6.1.6. Controlling relative humidity

Desiccation is usually not a severe issue in endothermic bees because they produce much metabolic water (Roberts and Harrison, 1999). However, it may influence respiratory measurements of larvae or pupae. To avoid desiccation during longer lasting measurements (e.g. overnight), the relative humidity in the measurement chamber can be adjusted by saturating the air with water vapour by passing it through flasks with distilled water immersed in a temperature controlled water bath prior to the respirometer chamber. The temperature of this water bath is adjusted to the dew point temperature (Tdewpoint, °C) that corresponds to the desired relative humidity (rHdes, %) at the desired temperature inside the measurement chamber (Tdes, °C) (Stabentheiner et al., 2012):


Tdewpoint = 234.15 × ln(VP/6.1078) / [17.08085 – ln(VP/6.1078)]       (Equation 3)

VP = rHdes × SVP / 100                                                         (Equation 4)

SVP = 6.1078 × exp(17.08085 × Tdes) / (234.175 + Tdes)                            (Equation 5)

where VP is vapour pressure (mbar), and SVP is the saturation vapour pressure (mbar).