While most DIY projects probably won’t require the use of a relative humidity (RH) meter, there are some situations where you may need to use one of these devices.
For example, you may want to double check the humidity conditions in your house after a moisture damage remediation company declares that they’re finished with their dry-out operations. High ambient humidity in a room could be an indication that there are still moisture-compromised materials in the area, and that dry-out operations aren’t complete.
There are other examples of times that you might want to use a thermo-hygrometer, or RH meter. Whatever task you end up using your RH meter for, however, you probably want to make sure that the readings you get are as accurate as possible.
Ensuring RH meter accuracy can be difficult, as these meters can be affected by a number of factors, such as:
- Placement of the sensor.
- Contaminants on the sensor element.
- Temperature of the room being tested.
The first issue is easily controlled, and different RH meters will usually have placement tips included with the meter to help you avoid problems.
The second issue is one of maintenance and storage. To prevent this issue, avoid keeping your meter’s sensor out when it isn’t in use, storing it in its case whenever possible. If a sensor does get too badly contaminated, you’ll need to replace it.
However, the third issue, the ambient temperature, is one that many DIY enthusiasts might miss when checking the accuracy of an RH meter.
How does the temperature affect the accuracy of an RH meter, and how can you deal with this issue?
Relative Humidity and Different Temperatures
When checking the relative humidity of the air in a space, it’s important to know that the RH reading is based on a percentage of how much humidity would be required to make the air saturated with moisture.
This is why the temperature of a space can have a large impact on the relative humidity reading. As noted by the climate.ncsu.edu website, “if the water vapor content stays the same and the temperature drops, the relative humidity increases… This is because colder air doesn’t require as much moisture to become saturated as warmer air.”
Basically, warmer air needs more moisture to reach its saturation point, while colder air has a much lower saturation threshold. This is why it’s called “relative” humidity.
Dealing with Temperature Extremes When Testing Relative Humidity
There are a few ways of dealing with high or low temperatures when checking the relative humidity:
- Control the Temperature of the Environment. If you’re checking RH values indoors and have climate control, simply set the room temperature to a value that is optimal for your RH meter and let the room temperature stabilize before taking your reading. This helps avoid the issue of temperature affecting RH altogether.
- Correct for Temperature When Taking Readings. If the temperature of the area where you’re taking readings is unusually high or low, try correcting for the current temperature in your RH reading. The amount of correction needed will most likely vary by the make and model of meter that you use, so contact your manufacturer for information about correcting for extreme temperatures.
- Take Multiple Readings. In situations where the temperature of your reading environment changes regularly throughout the day, try taking multiple readings when the temperature is high, low, and in-between. For example, you could test RH when it’s cool in the morning, at the height of the heat at 2 or 3 p.m., and at dusk when it’s temperate.
- Check Dew Point Instead. If your RH meter has a dew point setting, consider using that instead of the RH setting. This setting tells you what temperature the air would have to be cooled to in order to become saturated. The climate.ncsu.edu page notes that “Dew point is a more reliable indicator of humidity than relative humidity because dew point is not changed by a change in air temperature and doesn’t fluctuate much throughout the day.”
Some of these methods of dealing with high or low temperatures when measuring RH are easier than others, and a few require your moisture meter to have specific capabilities.
So, when checking RH on your own, be sure to know what the current temperature is, and how that can influence your relative humidity reading.