SWG run time calculations assistance

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Brett S said:
Because the water hardness meter is reading for TDS, not salinity.
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The readout is µS. They call it a "hardness meter" for some bewildering reason which is probably related to marketing. When you put it in a standard 45 µS calibration solution it reads 45 on the display. I'm just comparing because the controller is also using conductivity. As far as I know there's no way to test for actual salt levels electrically.

The salinity meter is just a conductivity meter with a different scale built-in.
 
sbcpool said:
It's not appreciable different. About 12 lbs.

It was as you suspected: The modifications on the returns made zero difference on FC production. The good thing is that now I can put valves on the returns and regulate the output of the pump.
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That will only reduce flow rate by about 3 GPM. Was the 10 & 12 PSI in pool or spa mode? They should be different pressure.

The salinity meter is just a conductivity meter with a different scale built-in.
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That is correct.
 
Earlier you posted that the voltage was around 20.5v. In the manual, they specify a range of 21-27v. At 6.5 amps, I would expect the voltage to be a little higher than that and not to reach the minimum specification until the current increased to 8 amps. One additional experiment would be to raise the salt level in the spa until the amperage approaches, but does not exceed, 8 amps and see what the voltage on the unit is as well as the production. This should be the most efficient operating point.
 
mas985 said:
Earlier you posted that the voltage was around 20.5v. In the manual, they specify a range of 21-27v. At 6.5 amps, I would expect the voltage to be a little higher than that and not to reach the minimum specification until the current increased to 8 amps. One additional experiment would be to raise the salt level in the spa until the amperage approaches, but does not exceed, 8 amps and see what the voltage on the unit is as well as the production. This should be the most efficient operating point.
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That's a great idea. I've been wondering whether everything's being fooled by high conductivity due to our hard water and years of topping off (still waiting for the Taylor 1766). We don't get enough rain to dilute the water, just lets me avoid topping off the pool with the hose sometimes.

Our tap water is about 450 µS, and I have to add about 500 - 600 gallons a week minimum in the summer and about the same every two weeks to every month in the off season. I don't know how old the pool water was when I bought the house, but it's been over 5 years with me topping it off. Each topping off would add about 10 - 11 µS to the pool. 4 months weekly and 8 months every three weeks (average) for 5 years would give something around 2,000 µS if I'm calculating correctly. That's without a grain of salt in the pool.
 
I think I'm going to drain the spa. Then I'll fill the spa with fresh new tap water and add 16 lbs of salt to bring it up to 3500 as a starting point and test the SWG that way. That should confirm or falsify any hypothesis about the issue being anything to do with other ions confounding salinity tests (or anything else with the pool's chemistry).
 
Fresh fill water usually has high chloramines which will use up some of the FC so before taking measurements make sure there are no CCs left. You may need to have a higher FC starting point.

Also, if testing during the day, don't forget the CYA.
 
That's the plan: get all the chemistry stable where it would be in the main pool. I may have to wait awhile for the water to get warm enough anyway. The heater isn't working at the moment.

I'm planning to add:
  • 105g of CYA, aiming for 50.
  • 4 oz. 12.5% bleach for 7 FC
  • 16 lbs. salt.
  • Adjust pH to match pool
 
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Spa drained/filled, chemicals added. Brewing now before I test levels. Water temp 71°F.

If I had been a little smarter I would have timed how long it took to drain the spa to get the pump's flow rate. It definitely wasn't 94 gpm. It took a good 15 minutes or more to drain ~525 gallons.

Being drained.
IMG_20200809_095738.jpg

Being filled.
IMG_20200809_100207.jpg
 
That would indicate it was 35 GPM which can't be right either. Are you sure the spa is 525 gallons. What is the diameter/depth?
 
I calculated it based on it being two circular slugs of water. The top layer is 19½ inches deep and 82 inches in diameter for ~445 gallons. The narrower bottom part is 14 inches deep and 46 inches in diameter for ~100 gallons. I couldn't drain it all the way to the bottom because it started sucking air. There was only about 3 or 4 inches of water in the bottom, or ~25 gallons left.

EDIT - After this experiment is done I will time how long it takes to lower the spa by 4 inches or something. 1 inch corresponds to about 23 gallons, so it should be easy to tell the difference between 1 minute and 3 minutes.
 
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That is a good idea. Just make sure you are draining back into the pool. If you drain outside of the pool, the head loss will be considerably different.
 
If it is that slow, one thing that might explain that and the lower than expected filter pressure is a clogged impeller. But then it isn't an issue of too high a flow rate either.
 
Well, shoot! After 18 minutes run time (0.3 hours) the FC rose from 7.0 to 10.0, or 10 per hour. That's exactly in line with what I was seeing before - about 1 lb./day.

The controller reported almost precisely 3500 ppm salt with ~22V and 6.2A.

I guess the good news is that anything having to do with water chemistry has been knocked out of the equation.
 
As a side note: the builder who used plain old 3/4' pipe for my returns didn't just use schedule 40 3/4", he used class 200 3/4" pipe. How very thoughtful of them. I will NEVER find a fitting that will fit the inside diameter of those return pipes. I'll have to wait until replaster to dig out around them and glue a proper fitting.
 

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