Relationship Bet Alkalinity with Total Dissolved Solids

TDS will be always lower, since you are using demin water for boilers.

The alkali addition to the feed water to avoid acidic corrosion, naturally will reflect as higher alkalinity in the condensate water.

Now based on your total alkalinity value of condensate water & quantity of recycled condensate water, you can reduce dosage of caustic to the input water.

TDS is measured in mg/L

Alkalinity is often measured in CaCO3 equivalents (mg/L CaCO3).

So you may not be comparing apples with apples. Convert the CaCO3 back to mg/L and it might make more sense.

http://books.google.com.au/books?id=8nDIGStFlyMC&pg=PA120&lpg=PA120&dq=alkalinity+measurement+CaCO3&source=bl&ots=1ROcVHwVwU&sig=Itao6zYnLccJHKyy2xyFDEwbmqg&hl=en&sa=X&ei=ILVPUfTuMOmeiAfTrIBY&redir_esc=y#v=onepage&q=alkalinity%20measurement%20CaCO3&f=false

Hope this helps.

Anthony

Total alkalinity higher than TDS can be attributed to maintaining chemistry with volatile additives and the lack of significant leaks into your condenser.

In most waters where alkalinity is measured as CaCO3, the value will be about 45% of the conductivity. However, there are exceptions such as acidic conditions. The conductivity can range between 0.6+/- 0.05 of the TDS for waters that are not "salty" or with excessive TDS (greater than 5000mg/L). TDS should always exceed the alkalinty and not the other way around. Alkalinity is the major anion balance to the cation balance ( hardness measured as CaCO3 is about 45% of the total conductivity).

Check your pH. Are you using true TDS measure or something that is based on the adjustment of conductivity. That is the way most of the pen stick type of meters work. Some of the stick pen TDS measures are based on 0.5 of the conductivity and some need to be adjusted for temperature. Conductivity is measured at a standard of 25 degrees C. If the water is very cold your TDS stick will read very low. That could account for your low TDS reading. This link will help explain the conductivity v temperature issue.