Re: Water maker Questions

Gary Silver

Hi Tom and All:

A few notes on desalination of seawater and my experience with my Dessalator 160 l/hr over the last 18 years of use:
For the following reason I measure EC (electrical conductivity)  not TDS for measuring my product water;

"EC stands for Electrical Conductivity and is measured micro-siemens per centimeter. TDS stands for Total Dissolved Solids and is measured in PPM or parts per million. TDS is acquired by taking the EC value and performing a calculation to determine the TDS value. Because TDS is actually a calculation it is really only a guess at what the nutrient concentration is. On top of that, there are three different conversion factors to determine TDS and different manufacturers use different conversion factors. In other words you could test the same solution with two different meters and get two totally different readings. But the EC is read the same by all meters the only difference is the conversion factor."

I take this to mean the EC is a primary measurement whereas TDS is a secondary calculated value (based on a formula).  That said, TDS meters are probably "pretty close".  I love the quote from the WHO that shows that there isn't a hard and fast number for TDS, it is a range and various palates will taste water in different ways. 

My experience : Water makers love to be used.  Lack of use is probably the most common cause of membrane failure.  Frequent (daily or every other day) use will prolong their life.  

I have never quite understood why operation in brackish water is damaging but I have heard that opinion often enough to believe that it is so. 

Since I monitor EC continuously I can speak to the trends associated with water maker use: 

1. Upon startup of my 160l/hr Dessalator water maker, the EC will start about where it left off when it was shut down (usually about 400 microSeimens/cm), this is due to the last produced product water being at the EC sensor,
2.  Within seconds the ED will rise to over 2,000 microsiemens (max recording range of my EC monitor), I have speculated on why this rises (osmosis vs reverse-osmosis taking place within the membrane capsule during the shut down period)
3.  Then over a period of about 2 minutes it will gradually fall to about 600 mS, at which point I will start to "Save" the product water.  I have a manual "Save / Discard" switch and utilize that to determine when to save product water.
4.  Then EC will fall continuously during production (the longer the water maker runs the lower it goes).    
5.  The higher the pressure that I run the water maker (up to 65 Bar, top of the green), the lower the EC / better the EC.  At bottom of the green the EC will usually be about 50-100 microSiemens/cm higher than when run at the top of the green.  

You do no favors to your water maker or product water by running it below the green arc.  I routinely run my membranes at 65 bar and I am only only my 3rd set of membranes in 18 years (NOTE: I use my boat only 3 months out of the year and have a automatic timer that flushes the membranes with carbon block filtered product water for 2 minutes every 48 hrs when I am not aboard the boat, so my experience probably isn't typical for a cruising sailboat).

So many good questions around the water maker.   Can't speak to "cleaning" the members as I have never found the need to do that. 

One last point: don't use sodium metabisulate to pickle your water maker unless you absolutely have no other option.  If you do use it, only use it at the lowest possible effect concentration and for the shortest period of time possible.  It is corrosive and can damage metal parts in you water maker (hence the reason I have a fresh water flush system and haven't pickled in in 13 years).

All the best to my fellow Amel owners who, like me, are on a never ending journey of learning.

Gary S. Silver, M.D.
s/v Liahona
Amel SM 2000  Hull #335
On the hard at Puerto Del Rey Marina, Puerto Rico

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