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How to measure indoor humidity & temperature:
Tool & instrument choices: this article describes tools and instruments used to measure indoor humidity, relative humidity, and temperature - also giving the dew point.
We discuss choices and accuracy of equipment used to measure indoor humidity level as a step in avoiding indoor condensation trouble, mold and other
indoor pathogen growth in buildings.
This article answers the question "How do I maintain to avoid mold and indoor air quality issues?"
We also discuss where and how to measure indoor humidity, what indoor humidity targets to set, and we explain
relative humidity, dew point, and moisture condensation in and on building materials.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Choices of Instruments to Measure Temperature and Relative Humidity Indoors - Hygrometers
When you buy an instrument to measure indoor humidity, you must either buy one which reads the result in relative humidity or you'll have to do some calculations to factor in temperature.
Relative humidity is defined as the ratio of partial pressure of water vapor to the saturated water vapor pressure in air at a given temperature, expressed as a percent.
Example: A cubic meter of air at 27°C has a saturation vapor pressure of 35 millibars (mb) or about 0.5 psi. [1 mb = 0.0145038 psi]
If, then we measure the actual water vapor pressure in that air as 17.5 (mb) or half of the saturation pressure or a relative humidity of 50%.
[Click to enlarge any image]
The old, amusing, but very
accurate sling psychrometer made an "end run" around the calculation problem by swinging a wet bulb thermometer
through the air, literally slinging it around in a circle at the end of a string.
caused the water on the bulb to evaporate. The instrument then measured the
temperature drop from evaporation and enabled reading of the relative humidity.
Portable hand-held hygrometers or thermo-hygrometers are widely available, accurate to +/- 2.5%, some more accurate to +/- 0.5%, and are used by many home inspectors, environmental inspectors, hygienists, as well as industrial specialists.
Manufacturers of portable hygrometers include the following:
Autotronic Enterprise Co., Ltd.,
General Electric Corporation,
Shincluster Electronics Co., Ltd.,
Tecpel Co. Ltd.,
Shown above is the Tecpel model DTM 550 hand-held thermo-hygrometer accurate to +/- 0.5%
Modern electronic instruments which read
relative humidity are available from many suppliers and read out immediately in relative humidity, or "RH".
We have found that the less costly instruments, perhaps in the $40. to $100 range, are not precise "lab-grade"
instruments, but they'll generally be within 1-3 percentage points of one another - accurate enough for our purposes,
particularly if the instrument is consistent in its behavior.
Since these instruments have to know the
temperature as well as the humidity in order to calculate the relative humidity of the
air, they'll usually give the temperature reading as a bonus along with the RH reading.
At left we show a low cost Radio Shack™ indoor-outdoor temperature and humidity monitor.
Variations in Indoor Relative Humidity by Building Area and Surface Type
The relative humidity, or "RH" will vary significantly in a building at a given moment, depending
on where you make your observations.
Here are some example RH measurements from
a recent investigation at a 1970's wood frame two story home in generally good condition, after an extensive
mold remediation and dryout project, where the owner had been running two dehumidifiers in the basement, and
where there were no building leaks:
RH 48% - Outdoors
RH 45% - Indoors, main floor kitchen, center of room, 5 ft. from floor
RH 49% - Indoors, basement, center of single large area, 5 ft. from floor
RH 57% - Indoors, basement, 1" from rear wall, 3/4" from floor
RH 49% - Indoors, basement, at intake side of the dehumidifier, with the machine operating
RH 35% - Indoors, basement, at the outlet side of the dehumidifier, with the machine operating
Notice, with no surprise, that the RH is higher close to the (cool) masonry surface? This explains our reasoning in suggesting a fairly low basement RH target for buildings if we're going to measure the RH
in the center of the room.
Some dehumidifiers have an RH meter built right into the machine, so it will
tell you what RH level it's seeing in its incoming air. But for operating efficiency you'll often run the machine
in the center of the room.
The target humidity for a building, if measured at room center, needs to be low enough to avoid
condensation out on cool surfaces at the room perimeter or floor.
How do you measure indoor humidity and relative indoor humidity?
Relative Humidity vs. Absolute Humidity in buildings
A variety of instruments can measure the amount of moisture in air, which we call "humidity." For example an inexpensive
indoor "weather station" often includes a "humidity" gauge along with a barometer and thermometer.
But just knowing
the level of moisture in air (absolute humidity) is not enough. Usually, the humidity targets
we use in these articles, and in academic
or scientific texts are numbers expressed as relative humidity which takes into account not only the absolute
water level in the air, but also the air temperature.
Relative humidity, by taking into account both the absolute humidity in the air and the temperature of the air,
is telling you the humidity level as a function of the maximum amount of water that the air is capable of
containing at a given temperature.
