I had a HM Digital TDS conductivity meter that I was happy with but after it went for a swim I replaced it with a cheep eBay TDS meter and wanted to check its accuracy. I got a reading of 3780ppm and from two different LPS's I got 4100ppm and 5750ppm as ppm or mg/L NaCl. I suspect the 5750ppm reading is suspect due to anything from a poor sample to a lack of calibration. My Waterco swing arm hydrometer is giving a reading of just over 4000ppm or ~1.0028s.g. Given my target salt level is 6000ppm, neither of the LPS's recommended adding any salt.
Salinity is a little different to most other pool parameters as we tend add an entire bag of salt instead of adjusting to a specific level. One 20kg bag of salt raises the salinity in my 29kL pool by ~570ppm of NaCl. This makes me think that salinity readings don't need to be super accurate but somewhere close would be nice, either the 5750ppm reading from before or the other two readings are way out and it makes me wonder about the other results. And so the search was on for a standard solution and preferably something I could make at home.
I kept a reef aquarium for many years and recall an article by Randy Holmes-Farley that describes the methods for preparing home made standard solutions for refractometers, hydrometers and TDS or conductivity meters. Given the quite low and tight range for pool salinity I think refractometers and the method using standard measures, cups, teaspoons and 2L Coke bottles is a bit useless. The average kitchen scale is not accurate enough for my purpose but I was able to get a half decent scale from eBay for under $10. Natural sea water and the article from Randy Holmes-Farley target a salinity of S=35, 35ppt or 35,000ppm. A dilution factor or 7 neatly converts the salinity to S=5, 5ppt or 5000ppm which I think is ideal.
The original article is found online hear;
Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
Without getting into specifics (refer to the article) conductivity and specific gravity standards are a bit different and not interchangeable.
Demineralised or distilled water would be the best and whilst it does cost a bit more than tap water I can get a enough for both solutions for around $5. My tap water has a TDS reading of ~90ppm so I could make a 5090ppm standard but I'll opt for demineralised water.
Using standard iodised table salt the recipes are;
Conductivity: for 5000ppm, 6.2gms of NaCL* in 1,274gms H2O^
Specific gravity: for 5000ppm, 6.2gms of NaCl* in 1,127gms H2O^
Where the conductivity standard is used for a TDS or conductivity meter and the specific gravity standard is used for a hydrometer.
*Iodised table salt is used for NaCl.
^For water (H2O) mgs = mls at 25degC.