In an embodiment, a method is provided. Each stop element has an outer surface that absorbs substantially all projection light impinging thereon, and a substantially rectangular circumference. , a thermal conductivity of greater than 0. Black liquor is withdrawn from the digester via a black liquor line 108 to the impregnation vessel 101. Both pairs of the fixed comb electrode and the corresponding movable comb electrode are configured such that a voltage can be applied therebetween independently. The CO2 getter can remove a relatively large amount of carbon dioxide from the cavity, and thus effectively prevents solder joint corrosion.