If we're trying to control mold and other indoor pathogens for which water is a gating
factor, it's relative humidity that is important. Why? Because water condenses out of air onto a
building surface (and thus supports mold or other indoor pathogens) only when the air at that surface contains
more water than it can hold at that temperature.
When warm, moist air contacts a cool surface, your basement drywall
near the floor, for example, the air touching that surface may cool and give up some of its moisture to condense
on the surface.
Watch out: We recommend use of dehumidifiers and
humidity instruments or humidity transmitters to monitor your building.
But no dehumidification system will be up to the task of preventing mold if a building has serious leaks, flooding, or water entry.
No dehumidifier, no "air cleaner," no "ozone generator," nor other magic machine, spray, or air treatment will correct a mold problem
in a building if there is a significant problem reservoir.
For that case, what's needed is to find the mold problem, remove it,
and correct its cause. And as a last warning, there are about 1.5 million mold species - some of them may be able to grow in very
dry or very wet or other inhospitable conditions.
Continue reading at HUMIDITY CONTROL & TARGETS INDOORS for an explanation of the proper indoor humidity levels we should be trying to achieve, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Professional Equipment is a retailer of inspection tools and test equipment - selling temperature, humidity, and RH measurement tools from under $20. to over $1,300. U.S.
Tecpel Co., Ltd., 4F-1, No 225, HePing East Road, Sec 3, Taipei 11056 Taiwan ROC 888-2-2737-5866
"Damp Indoor Spaces and Health", Institute of Medicine, National Academy of Sciences, 24 May 2004. - Web Search 6/23/2010 - original source: http://www.iom.edu/Reports/2004/Damp-Indoor-Spaces-and-Health.aspx
WHO Guidelines for Indoor Air Quality: Dampness and Mould (World Health Organization Europe), WHO Regional Office for Europe, ISBN-10: 9289041684, ISBN-13: 978-9289041683 When sufficient moisture is available, hundreds of species of bacteria and fungi -- particularly mold -- pollute indoor air. The most important effects of exposure to these pollutants are the increased prevalence of respiratory symptoms, allergies and asthma as well as disturbance of the immune system. Preventing (or minimizing) persistent dampness and microbial growth on interior surfaces and building structures is the most important means of avoiding harmful effects on health.
This book provides a comprehensive overview of the scientific evidence on the health problems associated with this ubiquitous pollution and provides WHO guidelines to protect public health. It also describes the conditions that determine the presence of mould and provides measures to control its growth indoors.
Olalekan F. Osanyintola, Carey J. Simonson, Moisture buffering capacity of hygroscopic building materials: Experimental facilities and energy impact, article within Energy and Buildings,
Research into dynamic moisture storage in hygroscopic building materials has renewed interest in the moisture buffering capacity of building materials and shown the potential for these materials to improve indoor humidity, thermal comfort and indoor air quality in buildings. This paper complements previous research by estimating the effect of hygroscopic materials on energy consumptions in buildings. The results show that it may be possible to reduce heating and cooling energy consumption by up to 5% and 30%, respectively, when applying hygroscopic materials with well-controlled HVAC systems. The paper also describes two different experimental facilities that can be used to measure accurately the moisture buffering capacity of hygroscopic building materials. These facilities provide different convective transfer coefficients between the hygroscopic material and ambient air, ranging from natural convection in small, sealed jars to fully developed laminar and turbulent forced convection. The paper presents a numerical model and property data for spruce plywood which will be used in a companion paper [O.F. Osanyintola, P. Talukdar, C.J. Simonson, Effect of initial conditions, boundary conditions and thickness on the moisture buffering capacity of spruce plywood, Energy and Buildings (2006), doi:10.1016/j.enbuild.2006.03.024.] to provide additional insight into the design of an experiment to measure the moisture buffering capacity of hygroscopic materials.
Moisture buffering capacity;
Indoor air quality;
Convective transfer coefficients;
Olalekan F. Osanyintola, Prabal Talukdar, Carey J. Simonson
Effect of initial conditions, boundary conditions and thickness on the moisture buffering capacity of spruce plywood
Energy and Buildings, Volume 38, Issue 10, October 2006, Pages 1283-1292
Kansas State University, department of plant pathology, extension plant pathology web page on wheat rust fungus: see http://www.oznet.ksu.edu/path-ext/factSheets/Wheat/Wheat%20Leaf%20Rust.asp
"A Brief Guide to Mold, Moisture, and Your Home",
U.S. Environmental Protection Agency US EPA - includes basic advice for building owners, occupants, and mold cleanup operations. See http://www.epa.gov/mold/moldguide.htm
US EPA - Mold Remediation in Schools and Commercial Building [ copy on file as /sickhouse/EPA_Mold_Remediation_in_Schools.pdf ] - US EPA
US EPA - Una Breva Guia a Moho - Hongo [on file as /sickhouse/EPA_Moho_Guia_sp.pdf - - en Espanol
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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