Salt test results are strange

[CFlaPool]

Ever since I started testing for salt, the SWG always reported lower readings than the salt drop test kit. For the past couple weeks, the SWG has maintained a level of 3600 but the test is indicating 2600-2800. So it is now lower than the SWG. We did have a lot of rain that cause pool level to rise 5 inches. That is when the test kit values started coming down. My Chromate Indicator reagent is old. I go thru silver nitrate a lot. Could the reagent age cause a lower reading? When I add the first few drops of nitrate, it does cause some clumping to develop in the sample. I do not recall that happening within the first few drops before.

Any ideas? You all are always so helpful.
-Greg

 

[JoyfulNoise]

Start with fresh reagents. Silver nitrate can breakdown on its own. Also note that the SWG internal salinity test is by electrical conductivity which is inherently less accurate than the chloride test. And the EC test by the SWG just isn’t that good to begin with. So there is a wide margin of error for it, about +/-400ppm. The drop test is only good to about +/-200ppm (+/- 1 drop) unless you use a larger test volume (like 20mL or 40mL) to get greater precision out of the test.

 

[CFlaPool]

Thanks JN. My silver nitrate is fresh. (I finally got around to wearing nitrile gloves to do the test, after having mysterious brown spots on fingers later in the day.) I guess I can get a new Chromate and see it that brings the two measurements closer together. My surprise was the flipping of the drop test now lower than the SWG report. Would deposits on the SWG cause a higher reading to be reported?

 

[JamesW]

Maybe an incorrect temperature reading from the cell.

 

[CFlaPool]

Our pool temp has reached 87-89 degrees, which is high even for central Florida. The pool water temp sensor is saying 89. Could that affect the IC-40 reported reading to the easyTouch?

 

[DrewLG]

The IC40 has its own temperature sensor. You can make it display its temperature reading: Pentair Intellichlor ICXX SWG - Further Reading

 

[CFlaPool]

The IC40 has its own temperature sensor. You can make it display its temperature reading: Pentair Intellichlor ICXX SWG - Further Reading

Thanks. I remember reading that thread a while ago and forgot about it.

I also saw this:

Cleaning a SWG cell with Vinegar Acetic Acid​

If the scale is stubborn then use cleaning vinegar (6% acetic acid … available in Home Depot). It’s milder than Muriatic Acid and won’t damage the ruthenium surface. Highly concentrated mineral acids are not good for the transition metal catalysts.

One question: can I use diluted 30% vinegar to clean the plates in the SWG? We have it for making weed/grass killer. Can I assume that full-strength might be harmful or unnecesary.

 

[CFlaPool]

Start with fresh reagents. Silver nitrate can breakdown on its own. Also note that the SWG internal salinity test is by electrical conductivity which is inherently less accurate than the chloride test. And the EC test by the SWG just isn’t that good to begin with. So there is a wide margin of error for it, about +/-400ppm. The drop test is only good to about +/-200ppm (+/- 1 drop) unless you use a larger test volume (like 20mL or 40mL) to get greater precision out of the test.

I went to TFT and ordered a new salt testing kit. I had bought the last bottle of silver nitrate elsewhere and maybe it was old stock.