Humidity – What Is It? 

Humidity is something we hear about daily in weather reports. Humidity is to blame for that muggy, steam-room feeling you experience on certain summer days.
 
Humidity can be measured in several ways, but relative humidity is the most common. In order to understand relative humidity, it is helpful to first understand absolute humidity.
 
Absolute humidity is the mass of water vapour divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain.
 
Relative humidity (RH) is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100% RH means that the air is totally saturated with water vapour and cannot hold any more, creating the possibility of rain. This doesn't mean that the relative humidity must be 100% in order for it to rain -- it must be 100% where the clouds are forming, but the relative humidity near the ground could be much less.
 

Humidity Effects on Humans 

Humans are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature. If the air is at 100% relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling ­us off. For example, if the air temperature is 24 °C and the relative humidity is 0%, the air temperature feels like 21 °C to our bodies. If the air temperature is 24 °C and the relative humidity is 100%, we feel like its 27 °C out.   People tend to feel most comfortable at about 45% RH. Humidifiers and dehumidifiers help to keep indoor humidity at a comfortable level.

 

Humidity – Effects on Buildings & Materials 

Mould spores are everywhere and are normally harmless, but under the right conditions of temperature and humidity mould and mildew can grow exponentially, causing damage to fabrics, paper and walls.  Museums must carefully control humidity for this reason.   80% RH is the point at which mould will grow rapidly. Time to the onset of visible mould at various RH percentages. Graph courtesy of Canadian Conservation Institute.

 

Condensation - Dew Point 

At given humidity, temperature and pressure conditions, the water vapour in the air will condense and form liquid water.  This is most often seen on cold mornings when a cool window (cooled by the outside air) condenses the suspended vapour to form quite surprising quantities of water.  In fact at 20 °C and 60% RH, 1 cubic metre of air holds 10 grams of water.  At 8 °C, it holds half that amount.   Also when a cold drink is removed from the refrigerator, vapour will condense on the bottle as it hits the warmer air. Dew Point is a measure of the temperature at which water condenses into liquid water at the same rate it evaporates.

 

How do we measure humidity? - Hygrometers 

3 main techniques are used for measuring humidity
 

1. Hair Tension
2. Wet & Dry thermometers
3. Electronic sensors (resistive or capacitive)

Hair Tension 

These devices use a human or animal hair under tension. The length of the hair changes with humidity and the length change may be magnified by a mechanism and/or indicated on a dial or scale. The traditional folk art device known as a weather house works on this principle. Whale bone may be used in place of hair.
 
In 1783, Swiss physicist and geologist, Horace Bénédict de Saussure built the first hair-tension hygrometer, using human hair.
 
It consisted of a human hair eight to ten inches long, fastened at one extremity to a screw and at the other passing over a pulley being strained tight by a silk thread and weight. The pulley is connected to an index which moves over a graduated scale. The instrument can be made more sensitive by removing oils from the hair, such as by first soaking the hair in diethyl ether.
 

Wet-Dry Thermometers

Wet dry thermometers can measure humidity by comparing the differences in temperature reported on a thermometer with a dry sensor and another with a wet one.  This works as the rate of evaporation from the wet sensor is relative to humidity.
 
‘Whirling’ or ‘sling’ psychrometers work in the same way but are spun through the air on a rope or mechanism to improve air flow over the sensors.
 

Electronics

Most commonly today, the sensors used are electronic.  The means of sensing may be resistive, capacitive or semi-conductor types. 
 
Most will measure up to 100% RH but cease operation when there is condensing moisture formed on the sensor.  Some specialised types have heaters within the probe to deal with condensing moisture.
 

Calibration – How do we know we’re reading correctly?

The simplest method of checking a humidity meter for accuracy is to use ‘Calibration Salts’.    It was discovered that the air above a saturated solution of certain salts, had a fixed humidity so could be used as a reference.  For example a saturated solution of Sodium Chloride will have a layer of air above it that is constant between 75.5% RH and 76.29% RH across a temperature range of 0 to 75 °C. Specific salts need to be purchased for each type of instrument as the sensor must fit perfectly into the receptacle.    This calibration method is also not easily traceable.  Many customers require calibrations that can be verified and are traceable through an unbroken chain of traceability back to national and international standards.    An IANZ accredited calibration laboratory exceeds all these requirements.