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In my expereince, it is impossible to give people a "timeline" on water longevity unless you have a clear understanding on the source water, potential usage of the spa and its initial TDS reading.If your TDS starts off at 150, sure you may get close to 6 months (which is pushing the envelope a bit IMO), but if your TDS is starting off at 800, you may only get 2-3 months (if that). We had many people on well water that had initial readings of 1000+ TDS that only got 4-6 weeks out of a fresh fill.The best analogy for TDS is a coffee cup. You can keep adding sugar to it but at some point the coffee becomes saturated and the sugar will not dissolve and instead, sit in the bottom of the cup. Anything past 1500 TDS in a spa prevents your chemicals from properly doing their job as the water is now becoming saturated essencially throwing good money after bad.The mistake many people make is that they base their water quality off of clarity. Battery acid is also clear...
Acceptable TDS levels can be a confusing number. ANSI/NSPI-4 1999 changed the standard to a maximum of 1500 ppm greater than pool/spa startup. The NSPI guidelines show 300 pm minimum, 1000-2000 ppm ideal and 3000 ppm maximum with 1500 ppm maximum at pool/spa startup. To get value from a TDS measurement, you need to know what the TDS level of your water is at a new fill. Add 1500 ppm to that number to get to the level you should change your water. 3000 ppm is the absolute upper limit.
I haven't read this data but my opinion is that a spa and a pool are entirely different creatures when it comes to what is exceptable TDS readings and should not have similar data or requirements for suggested draining. The drastic difference in volume of water alone would be enough to suggest differing requirements of TDS IMO.I have always suggested that 1500ppm is MAXIMUM for a spa and 2500-3000ppm for a pool but that's just what I was taught many years ago. I'm guessing water hasn't changed much over that period of time...
I have found this to be true on all the water parameters between pools and spas. I had a heck of a time getting my water to stay nice in the spa even though I had 4 years of water care under my belt. After about 6 months (yes, I'm dense) it dawned on my that I had to change the way I took care of my spa water.TDS in a pool may not be as problematic for most people, winter has us dumping out the water in our pools. TDS shouldn't be problematic in our spas as long as we follow the dump and refill every 3-4 months. I guess guess using MPS may get your TDS higher during that time but as long as the water is managable you'll be a short time away from a refill.Although I don't measure TDS there are definate signs of it - ozone bubbles getting really small is my indication I'm getting close to a water change. I'm at that point now since I had to fight that bacteria problem in my tub, I was planning on refilling in March - I may do it sooner if this weather stays the way it has.
Is there a TDS kit you can buy or do you have to take a H2O sample to your dealer to test?
I completely agree...a pool is a whole lot easier to maintain than a spa. As Steve mentioned earlier, source water can make a lot of difference in water care, especially regarding TDS. Most start up water issues can be adjusted but if your TDS starts high, you will most likely have a more difficult time with your water and may need water changes sooner than another might. All water has some TDS or at least should have some TDS. For instance, the calcium in your source water will be part of the makeup of your start up TDS. This is why the standards state a minimum of 300 ppm TDS to start. Ideal water gets changed at 1000 - 2000 ppm TDS. If you start at 300, add 1500...at 1800 ppm you're in very good shape. If your source water has 800 ppm TDS, to stay in the ideal range you should only add a maximum of 1200 ppm. The worst acceptable source water, according to NSPI, would be 1500 ppm of which you could keep until TDS levels reach 3000 ppm....but this would be a worse case situation...but it may be a situation some people need to deal with.