Why Your Salt System Reading and Test Kit Disagree

It's one of the most common points of confusion for a saltwater pool owner. You diligently test your pool water and get a salt reading of 2800 ppm. You use our Pool Salt Calculator to figure out how much salt to add. But when you look at your salt chlorinator's control panel, the digital display reads 3300 ppm. Which one is right? Why is there a difference? And which reading should you trust?

This guide will explain the technology behind these readings, the common causes for discrepancies, and provide a clear answer on which number you should rely on for accurate pool maintenance.

How Your Salt System "Measures" Salt

The first thing to understand is that your Saltwater Chlorine Generator (SWCG) does not directly measure salt. It can't count the individual salt particles in the water. Instead, it estimates the salt level based on **electrical conductivity** and **water temperature**.

• Conductivity: Pure water is a poor conductor of electricity. When salt (an electrolyte) is dissolved in water, it allows electricity to flow more easily. The more salt, the higher the conductivity. Your salt cell passes a small electrical current through the water and measures how easily it flows.
• Temperature: Water temperature significantly affects conductivity. Colder water is less conductive than warmer water, even at the same salt level.

The SWCG's software takes the conductivity reading and the temperature reading from its sensors, and then uses a pre-programmed algorithm to convert those two values into an estimated salt ppm. This is not a direct measurement; it's a calculated guess.

Why the Chlorinator's Reading is Often Wrong

Because the system's reading is an estimate based on multiple factors, there are several reasons why it can be inaccurate:

1. Dirty Salt Cell

This is the number one cause of inaccurate readings. Over time, calcium and other minerals form a white, insulating scale on the cell's metal plates. This scale physically blocks the electrical current, making the conductivity appear lower than it actually is. The system misinterprets this as low salt and may display a "Low Salt" error, even when the real salt level is perfect. Cleaning the cell will often correct this issue.

2. Sensor Age and Calibration Drift

Like any electronic sensor, the temperature and conductivity sensors in the salt cell can lose accuracy over time. They can drift from their factory calibration, leading to consistently high or low readings.

3. Total Dissolved Solids (TDS)

Your chlorinator assumes that all conductivity in the water is due to salt. However, other dissolved minerals and solids also contribute to conductivity. This includes calcium, carbonates, and other substances that build up in the water over time. High TDS can make the water more conductive, causing the system to report a salt level that is artificially high.

4. Software Algorithm Limitations

The conversion algorithm is just an approximation. It may not perfectly account for the complex interplay between temperature, salt, and other dissolved solids, leading to inherent inaccuracies.

How Your Test Kit Measures Salt

Independent salt testing methods are designed to be more direct and less susceptible to these interferences.

• Digital Testers: These also measure conductivity, but they are purpose-built for this single task. They often have more sophisticated sensors and calibration routines than the all-in-one sensors in a salt cell. When properly calibrated with a standard solution, they provide a very accurate reading.
• Test Strips & Liquid Kits: These use a chemical reaction to measure the chloride concentration in the water. For example, titration kits use a reagent (silver nitrate) that reacts with the chloride ions. This method is not affected by cell scaling or other dissolved solids (TDS), making it a reliable way to get a true salt reading. Our guide on how to measure salt levels provides more detail.

The Final Verdict: Which Reading to Trust?

Always trust your independent, properly calibrated test kit over the reading on your salt chlorinator.

Think of the chlorinator's display as a convenient but general reference—like the fuel gauge in your car, it gives you a good idea but isn't perfectly precise. Your independent test kit is like dipping a measuring stick directly into the gas tank—it gives you the true measurement.

When you use our Pool Salt Calculator, you should always use the value from your digital tester or liquid test kit in the "Current Salt Level" field. Do not use the number from your system's display. This ensures you are adding the correct amount of salt to reach the true desired ppm, which will allow your (clean and functioning) salt cell to operate at peak efficiency.