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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. |
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. |
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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. |
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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. |