RO/DI inline TDS meter placement.

Hey guys. I got a used RO/DI filter and I just bought new replacement filters and an a dual inline TDS meter. Now I need to know where I should put the two sensors.

Before the filter and after?
Before the filter and before the DI so I know when to change the membrane? The DI changes colors and the prefilters get changed every 6 months. The instructions say to change the membrane when the TDS after the membrane (pre-di cartridge) is over a certain precentage of the tap water TDS reading.

 

Probe IN goes post RO and Probe OUT goes post DI.
Basically between the membrane and the DI filter and at the end of the treatment process.
The RO membrane condition and the DI resin condition are the two things you need to monitor the most. Tap water TDS is a concern but it does not change much and you cannot control it, you can control the condition of your membrane and DI.

You should change the DI when it is no longer reading 0 TDS on a consistent basis. The reason being DI resins start to release weakly ionized substances even before the resin is completely exhausted. These can include all forms of ammonia like nitrates as well as phosphates and silicates and in much larger quantities than in the tap water originally since they have been concentrating for hundreds of gallons. A hobbyist grade TDS meter may not even detect some at low levels and color changing resins can be very inconsistent. Use the meter. I actuall prefer a good handheld meter over the inlines since it gives you versatility and portability. You can measure the tap water, RO only, RO/DI, top off storage container, the neighbors, the LFS, bottled water, the vending machine etc. Much more versatile and more accurate too since it is really temperature compensated unlike the inlines that measure air temperature not water temperature and they are rarely the same. I have two of the dual inlines but usually depend on my handheld for accuracy, especially at low ranges like RO and DI.

 

Ok thanks for the help. That makes sense to do it post membrane and post DI.