Watermaker TDS

[thinwater]

Once you get below 250 ppm, there really isn't an argument that less is better. In fact, WHO (World Health Organization) standards suggest that your RO water is already deficient in calcium and magnesium.


[Yeah, this is just to stir you up, or at least to put thing in perspective.]

[belizesailor]

Originally Posted by Tellie

You know me well enough, my last line about the 17 year old was my usual poor attempt at humor.

But you are asking very good questions. My point, in a nut shell, was the fact that all membranes are different to some extent even in the same roll. It makes it difficult to judge just by the TDS readings the over all condition of a membrane. It's true most reputable watermaker companies will say anything under 500 is good. I believe anything over 500 TDS is usually an indication that something is amiss. But that doesn't always mean it's the membrane. Flows and pressures are important to get accurate. As you know a feed pump that is giving issues will effect the TDS readings of a membrane. Fix the feed pump in time and the TDS reading will most likely return to normal. Charting your TDS without charting the flows and pressures as well only gives you one third of the equation. Temperature and salinity of course make a difference. But salinity of the water you're floating in is also a number that needs to be known. This requires a more sophisticated meter that can read accurately such high salinity content which most typical cruisers are unlikely to have due to the cost. As you mentioned, your older membranes were producing a lower TDS reading than your new ones. I see this all the time. This is not an indication that your new membranes are not as good as your older ones or that they will fail sooner. Most likely the new ones will last longer than the older ones even though the older ones are making better quality water. Which leads the question of failure. It is very rare for a membrane that is properly being taken care of to fail catastrophically. They usually slowly fade away as the TDS readings increase over time. Charting the TDS readings as you do is the best way to determine in a good working system if the membrane is starting to show signs of deterioration. A lot depends on where you are cruising. A membrane in the Pacific will usually perform better than the same membrane in the Atlantic. We see some of the saltier waters in the world here on the Atlantic side. The salinity here can range from 32,000ppms to 36,000 ppms. So a difference in product water of 100-150 ppms +/- is an infinitesimal difference really and I wouldn't flinch on the difference if analyzing a system. Also cleaning a membrane in my opinion is a waste of time. Most membranes I've tried to clean over the years were unsuccessful and those that did show improvement had a very short life soon after cleaning. Today most standard membranes are cheap enough and replacement is a better idea and investment on your time. Running your pressures up to 900psi is not a good idea either. Yes this can certainly lower the TDS readings and even increase production but at these pressures and above you risk scaling and slowly damaging your membrane leading to an earlier death sooner than later. You are also correct on carrying a spare membrane. The self life of a membrane outside of a working watermaker is usually a little over a year or so. By the time the membrane in the system goes bad a few years down the road the spare membrane is worthless as well. Yes there are a lot of $200 membranes out there for sale on Ebay and such and there is absolutely no way to know what you are getting. The term "New Old" comes up a lot and they should be avoided like the plague. Unfortunately, there are still systems out there that use proprietary membranes and people with those systems are stuck many times with the higher cost of membrane replacement.



PS, There is some truth to the 17 year old. My Dad bought a new Cuda when I was in high school that was special ordered by someone else and the deal fell through. He got a really good price on it and decided on the Cuda instead of the station wagon he was going to get. He came home with it and asked me what a 426 Hemi was. We got a new station wagon about the time I turned 18.

Thanks for such a comprehensive anwer Tellie!

(Similar story: My dad sold the lake house and the RV just before I got my driver's liscence...years later I understood why!)

[belizesailor]

Originally Posted by thinwater

Once you get below 250 ppm, there really isn't an argument that less is better. In fact, WHO (World Health Organization) standards suggest that your RO water is already deficient in calcium and magnesium.


[Yeah, this is just to stir you up, or at least to put thing in perspective.]

Ive done some reading on this issue because an aquaintence was going on about it. My take aways are that there is little hard research on the subject (as is the case too with very high TDS) and that the category of water subject to the most speculative concern is what the WHO referes too as "demineralized" water (< 50 PPM) and then only if it is the primary source of long term drinking water.

[Bill O]

Originally Posted by thinwater

Once you get below 250 ppm, there really isn't an argument that less is better. In fact, WHO (World Health Organization) standards suggest that your RO water is already deficient in calcium and magnesium.


[Yeah, this is just to stir you up, or at least to put thing in perspective.]

So where did the table come from (source)? Find it a little sketchy that they can't spell analyte nor magnesium correctly.

[Pizzazz]

It is very easy to calculate estimated TDS. Most seawater membranes advertise seawater rejection around 99.4%. If seawater is 32,000 ppm, this would imply average fresh water ppm of 32,000 x 0.006 = 192 ppm. There are many factors affecting the perfect conditions such as temperature, flow rate, salinity, quality of the membrane, etc. Fortunately they often cancel each other and ppm stays around 200 ppm. If you are getting significantly higher, most likely the membrane is old, was not preserved properly or was left to dry out.

If you have a clogged membrane (due to insufficient flux), it is more likely that the output will fall but the quality will remain good. Some membranes can last 10+ years if preserved well, others go bad after six months. Most vendors on Amazon will send you a replacement membrane if you get a bad one.

