What is “Relative Humidity” and How Is It Measured in Q-FOG Testers?

Humidity is a general term that describes the amount of water vapor in the air. Humidity is a critical element of the outdoor environment and contributes to the material degradation in both weathering and corrosion.

Humidity can be expressed as either an Absolute Humidity or a Relative Humidity (RH). Absolute humidity is the mass of water vapor in a given volume of air, expressed as g/m³. Relative Humidity (RH) represents the amount of water vapor in the air vs. how much it would contain if fully saturated, expressed as a percentage. Relative humidity is used much more commonly both in determining human comfort level and when describing natural and accelerated weathering.

Relative humidity can be measured a number of ways. Q-Lab uses two methods in our testers - electronic measurement with a digital hygrometer is used in Q-SUN xenon arc testers and mechanical measurement with a wet bulb/dry bulb hygrometer is used in Q-FOG corrosion test chambers.

Digital hygrometers are relatively common in everyday life. A digital hygrometer doesn’t require significant air flow, which makes it ideal for use in the Q-SUN tester and for ambient lab measurements. Digital hygrometers are readily available and simple to package.

A wet/dry bulb hygrometer uses thermometers, which makes it relatively easy to calibrate compared to a digital hygrometer. The wet/dry bulb requires a lot of air flow, which isn’t a problem in the Q-FOG tester’s blower module, and is also easy to maintain free from corrosion. Salt fog would degrade and eventually destroy a digital hygrometer if it were used in a Q-FOG chamber.

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