When an adsorber is used, the silica gel may discolour differently due to environmental conditions and operating parameters. But why does the silica gel not always change its colour from orange to green, but brown?
Function and time of change
Adsorbers without valves have air inlet openings in the base and allow air (and thus moisture) to flow continuously to the desiccant. If the granulate is loaded slowly and constantly (mounted on a system with very weak or no air movement), the load will change colour evenly over the entire body. This is because the silica gel wants to reach equilibrium and thus distributes the water. The orange shade changes to a dark green shade with water, but only when the silica gel is loaded with approx. 15 to 20%. In the step before, the orange becomes darker and darker and appears slightly brownish.
Figure 1: Left – adsorber in new condition; Right – adsorber after use
A brownish to black colour is also caused by oil aerosols or other vapours. This means that when the upper activated carbon disc (black layer) can no longer absorb the escaping particles, they will slowly settle over the silica gel. In the process, silica gel will turn into a comparable colour shade as shown in the picture.