You calibrate your meter by using distilled water and sea salt. Make a mix that gives you 200 ppm, then adjust the screw to match that on the display. Then go to 400 ppm and see if it is linear. Most of the time the discrepancies are small.

ps. I am getting 95 ppm to 160 ppm based on pressure 800-900 psi, temperature and recovery ratio. That is in Marina Del Rey.

SV Pizzazz

[Bill O]

Originally Posted by Pizzazz

You calibrate your meter by using distilled water and sea salt. Make a mix that gives you 200 ppm, then adjust the screw to match that on the display. Then go to 400 ppm and see if it is linear. Most of the time the discrepancies are small.

... and your ppm of your distilled water used for dilution is what (not always zero) and sea salt is pure? Fairly large dilution to get to even 1000 ppm. To make a accurate calibration std would need an analytical balance, volumetric flasks, etc. for anything close to accurate and doubt anyone is willing to go through the hassle. Maybe "ballpark" or greater is close enough for most, but hard to do a good comparison of data w/random errors.

[Paul L]

[QUOTE=Pizzazz;3078847........

ps. I am getting 95 ppm to 160 ppm based on pressure 800-900 psi, temperature and recovery ratio. That is in Marina Del Rey.

SV Pizzazz[/QUOTE]

Which has cold water and significantly less salty ocean than the Atlantic. Take the same unit to the Carib side of Panama and it would probably go up another 60-100.

[Davo1404]

Rainman 6 months Old. North Queensland. Water temp around 24c. TDs between 160- 190.

[Marathon1150]

Our brand new, out of the box Spectra Ventura delivered about 350ppm initially while in the Sea of Cortez. I complained to Spectra because this seemed high compared to "the neighbours" and despite their claim that anything less than 500 is "excellent". I received an email from a VP who promised to replace the membrane if the product TDS did not decline to about 200 in 6 months. It went down to less than 200 in the Sea of Cortez in less than 6 months and stayed there over a period of about 2 years. Crossing the Pacific it has gone up slightly. I record the number every time that we run the unit but I don't have the data with me.

I also calibrate the TDS meter. I note another post that identifies one of the problems with standard solutions available to calibrate the meter. They are usually 1000ppm which is significantly higher than the target range and is a single point. It is normally best to calibrate instruments such as this with at least two points that bracket measured values.

[thinwater]

Originally Posted by Bill O

So where did the table come from (source)? Find it a little sketchy that they can't spell analyte nor magnesium correctly.

I'm a terrible speller and a poor typist. It was my draft of a table that was later proofed and published.

But feel free to look up the numbers or run your own tests. My lab work is better than my typing.




---

Yes, the WHO standard is a question mark. I just posted it because it is out there. I'm not judging it. You can Google the underlying documents.

[belizesailor]

Re meter calibration:

Are TDS meter responses not linear? Does calibrating to 1000 ppm result in less accurate readings at lower PPM?

Amazon sells some multipoint calibration solution kits which cover lower ranges.

https://www.amazon.com/-/es/Conducti...+calibr&sr=8-6


Also, diluting a 1000 ppm solution to a lower reference point should be straight forward. Especially given that in this application accuracy is not super critical.


Gentlemen, sync your meters! [emoji6]

[newhaul]

Originally Posted by belizesailor

Re meter calibration:

Are TDS meter responses not linear? Does calibrating to 1000 ppm result in less accurate readings at lower PPM?

Amazon sells some multipoint calibration solution kits which cover lower ranges.

https://www.amazon.com/-/es/Conducti...+calibr&sr=8-6


Also, diluting a 1000 ppm solution to a lower reference point should be straight forward. Especially given that in this application accuracy is not super critical.


Gentlemen, sync your meters! [emoji6]

there is one of those water to go places close to my marina they have several TDS meters .
I have them use at least 3 of their meters on a couple different samples of water then I use mine on the same samples and calibrate accordingly. I figure if I average 4 different meters on the same sample I'm close enough. That and a UVC light between watermaker and tank I'm good.

[Bill O]

Originally Posted by belizesailor

Re meter calibration:

Are TDS meter responses not linear? Does calibrating to 1000 ppm result in less accurate readings at lower PPM?

Amazon sells some multipoint calibration solution kits which cover lower ranges.

https://www.amazon.com/-/es/Conducti...+calibr&sr=8-6


Also, diluting a 1000 ppm solution to a lower reference point should be straight forward. Especially given that in this application accuracy is not super critical.


Gentlemen, sync your meters! [emoji6]

Well after some thought on the whole matter of calibration of a TDS meter, will take a step backward from my previous comment and say in the grand scheme with all the random variables (scientific errors) that can occur in the whole RO water making process, it doesn't really matter for this application.
We aren't doing precise analytical chemistry measurements here, therefore a slight deviation with the meter of 10 or 50 ppm isn't of much concern when we are trying to hit a range of 500 ppm or less. For example, many TDS meters are not temperature compensated, so unless we are doing all measurements (calibration and test measurements) at 25C/77F they will be off.
There are more variables, but won't bore you w/my career in anal chemistry, reference materials used for calibration, etc.. Carry on!

[belizesailor]

Just for future reference: the HM Digital TDS-3 meter I received with my Spectra is temperture compensated. HM also sells a 342 ppm calibration solution (I received a packet of that too). The meter is now 16 years old and still looks and works like new.

http://hmdigital.com/product/tds-3/


Now $15 on Amazon. A damn good buy.