A hygrometer measures humidity, the amount of water vapor in the air. Most of the ones we sell read relative humidity, or RH, shown as a percentage. That number tells you how much moisture the air is holding compared with the most it could hold at that temperature.

That last part trips people up. Warm air can hold more water than cold air, so the same amount of moisture shows a lower RH on a warm afternoon than on a cool night. It's why a humidity reading almost always comes with a temperature reading, and why nearly every hygrometer we carry is really a thermo-hygrometer.

How the different types work

Digital hygrometers. Most modern meters use a capacitive sensor. It's a thin layer of material that soaks up and gives off moisture, sitting between two electrodes. As the layer takes on water its capacitance changes, and the meter turns that change into an RH reading. Some sensors are resistive instead and track a change in electrical resistance. Either way you get a fast reading, and many models will log data or calculate dew point for you.

Mechanical hygrometers. The classic dial hygrometer uses a material that changes length as humidity changes. Human hair was the original. It sounds odd, but hair really does stretch a little as it absorbs moisture. These days it's usually a synthetic element or a coil. They need no batteries and look nice on a wall. But they drift, and they're slower and less precise than a good digital meter.

Psychrometers. A psychrometer uses two thermometers. One stays dry. The other has its bulb wrapped in a wet wick, and water evaporating from that wick cools it down. The drier the air, the more it cools. Compare the wet-bulb and dry-bulb readings and you can work out relative humidity and dew point. A digital psychrometer does that math for you. The SAM990DW is one example, and it shows dew point right on the display.

A quick word on dew point

Dew point is the temperature the air would have to cool to before moisture starts condensing out of it. If a cold surface in your building sits below the dew point, it's going to get wet. Think of a cold-water pipe, a basement wall or a window in January. If you're fighting condensation or mold, dew point often tells you more than RH alone.

Where they get used

Hygrometers turn up in homes, museums and archives, greenhouses, warehouses, food plants and just about every HVAC tech's truck. The reasons vary. Wood floors and musical instruments crack when the air gets too dry. Paper, artwork and stored goods suffer when it's too damp. Growers watch humidity because it affects plant health and disease. And plenty of people simply want to know why the basement smells musty.

For one room, a small wall or desk unit is usually all you need. For walking a building or checking how an HVAC system is performing, a handheld meter with a probe makes more sense. Cooper-Atkins' SRH77A-E reads temperature, humidity and wet bulb, and it's a solid all-around choice. If you need to know what humidity did overnight or over a whole month, skip the meter and look at a data logger.

Accuracy and calibration

Humidity sensors tend to drift more than temperature sensors, especially after a few years in dusty or damp spots. If your readings go into quality records or a customer requirement, plan on checking them on a regular schedule. Many of our instruments can be ordered with a NIST traceable certificate, and our in-house lab handles calibration and repair. You can get started on our calibration and repair page, or call and ask for ext 3 with any questions.

Our current lineup is in the hygrometers collection, with more options under humidity instruments. Not sure which one fits? Give us a call and tell us what you're trying to keep an eye on.