Radio shack hygrometer (C) Daniel Friedman Hygrometers & Thermometers
Guide to Tools for Measuring Indoor Humidity & Temperature

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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

Tecpel hand held hygrometer

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.

This motion 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:

Shown above is the Tecpel model DTM 550 hand-held thermo-hygrometer accurate to +/- 0.5%

Radio shack hygrometer (C) Daniel Friedman

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:

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.


Or see DEW POINT TABLE - CONDENSATION POINT GUIDE for an explanation of dew points and relative humidity in buildings.

Or see these

Humidity & Moisture Articles

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HUMIDITY MEASUREMENT INSTRUMENTS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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