Testing for salt

I have a 22000 gal IG vinyl pool with a SWG. We just installed the SWG this spring and for the most part it's been great. But lately I am having trouble keeping my chlorine level up. I tested the salt level with test strips and my strip said I had 4790ppm of salt. I have an Atlantis SWG and they recommend a level between 4000-5000 ppm. I took a sample to the pool store and they said I had 3780 ppm. My neighbor has an salinity meter and his read 3900 ppm. Which do I believe? I just noticed my salt strips expired 6/2010. Could that make them read high?
Here are my levels yesterday:
CL .5
Alk 90
PH 7.6
CYA 50

I will let one of the SWCG experts chime in about the salt level, but if the salt is okay, then dropping chlorine levels could indicate an algae bloom trying to happen. Have you tested for combined chlorine? Has the chlorine level been that low for very long? You might want to consider shocking the pool with plain, unscented bleach to see if that stops the chlorine loss.

Janet

Regarding the expiration date, its not recommended to trust it after the expiration date. Even within the usable date, salt strips are questionable. IMO, it really depends on how you store and use the salt strips. Ensuring the bottle is always dry and your hands are dry when removing a strip, and finally, storing the bottle indoor and out of sunlight, will ensure a fair amount of accuracy.
Your cya level is a little on the low side and should be increased to around 75 ppm.

You can try to get a water sample directly after the cell, by loosening up the union after the cell, and after a few minutes of the cell being activated. Test the chlorine level of this water sample and compare it to the pool water. It should come out 1 to 5 ppm high than your pool water sample. This would mean its generating chlorine but being consumed quickly. Which ultimately means you have a high chlorine demand that would be satisfied by providing copious amounts of chlorine to shock levels. You may need to do this several days in a row until you can see that your chlorine residual is holding.

If you have signs of Algae starting, you may want to check your phosphate levels as well. There are phosphate treatments available, if you're high. Phosphates are algae food and helps them thrive in moderate levels of chlorine.

A phosphate reduction using a phosphate remover is not necessary if you kill off the algae by shocking the pool and then maintain an appropriate FC for your CYA level as shown in Ben's Best Guess CYA chart.

First of all, I wouldn't trust the pool store readings, especially the CYA reading. Get your own good test kit: the Taylor K-2006. If your CYA level is truly 50 ppm, then your FC level probably dropped below 3 ppm for a time then algae was probably able to grow faster than chlorine could kill it so increased chlorine demand greater than your SWCG output, but I suspect your CYA level may actually be even higher.

Note that pool store and SWCG manufacturer recommendations of 1-3 ppm FC are NOT sufficient to prevent algae in typical pools with higher CYA level -- usually around 60-80 ppm CYA in SWCG pools as recommended by most manufacturers. You need an absolute minimum of 4 ppm FC in such pools and possibly higher (Ben's chart shows 5 ppm minimum).

The Goldline salt tester (Goldline just puts their name on it) is about 100 dollars. You could easily wind up spending that (and more) by using test strips that are giving you faulty readings. I haven't used them, and really can't comment on them, other than reading threads here and elsewhere where the readings have been off, causing owners to over salt pools thereby requiring partial drain and refill (charges that could be saved by having an accurate salt tester). I've also seen threads here stating cons with the testers saying that, "But you have to calibrate them all the time". Iv'e had mine for years and have only needed to calibrate it once (you do need to check it often, but that is easy.) I have to check salt content all the time, so it don't make sense for me to use test strips. I understand that the price works out to about 1 dollar a strip. Too expensive for me.