You only need one converter facility on a planet.
When you get better tech, upgrade
the facility. Its also much cheaper and faster to upgrade.
Once the facility has been on the planet for the specified time, the atmosphere gets changed into whatever the majority race breathes.
Most abilities in SE4 either add or take the maximum of the available components. Perhaps you were seeing an additive effect on the converter abilities? 30+25 = 55 turns to conVersion.
Atmosphere converters are quite a special case, in that more of the ability is not a good thing
[ March 30, 2003, 14:29: Message edited by: Suicide Junkie ]