send me a private Email (email@example.com) asking for it. This program is for a personal computer (PC) only, it won't work on a Mac.
7. THE IDEAL GUITAR CASE
It's time for some manufacturer to design and sell the ideal guitar case. Besides inclusion of room for a folding footstool, music stand, and some music, it should:
1. Be airtight to prevent moisture from entering or leaving the inside.
2. Be reasonably well insulated to prevent rapid shifts in temperature which, as you now know, can cause rapid shifts in humidity.
3. Have an electronic temperature and humidity meter installed in the case and preferably visible through a clear window on the case so it can be read without having to open the case.
4. Have an alarm that will alert you if any temperature or humidity problems are occurring that could affect your guitar while it is in the case.
5. Have a built-in humidifier and dehumidifier, both of which can be turned on or off and adjusted from outside of the case.
I hope this information is of value to you. If there are any errors or omissions, problems with clarity, etc., please let me know. I have a thick hide.
ABOUT THE AUTHOR
Bill Schreiner didn't know what he wanted to be when he grew up but after 6.5 years of college he ended up with a degree in Mechanical Engineering from the University of Wisconsin, USA. He is a member of the National Society of Professional Engineers. He is curious about everything, gets bored quite easily, and is probably the only engineer in the world who has studied Vertebrate Embryology.
He was a Research Engineer and a Service Dept. Staff Engineer for Caterpillar, Inc.
He was a Foreign Service Representative for Caterpillar Americas, S.A., which is where he discovered the classical guitar.
He was President and CEO of TEAM Electronics of Green Bay, Inc.
He was an Industrial Applications Engineer and Marketing Supervisor for Wisconsin Public Service Corp., a gas and electric utility.
He still doesn't know what he wants to be when he grows up.
Humidity Control by Ivan Stepnich, Business News Publishing Co. Troy, Mich, 1988
Psychrometric chart, Willis Carrier, 1911
Engineering Applications Specialists, Inc. Chart/PC
Mark's Standard Handbook for Mechanical Engineers 8th Edition
Handbook of Air Conditioning Heating and Ventilating Editors Stamper and Koral
See also luthier David Schramm's page on humidity for more information on this topic.
© Roger G. Thurman 1998-2002
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