Whats on your hands?

['Ric]

Hmmm... What *should* my TDS reading be?

[YBeNormal]

Hmmm... What *should* my TDS reading be?

 

That's a tough question. TDS is something we use in an attempt to measure the purity of water. What we are really measuring is conductivity of the water and the actual measurement is dependent upon the number of mobile charged ions in the water sample (e.g. chloride and sulfate). Uncharged or neutral ions (e.g. silica and ammonia) will not register on a TDS meter.

 

RO/DI units have several parts that each work to remove certain impurities. The sediment filter removes larger suspended particles, typically 1 or 5 micron. Carbon filter(s) in the next chamber(s) remove smaller suspended particles and many chemicals such as chlorine and chloramine. The RO membrane removes most of the remaining impurities but allows a small percentage to pass and this is subsequently mopped up by the DI resin. If you have a good RO/DI unit that is properly maintained, you should typically see zero TDS readings after the DI chamber.

 

When you see the TDS readings begin to rise, consider this to be a warning that the DI resin and/or membrane are wearing out (prefilters do not lower TDS of the source water). Without subjecting the water sample to lab testing, you have no way of knowing if the TDS readings are caused by harmless impurities such as carbonates or harmful impurities such as chloride and the safest bet is to service the RO/DI unit.