You may feel the humidity on a sticky hot day, but knowing how humid requires a scientific instrument. So, what instrument measures relative humidity?
This scientific article defines relative humidity and provides exceptional examples of devices capable of measuring it.
An Introduction to Relative Humidity
There are two definitions you need to know about humidity levels:
- Absolute humidity: the amount of water vapour expressed as moisture per cubic metre of air (g/m3). Absolute humidity is a measure that’s independent of the temperature.
- Relative humidity: the percentage of moisture the air can hold (potentially) expressed as a percentage. Temperature influences relative humidity. When temperature increases, relative humidity decreases and vice-versa. The relationship between temperature and humidity is not linear - see figure 1 (below).
Hot air can hold more water vapour than cold air. As water vapour fills up an atmospheric volume, relative humidity level increases. 100% relative humidity occurs when air holds the maximum amount of water vapour at a given temperature. Any moisture above the dew point cannot remain in the air as water vapour. Condensation will form as rain, fog, or dew. This occurrence is known as the saturation or dew point.
Figure 1 Relationship between vapour content, temperature and relative humidity
Applications for Measuring Relative Humidity
Relative humidity is a common measurement in the following situations:
- Homes
- Greenhouses
- OH&S in workplaces
- Museums
- Cleanrooms and laboratories; and
- Warehouses.
What Instruments Measure Relative Humidity? Product Examples
There is an abundance of instruments that measure relative humidity. These devices have a long list of names - psychrometer, hygrometer, and many more - complicating matters further.
As promised, and as a hack to understanding the pros and cons of different equipment, Instrument Choice Scientists have put together examples of instruments that measure relative humidity excellently (see below).
Q: What Instrument Measures Relative Humidity?A: Ambient Humidity MetersAmbient humidity meters offer a range of deployment options - indoor, outdoor, handheld and desktop models, collecting relative humidity information and displaying results on an LCD.
Product Example: 
Humidity & Temperature Monitor
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Q: What Instrument Measures Relative Humidity?A: Humidity LoggersHumidity loggers track humidity levels over extended periods.
Product Example: 
WBGT Meter SD Card with Data Logger
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Q: What Instrument Measures Relative Humidity?A: Alarm Humidity MetersThis type of humidity meter features alarm functions that alert you when humidity levels fall outside your preset “acceptable” range.
Product Example: 
Hygrometer, Temperature, Dew Point Meter With Led Alarm
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Q: What Instrument Measures Relative Humidity?A: Analog HygrometersAnalogue Hygrometers display wet bulb and ambient temperature readings, which can be used to calculate relative humidity. As a bonus, they do not require batteries. 
Product Example: Mason's Hygrometer Red Liquid
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Q: What Instrument Measures Relative Humidity?A: PsychrometersPsychrometers feature many similarities to analogue hygrometers (above). They are low cost and battery-free relative humidity meters featuring a wet and dry bulb. To produce readings capable of determining relative humidity after first whirling the device. Psychrometers are found in laboratories, educational institutions and used in fieldwork.
Product Example: 
Sling Psychrometer -5-50 Celsius
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Conclusion – What Instrument Measures Relative Humidity?
While there are countless names for the instruments measuring relative humidity, the most common terms are hygrometer, humidity meter and psychrometer.
Browse the full range of relative Humidity measuring instruments here
Or, for more assistance finding the best meter for your application, speak with an Instrument Choice Scientist! We’re here to help! Call 1300 737 871 or email customer-service@instrumentchoice.com.au.