How to measure the amount of salt in your swimming pool

Among the people who have a salt chlorinator, some use a TDS digital water test meter (Total Dissolved Solids), to measure the quantity of dissolved solids in the water. This is not a good choice for measuring the sel amount of swimming poor.

A TDS measures the conductivity of water and expresses it in siemens, which will often be translated for you into grams per litre. To perform this measurement, the device uses a conductivity probe composed of 2 graphite or other electrodes, a slight current will pass from one electrode to the other. Depending on the current, i.e. the quantity of electrons circulating between the 2 electrodes, the resistivity of the water is translated to define the conductivity.

However, when we talk about resistivity, this does not mean that it is the quantity of salt (NaCl) dissolved, but the resistance that water opposes to the passage of a current. This resistance can vary depending on the amount of salt, it is true, but also on other minerals present in the water. But that's not all. The water can be made more resistive by the presence of microorganisms that you cannot see, but which will soon appear when the water turns green. We are talking here about the turbidity of the water due to pollens, spores, etc…

Which means that measuring the amount of salt with a TDS meter is wrong. However, if the water is well filtered, well sterilized, the amount of salt is close to 6g/l, the better the measurement and the lower the error. It is common to see an error of 1.5g/l, and for a chlorinator that works with 3g/l of salt, this is a big mistake.

Therefore, the measurement of salt must be done by a chemical reagent. To date there are several brands, but the only one that has regularly given me satisfaction is the Aquacheck salt strip. The max error I have seen so far is 0.2g/l and is often closer to 0.1g/l; it depends if it is new or not, soiled or not.

When to measure salt?

In the spring, if there has been no filtration, the salt density will be higher at the bottom of the pool than at the surface. Therefore, the filtration must be activated after taking care to disconnect the salt chlorinator. This is also the period when you clean your pool, possibly when the robot passes. If you pass the robot, do not forget to put flocculant socks in the skimmers.

After 1 to 2 days of filtration, you backwash the filter. You will set the amount of salt with a strip. Then you top up. You restart the filtration and wait 48 hours of filtration before reconnecting the chlorinator.

Note

This is the time when you have to do a chlorine shock with bleach or calcium hypochlorite. If necessary, correct the pH if it is too high. If it is low but not excessive, do not touch anything, electrolysis will make it rise. Also be aware that winter rains make the water very soft and acidic, wait a bit before correcting the pH.

If you had to raise the pH because after a month of electrolysis it is still below 6, do a chlorine shock, turn off the electrolyser, add soda ash (pH+). Turn on the filtration every day. Switch the chlorinator back on after 4-5 days.

If the TH is too low (10°F) calcium can be added by adding calcium chloride (about 11 g/m3 /°F) or bicarbonate (16.8 g/m3 /°F) possibly carbonate. The TAC+ will do. Same operation as for pH+. So please pay attention to this.