In high school I helped record daily weather measurements from instruments on the second-story roof. We used readings from wet- and dry-bulb thermometers to determine relative humidity. A psychrometric chart let us read off the percent relative humidity value. Today, relative-humidity sensors make measurements directly and produce a voltage proportional to the percent relative humidity (%RH).

Honeywell, for example, has several %RH sensors that use a polyimide sensing material that reduces temperature dependencies and yields RH measurements to within 3.5 %RH. The sensors come in a variety of packages from single inline packages to TO-5 JEDEC-style "cans" as well as surface-mount packages. The sensors' small sizes make them interesting for use in remote environment monitors, medical equipment, HVAC equipment, and similar applications. I might include a humidity sensor in a remote weather station I hope to design and build one of these days.

Unlike small temperature or pressure sensors that react to change almost immediately, RH sensors have a longer response time because they must absorb or release moisture from the sensor element. In the Honeywell devices, response times range from 5 to 50 seconds, depending on the model chosen. Response times all refer to 1/e in the device data sheets. That means the response time of the sensor equals the time to reach 1-(1/e), or 0.632 (63.2%) of the final reading. (The value of e, the natural logarithm base, equals 2.718.) For the Honeywell sensors, humidity response times also refer to measurements made in slowly moving air.

What about that voltage output? Well, you don't have to use complicated nth-order equations or extensive look-up tables because the sensors produce an output that provides a very close fit with a straight line. Sensors may have a temperature-dependent temperature offset, but you still have a straight line, so simple math will correct for temperature changes. The Honeywell HIH-4602 A- and C-series sensors include a temperature sensor so you also can calculate dew point and adjust for the temperature offset.

The sensors operate between -40 degrees and +85 degrees C and they run on a voltage between 4 and 5.8 volts, so their output will easily connect with a microcontroller input. Prices range from about $15 to $20 each down to below $10 in large quantities.

Honeywell offers developers several useful "Application Sheets" such as "Humidity Sensor Performance Characteristics," and "Humidity Sensor Moisture and Psychrometrics," via its humidity-sensor Web site at: www.sensing.honeywell.com. If you land on the main Honeywell sensors page, select the "Sensors" tab, click on "Humidity Sensors." On the "Humidity Sensor Line Guide" page, select, "Product sheets, installation instructions and other documentation."

Have you used an electronic %RH sensor or do you plan to? I welcome your comments.

--Jon Titus

References:
You can find a psychrometric calculator at: www.uigi.com/WebPsycH.html
and you can obtain a psychrometric chart at www.uigi.com/UIGI_IP.PDF

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