Caravan and travel filters range from single to twin systems and from standard to slimline units. The unit required depends upon such factors as whether the filter is to be installed in the van or used externally, if chlorine is to be removed when filling the caravan tanks or left in and removed at the sink, or if more standard cartridges are preferred so they can be obtained anywhere around the country. Twin systems are generally better as they have a cheaper, disposable sediment filter to protect the more expensive carbon filter.