Thanks for your thoughts. I have inquired of several sources about these issues and this
is what I have distilled:
There are two separate phenomenon both of which are applicable to any RO membrane
1. TDS CREEP. Imagine that on once side of the membrane you have very high TDS
(seawater with about 32,000 ppm TDS) and on the other side you have product water with
a TDS of about 400-500 ppm. Water is forced across the membrane by the 55 bar (800
PSI) pressure gradient established by the high-pressure piston pump. This high-pressure
gradient overcomes the natural osmosis and drives the physics of osmosis in the reverse
direction (RO). When the system is shut down the pressure equalizes on both sides of the
membrane but there remains still a very high gradient in solute (TDS) across the
membrane. At this point natural osmosis begins to occur with water being drawn back
across the membrane to the high TDS side in an effort to try to equalize the osmotic
pressure on both sides of the membrane. The longer the membrane sits idle the more
water is drawn back across to the high TDS side. As more and more water is drawn back
across the membrane there is an ever-diminishing amount of pure water on the low TDS
side (but still the same amount of solute) so the TDS measured in ppm actually increases.
When the system is restarted there will be a brief period of relatively high salinity product
water. My experience with my continuous EC meter is that this high salinity, TDS Creep
water with a TDS of about 2500 to 4000, only last less than a minute before it is diluted
with the new 400-500 ppm TDS product water.
2. PRESSURE DEPENDENCY OF PRODUCT WATER TDS. That is, as the pressure of the RO
process is increased the TDS of the product water decreases. This phenomenon is much
less clearly understood. Rod Boreham gave me a similar description to what he gave you
but he also implied that without pressurized water on the chevron seal in the tube that
seawater could leak by. This just didn't seem logical to me for two reasons; a) the chevron
seal that seals the seawater side of the membrane in the tube from the product water side
is very tight. It is all you can do to get the membrane in or out of the tube because it is so
tight. Also, b) the membranes are robotically produced (hence their dramatically reduced
price and better performance than the old style membranes) and the edges and seams are
absolutely sealed. Again, what I can best understand from several sources is that on a
molecular level the angstrom size pores, that allow water to pass and not the solutes,
actually are squeezed even tighter (made smaller) when under pressure and up to the
rupturing pressure of the membrane the pores become smaller and smaller. This allows
less and less solute to pass. All RO membranes exhibit this quality and there is an
optimum pressure for highest quality water and least potential for harm to the membrane.
By running in the green zone the highest quality water will be produced while staying
safely below membrane rupture point.
Related to the salinity sensor and other questions I have raised on this forum regarding
the Dessalator control system suffice it to say that I am still trying to ferret out the truth. I
have been in touch with Amel, Joel, Dessalator and Rod Boreham. I have gone around and
around with Dessalator. Rod has attempted to be helpful but his information comes from
Dessalator and either because of language barriers or just reluctance by Dessalator to be
fully forthcoming he has been unable to come up with a satisfactory answers for me. I
have asked Dessalator for schematics and logic diagrams for their circuit board so that I
might understand the system and know how to trouble shoot it but they tell me that they
have neither available because the board was produced by a subcontractor that is no
longer in business.
This much I know for sure:
1) Amel believed that the system operated as advertised when we took delivery of our
boat. I am still unable to determine if indeed it did live up to Dessalator's claims.
2) There evidently is no way to know if the original equipment salinity sensor is working
correctly or not. The two-minute timer idea may or may not be accurate. I have so far
received no assurance from Dessalator that a properly functioning machine will sense and
divert high TDS water during water production following system start-up.
3) I reiterate my belief that the Dessalator monitoring system cannot be relied on and that
a secondary monitoring system is necessary. I will stick with that statement until
Dessalator or Amel can tell me how one can determine if their original equipment salinity
monitor is functional.
4) Amel has been very good to work with and remains one of the finest companies I have
ever done business with.
Gary,pressure is below the green range.
slightly larger diameter in the center than at the two ends. The reason for this is to allow
for distortion when the pressure is applied.
side to the other. They are rolled corner to corner. This creates angular seams along the
length of the tube. They are actually rolled with enough slack to account for a perfect fit
under pressure. If you run the water maker below the prescribed pressure, you are
squeezing salt water past the unsealed seams. In other words, they have to be fully
pressurized to seal properly. The good news is that your EC appears to be finding it.
green zone. Any higher or lower will result in problems. You should check with Rod to be
sure though. I think I interpretted his recommendations correctly, but one never knows.