What should one look for when considering a rodi system?
Besides gallons what separates an $80 from a $200 Unit?
A normal RO/DI unit is made up of 2 prefilters (1 sediment and 1 carbon), a RO membrane and then a DI cartridge. The sediment filter is there to protect the carbon filter. It will stop any larger items from getting to the carbon filter and clogging it up too early. The carbon takes out any chemicals or breaks them down into less harmful ones. The carbon's main job is to protect the RO membrane from chlorine since chlorine will destroy a ro membrane. The ro membrane then takes out most of the stuff in your water at a rate of 95% (what is usually guaranteed) up to 99%. The DI then takes care of anything else.
The canisters and plumbing is all generic stuff. What makes up the price difference is a lot of things already mentioned. GPD of the RO membrane, the kinds of carbon filters used and the micron rating on the filters. They usually run from 5 micron down to .5 micron. Just a quick hit, the micron rating of a normal ro membrane is .0001.
The rule of thumb for changing the prefilters is 6 months, the ro membrane should last 5+ years if you don't let chlorine get to it. You change the DI when your water is reading above 0tds. Some people add a water pressure gauge to their unit and don't change the filters until the water pressure drops off. This tells them that the filters are getting clogged up and are not running efficiently. Since you have a fairly small tank, I'm guessing you may be able to go a year on changing the prefilters.
Some things to consider. Get a water quality report from your provider, I'm assuming you are not on well water. You mainly are looking to see if they use chlorine or chloramines in the water. All the cheap units will be setup for chlorine. If you have chloramines then you will best be served getting a special carbon filter that breaks down chloramines. These of course are more expensive. Your probably could get away with a regular carbon filter for chloramines but you would need 2 of them to really get the job done. Chloramines will not hurt the ro membrane but the di will have to take care of the chloramines which means it will be used up quicker. You should also think about investing in a hand held tds meter. You can get one for $15 to $20 on
amazon. A lot of people go for the ones that you hookup to the ro unit but those do not compensate for temperature. You can go with either, but you will need one of them so you can measure your tds after the di.