Measuring (and Understanding) Humidity

Author’s Note: I can’t even start this blog before thanking Lew Harriman of Mason-Grant Consulting. Lew very patiently and gently hammered me into a much better understanding of humidity in air and its measurement. While any errors or lack of clarity regarding humidity and its measurement are mine, much of the insight and many of the resources mentioned here are Lew’s.

When I am teaching building science, I routinely ask building professionals how many recommend or provide their customers with a way to measure humidity, as well as what they think the air temperature and humidity is in the classroom. Most say they don’t provide any type of humidity sensor to their homeowners. And most will get the air temperature in the room within a +/- 2 degrees F accuracy, but guess at the humidity in the room — either dewpoint or more likely relative humidity (RH) — with accuracy of +/- 10%, or even more.

So, how important is it that we as building professionals — and our customers as home operators — know what the real moisture content of the air around us is? The answer — as with many building science questions — is, it depends.

Hygrothermal Rule Number One: Add understanding (and use) of dew point to RH

Most of us are familiar and comfortable with the term relative humidity (RH) when dealing with vapor in air. But RH is really only half of a measurement of vapor in air; it always needs a corresponding air temperature measurement to be complete and useful.

This is why most mechanical engineers strongly prefer to use the dewpoint temperature of air to characterize humidity; that is, the temperature at which the air is fully saturated and condensation occurs. The dewpoint temperature of air is a singular way of expressing humidity in air that is unaffected by the frequent changes in air temperature that happen in buildings.

A great way to make the shift to include dew point in the way we think and work with humidity is to have one of these apps on our smartphones:

• For iPhones: PsyCalc - Psychometric calculator smartphone app (iOS) — $8.
• For Android: AirPro - Android app.

Both apps include unit conversions from SI (metric) to I-P and vice-versa (another translation that I, at least, can often use help with). The other key thing about these apps is that as psychrometric tools, they remind us that discussion of humidity in air must include heat or temperature; hence this blog about humidity having hygrothermal rules.

Hygrothermal Rule Number Two: In terms of thermal comfort, humidity is most important at the farther reaches but not so much in the middle

In my experience, many of us — even most of us — don't really care about humidity until it is less than about 30% (at which point we get static electricity and significant drying of mucous membranes and our eyes) or greater than about 60% (at which point we begin to perspire to stay comfortable, even when at rest). It can be helpful to work this through with something like the CBE Thermal Comfort Tool.

Hygrothermal Rule Number Three: In terms of building durability, it is key to think and work in terms of dew point rather than RH

Many of us in the building community just need to be dope-slapped on this point; it’s when surface temperatures approach or reach dewpoint that all hell breaks loose in buildings. I can’t do this perspective greater justice than strongly recommending that all of us not just view, but study, Lew Harriman’s YouTube discussion: “Dew Point v. RH Control for Commercial Buildings.” It's just under 20 minutes — so it's tight and to the point.

Equipment for measuring humidity

Measuring humidity is no less of a struggle than understanding it. There are a lot of aspects of the equipment’s performance to consider:

• how fast a humidity sensor reacts to humidity changes,
• the range over which they maintain the same level of accuracy,
• differences in measurement accuracy based on whether humidity is increasing or decreasing (hysteresis),
• third-party certification of the sensor’s accuracy, and,
• the need for (and ease of) calibration.

Below is a list of generalizations about humidity sensors and hygrometers, cutting to the chase on the really complicated and difficult topic of measuring humidity.

1. It’s a real shame that just about all electronic humidity sensors display RH to 0.1%, since even really sophisticated and expensive equipment is only accurate to +/- 1%. Ignore that bloody decimal point number!
2. Measuring dewpoint temperature directly is done with sensors such as chilled-mirror hygrometers. These are not really appropriate for our purposes but can be important in calibrating other sensors.
3. Simple dial hygrometers are based on metal-paper coils: the coil tightens or loosens with changes in the moisture content of the metal-paper medium, with the coil connected to the dial. We don't recommend these sensors for either occupants or building professsionals. While these units are inexpensive, their accuracy is in the range of +/- 10%, and they don’t report dew point.
4. Sling psychrometers: These use the temperature difference between paired wet-bulb and dry-bulb thermometers to then calculate corresponding RH. These hygrometers run about $50 to $60. They generally read up to 5% high because of slowed evaporation from less-than-perfectly clean wick, poor contact of the wick to the thermometer bulb, less than complete evaporation from the wick. And of course, they don’t include dewpoint temperature readout.
5. The most common hygrometers/humidity sensors are electronic and work this way: hygroscopic materials take up and release water vapor and as they do, their electrical conductivity changes so that capacitors or resistors can be used to correlate to RH. Each of these sensors also lends themselves to data logging in addition to digital readout.
6. Resistive humidity sensors are not nearly as common as capacitive and it’s the latter that comes with a bit of a problem. Capacitive sensors use electrodes separated by a dielectric polymer film that responds to moisture content. The huge range of quality of the electrodes and film mean a huge range of accuracy, response time, and cost for capacitive sensors. But since they are all digital and display to a tenth of a decimal point, it can be hard to tell them apart, cost notwithstanding.

Finally, here is Lew’s list of handheld dewpoint meters that he recommends to building professionals, meters that have the best combination of accuracy and price, and that come with a certificate of their accuracy:

• Control Company (includes certificate) — $150. “I have had very good luck with Control Company hygrometers. They always seem to be better than the specs, and this one is very inexpensive, and has a remote probe — very useful for checking inside small places and inside ducts.”
• Kestrel 5200 HVAC — $269. (My favorite.) “This is the updated version of the one I carry at all times (my Kestrel 4200 is no longer manufactured). This newer version actually is slower to respond, but better protected. It’s accuracy is better than stated (based on measurements against calibration salts). This has a backlight, and you can choose three variables to display on each of three ‘user screens.’ Great for documenting the reading with your cell phone camera.”
• Omega RH 650 — $265. Multifunction, including material moisture content probe.
• Fluke 971 — $315. “Trusted name, reasonable accuracy, dew point and wet bulb, good sensor protection for dirty toolbags!”

Lew has this one recommendation for a desktop unit for homeowners/occupants:

• DH Gate — $20 or less from China. “Includes outdoor sensor. Probably not very fast-responding, but probably good enough accuracy for long-term awareness of high vs. low dew point.”

And despite how much heartburn this might cause Lew, I am going to make just one recommendation for a desktop sensor that does not include dew point, for those occupants and homeowners for whom including dewpoint may cause more confusion than it is meant to prevent.

• NIST Certified from the Weather Store 4096 (Traceable) — $68.