Psychrometrics
Also known as: psychrometric chart, psychrometry
Short definitionThe branch of calculation that treats air together with the water vapour in it. It gives the link between temperature, relative humidity, dew point, humidity ratio and enthalpy on the psychrometric chart.
When a cold room door opens, it is not only warm air that comes in; moisture comes in too. Psychrometrics tells how much heat and how much frost that moisture will bring. You mark two properties of the air on the chart, for example its temperature and relative humidity; dew point, wet bulb temperature, grams of water per kilogram and enthalpy are read from the same point.
Summer air at 30 °C and 50% humidity carries 13.3 g of water per kilogram, while in a room at 0 °C and 90% humidity the figure is 3.4 g. The roughly 10 g in between turns to frost on the evaporator and sets the need for defrost. The larger the difference between evaporator and room, the drier the room; for fruit and vegetables that need humidity, 4–6 K is chosen. If the outer face of a panel is colder than the dew point of the surrounding air, it starts to sweat. In a city at high altitude a sea-level chart is slightly off.