**5. Humidification**

Most incubators are humidified with water trays or tanks, which usually increase the relative humidity close to 100% to prevent cultures from losing water and concentrating the media. This is fortunate because CO₂ normally does not transverse dry membranes very readily. By hydrating the atmosphere, CO₂ can dissolve into the water vapour (giving H2CO3) and then diffuse easily across the membrane [41].

Unfortunately, the combination of high humidity and the warm temperatures inside an incubator are strongly conducive to infections and so regular thorough cleaning is necessary to prevent contamination of the cultures.

Microbial infection can be mitigated if the humidification of the incubator chamber can be reduced. For that reason, a culture vessel has been designed that can humidified itself. This permits the incubator to be run in a 'dry state' (i.e. with only ambient humidity). Assuming that the air around the incubator contains 40% relative humidity, (at 20 °C and normal pressure), then when this air enters the incubator and is warmed up to 37 °C, its relative humidity will fall to 13.9% making microorganism growth more difficult.

The culture vessel is humidified by placing water beads in a circumferential chamber around the gas exchange membrane and allowing air exchange into this

#### **Figure 6.**

*Loss of water from water beads in a culture vessel in a clinostat incubator at 37 °C. Two culture vessels, one containing hydrated water beads (blue line) and the other containing unhydrated beads (orange line) were rotated at 20 rpm in a clinostat incubator at 37 °C and weighed on a daily basis.*

chamber. In use, the hydrated beads release water linearly with time to the atmosphere (1 mL water gives about 1.67 L water vapour) and maintain a very high humidity close to the membrane and facilitating gas exchange (**Figure 6**).
