Total disolvable Solids (TDS)

[999]

Today i received my TDS meter in the post.

Tested it on tap water = 197 ppm

Filtered water = 137 ppm

Hot tub water just over three months old run on bromine = 1650 ppm

What are acceptable and maximum levels.

[Tman122]

How does the water feel and look? How long does it take for the bubbles to dissapate after you turn the jets off? An excepatble level would be what you think an exceptable level is.

[999]

I am changing the water next week anyway. The alkalinity and PH have been up and down but otherwise the water looks okay.

I am sure there is an acceptable range. A Google search has thrown up a few results. 1000-2000 is okay, over 2000 should be changed. 3000 is unacceptable.

 I was just interested to see what others work to. Maybe its not something many people check or are interested in checking, but that is what the forum is all about, sharing information and good practice.

[TwinCitiesHotSpring]

anything over the 1600 range and I change or do a "partial", like someone stated in the other post, nothing feels as good as some nice fresh water

[Jacuzzi Jim]

  Ive never really gotten into the whole TDS thing, if you are wondering or have to check and see if it's high you should probably be changing the water anyway.   Less water is in short supply which we don't really have that problem t0 often in the PNW.

[999]

Its currently -3 degees and snowing. I know i want to change the water but its a case of finding a good day.

[999]

You wouldn't know that the water needs changing just by looking at it. It is still crystal clear.

[chem geek]

Using TDS to determine when to change the water is not the best approach and is over-simplistic (and not very accurate anyway since some TDS measuring systems aren't very good).  TDS is a very rough proxy of cumulative spa use because one presumes that one adds oxidizer chemicals (chlorine, bromine, MPS) to oxidize bather waste and one does so roughly in proportion to the bather load.  The problem is that the TDS will increase from the background chlorine (and acid/base/alkalinity chemical additions for pH/TA control) even with no bather load so that the water has absolutely no problems at all.  For saltwater chlorine generators, for example, one can explicitly add 1500-2000 ppm or so for spas (except for HotSpring® ACE® that uses 1000 ppm) and 3000 ppm or so for pools.  TDS itself isn't really an important parameter -- it's what TDS is composed of that is more relevant.

For example, if one were to use Dichor-only and not have an ozonator, then one usually needs to replace the water about twice as soon as if one were to instead use mostly bleach (after initially using Dichor to build up some CYA).  The problem is due to the continued buildup of CYA from Dichlor which makes chlorine less effective so eventually slows down its oxidation of bather waste so it can no longer keep up in more heavily used spas, but the TDS rise is similar in both cases after accounting for pH/TA adjusting chemical additions.  So the TDS level that triggers water replacement using Dichlor-only is roughly half that when using mostly bleach, but that's because it's not really TDS that was the problem but rather the CYA level.

For many years, the pool industry blamed TDS for the problems of algae in pool water, but it was really the buildup of CYA that was the problem and the real parameter to look at.  Hypochlorite sources of chlorine will build up chloride (salt) in the pool water and increase TDS, but if there is low bather load in the pool then the water can remain in pristine shape regardless of TDS buildup.  Saltwater chlorine generator pools have 3000 ppm all the time.  They don't build up salt because the salt is used to produce chlorine and then mostly gets used up to become chloride again so in a sense is recycled.

The only real use for TDS measurement, or more simply the salt level (chloride), is to prevent it from getting too high to prevent metal corrosion or salt splash-out problems on porous stone materials.  It is also used to make sure there is the proper amount of salt for saltwater chlorine generators to operate.

In your spa, your ozonator is taking over some of the oxidation role and also helping to oxidize some additional chemicals that chlorine or bromine may not.  If the water is clear, isn't foaming, and doesn't have any objectionable characteristics, then the only reason to change it if the TDS is higher is if it is so high that you are concerned about the increased risk of metal corrosion.  This risk varies by spa since it is a function of the quality of metal components that are used.  Most all spas will say they handle up to 1500 ppm and some say 2000 ppm is OK.

[999]

Chem geek, thanks for such a comprehensive answer.

My water is still clear, feels okay and I have no particular problems. It also clears quite quickly after using the jets and anyone looking at it would think it was new or newish.

I have a problem with the bromine level going very high when shocking with MPS and more recently I have had to adjust alkalinity and PH more often than usual. Had quite a bit of foam after I last shocked it and run it with the jets on and cover open. I also noticed that the ozonator bubbles seem like they are smaller now. I'm sure they were bigger 3 months ago. I have had itchy dry skin on my legs recently as well.

I had a figure of 3-4 months in mind to change the water and it is now 100 days old, so perhaps that bothering me.

I think the fact that the TDS is 1650 ppm has just reinforced what I already know, that I want to change it. I am glad I bought the TDS meter for £7 (Which is about the price of two pints of beer here in the UK.)

Thanks for everones contributions, and sorry for labouring the point, but I am still learning.

[chem geek]

I have a problem with the bromine level going very high when shocking with MPS and more recently I have had to adjust alkalinity and PH more often than usual. Had quite a bit of foam after I last shocked it and run it with the jets on and cover open. I also noticed that the ozonator bubbles seem like they are smaller now. I'm sure they were bigger 3 months ago. I have had itchy dry skin on my legs recently as well.

Well, those might be reasons to consider a water change, regardless of TDS level.  As Tman122 wrote in another thread, there's nothing like fresh water.