Salt testing question

[newnick]

How accurate are tests for salt?
We have a 24' round pool thats suppose to be 4' deep which comes out to 13500.
The pool people gave us 12- 40lb bags of salt, of which I only used 9 because thats the amount I came up with after reading the instructions for the Aquatrol unit.
The Aquatrol has been showing 3400-3500 ppm of salt on average, sometimes 3300.
While attemting to get the cya and ta in the correct range I decided to measure the pool depth and calculate the gallons more accurately. I found that the depth is more like 3.79 feet deep which lowered the amount of water to about 12800 and thats what I've been using to add stabilizer and acid. My wife took a sample in to the pool place because our Aquatrol cell has devoloped a leak just below the top section and she wanted to show them a picture of it to see what they thought about it and to see about getting it checked out. They ran the sample and came up with 4620ppm of salt. The girl said she ran two different types of tests and averaged them to come up with this number. The way I figure it using the pool calculater I could be in the range of 3350, which aggrees with the Aquatrol reading. The Aquatrol is working fine with no warning lites on and the numbers are all in the correct range as far as the manual states.
Do ya'll think the pool places tests are way off?

 

[JasonLion]

I think a salt level around 3,800-4,000 is likely, and 3,600-4,600 is possible. You will always start with some salt in the water, often 500 ppm or more. Even the best salt tests are +-10%, and many of them are much worse.

In the end, if the Aquatrol is happy, there is no reason to worry about it.

 

[newnick]

Thanks, thats what I needed to hear.

 

[newnick]

Wifey took a sample to another ps today and they got 2500ppm, go figure.

 

[Guest]

How accurate are tests for salt?

Depends on what you are using for testing. Most SWGs are considered accurate if the readings they give for salt are within about 600 ppm of a chemical test or properly calibrated meter.. They are actually measuring conductivity. A properly calibrated, temperature compensated salt meter can be accurate to 100 ppm BUT if it's not calibrated properly can be WAY off. Personally I have only found the MyronL, Eutech, and Goldline (a rebranded Eutech) meters to be reliable. The ones from Hanna and LaMotte are worthless, IMHO.
Chemical tests are going to be as good as the tester.
Salt test strips are very accurate and very hard to do improperly. The biggest problem with them is not waiting long enough. Sometimes they can take a full 10 minutes to register properly. Titration tests for salt are tricky at best and it's very easy to go past the endpoint and get a much higher salt (actually chloride) reading than actually exists in the water.