They also make the Zenith, same principle but more costly (and perhaps discontinued). This head works differently than any other manual head on the market. The throne itself is very simple, a bowl with a hose coming out the bottom - with one critical added feature: the seat and lid have a seal that is vacuum tight when down. It is supplied with a Henderson lever diaphragm pump, same one used for bilges. There is an additional small hose inlet in the top, which is connected to a source of sea or fresh water. To operate, close the lid and pump. The pump evacuates the bowl, at the same time pulling in rinse water to replace the vacuum created. Very few moving parts, no valves to operate, no rotating or sliding seals. There is no chance for inlet and outlet water to mix. The only thing that might ever require service is the pump (rebuild kits include all the rubber parts) and the seat seal. I have two installed and (once installed correctly) they have been trouble free and maintenance free going on 7 years now. It is sooo much simpler and more reliable than the average (or even expensive) Raritan or Groco. I think you could plan on a rebuild of the pump every 10 or 15 years, if needed. Those pumps are very easy to rebuild, 15 - 20 minutes. I carry a pump rebuild kit and a spare seat seal, but have yet to need them.

If you decide to go this way, I have a number of tips on installation. Mainly the pump needs to be oriented properly and mounted at the right height relative to the bowl.

http://www.lavac.com/popular-model.htm