Practical guide: how to analyse and control your pool water?
In this article, you'll find a practical guide explaining the parameters and frequency of measurements you need to take to analyse your pool water. Then we give you precise answers about the thresholds to be respected for each parameter in order to maintain water quality. Finally, we present the different measuring tools with their advantages and disadvantages.
What parameters should be checked in swimming pool water?
To maintain the quality of your pool water, it is important to check the three parameters that affect water balance (pH, TAC and TH), as well as other parameters such as disinfection (chlorine level or ORP), temperature, stabiliser level, and salt level or salinity for saltwater pools
Target values
Minimum measurement frequency
Variability
pH
7,0 – 7,4
1 à 2 fois / semaine
very high
Free chlorine level
1,5 – 3,0 mg/L
1 à 2 fois / semaine
élevée
ORP
650 – 750 mV
1 à 2 fois / semaine
average
TAC
80 – 120 mg/L
1 fois / mois
low
TH
150 – 250 mg/L
1 à 2 fois / saison
very low
Stabiliser
20 – 50 mg/L
1 fois / mois
low
Salt content
3,0 – 5,0 g/L
1 à 2 fois / mois
low
Temperature
24 – 28 °C
1 fois / jour
medium to high
Parameters that influence water balance
The balance of water in a swimming pool depends on 3 key parameters: pH, TAC and TH. These 3 factors are closely linked. Water balance is essential to ensure water hygiene, bather comfort and equipment protection.
pH
For chlorine disinfection, it is between 6.9 and 7.4
For bromine or salt disinfection, it is between 7.5 and 7.8
Un pH trop haut >7,8 neutralise l’action du chlore (fait chuter l’efficacité de votre chlore de plus de 50%) et irrite la peau
Un pH trop bas devient corrosif pour les équipements et le revêtement
TAC
TAC (total alkalinity) or water alkalinity
Le TAC doit se situer entre 80 et 150 mg/litre ou ppm. Un bon équilibre de l’alcalinité détermine le pouvoir tampon de l’eau et impacte la stabilité du pH et l’acidité de l’eau.
Si le TAC est > 200, l’eau est tamponnée. malgré l’ajout de pH+ ou pH-, le pH reste stable.
If the TAC is < 100, the pH fluctuates wildly.
At the start of the season, the TAC is usually around 200 and ends up at 80–100. Whenever the water is agitated (swimming, fountains, cleaning), carbon dioxide escapes. To compensate for this loss, the water converts its bicarbonates into CO2. Furthermore, to maintain a stable pH (which is often falling), the addition of pH- ‘eats away’ at the alkalinity reserves over time.
TH
TH (hydrotimetric titre) or water hardness.
Le TH mesure la dureté de l’eau. Exprimé en degré français (°f). Idéalement il doit se situer entre 10°f et 20°f.
Below 10°F, the water is too soft and tends to dissolve the mineral components in your pond.
Above 20°F, the water is too hard. It then becomes cloudy and milky, and limescale deposits appear on the walls and equipment.
The TH depends on the water source and the region. For example, in the south of France, TH > 300; in Brittany, TH < 100
The total hardness (TH) remains relatively stable; however, the behaviour of the limescale changes radically depending on the water temperature and pH. For example: if your total hardness is the same in May as it is in August, the limescale will precipitate more readily in the form of scale when the water is at 30°C or has a pH of 7.8.
Afin de maintenir l’équilibre de l’eau, il est nécessaire de contrôler ces 3 paramètres. Tout l’enjeu est de maintenir un équilibre qui se joue entre trois différents paramètres : le pH, l’alcalinité et la dureté de l’eau.
The ORP/Redox value determines the effectiveness of your disinfectant (bromine, chlorine). It is a more reliable indicator than the free or total chlorine level.
For optimum disinfection of water at 25°C, ideal ORP measurements should be between 650 and 750mV. This is one of the parameters that varies most rapidly because it depends on pH and UV values.
This is one of the parameters that changes most rapidly, as it depends on pH levels and UV radiation.
The chlorine level
Chlorine is most effective at a pH of between 7.0 and 7.4
Below 1.5 mg/L, bacteria and algae can multiply faster than the chlorine can kill them. The water can turn green within a few hours, especially if the temperature exceeds 28°C.
Above 3.0 mg/L: the excess concentration becomes ineffective and irritating to the mucous membranes (red eyes, dry skin), discolours swimwear and, in the long term, accelerates the ageing of the liner and pump seals.
This is one of the parameters that changes most rapidly, as it depends on pH levels and UV radiation.
Why can’t we compare chlorine levels and ORP?
It’s a common misunderstanding, but these two measures do not mean the same thing.
The chlorine level indicates the concentration of chlorine molecules in the water. It corresponds to the disinfecting power of the chlorine, which is affected by the pH and the stabiliser.
The ORP measures the effectiveness of disinfection
Example: when the chlorine level is just right at 2 mg/L, the pH rises to 8, but the ORP drops from 700 to 500 mV
Other parameters to be measured
Water temperature
The water temperature must also be kept under control.
Heat accelerates chemical reactions. Bacteria and microorganisms multiply exponentially, consuming chlorine much faster than it is produced or added. Furthermore, UV light penetrates warm water more easily, breaking down free chlorine if the stabiliser level is not optimal.
As soon as the water temperature exceeds 28°C, it is recommended that you switch the filtration system to continuous mode (24 hours a day). The standard formula ‘Temperature / 2’ is no longer sufficient, as the risk of mustard or green algae blooms becomes critical.
The salt content
Thanks to the electrolyser, the salt enables the production of natural chlorine through a chemical reaction.
A constant salt concentration is required to prevent damage to the plates in the electrolyser cell.
The salt concentration inevitably decreases over the course of the season: dilution due to the addition of water (evaporation, discharge from filter backwashing, rain)
Stabiliser levels
The stabiliser level (cyanuric acid) is a parameter that needs to be measured when a chlorinated swimming pool is treated with stabilised chlorine (also known as multi-purpose chlorine) or when a salt-treated swimming pool has stabiliser added.
The ideal concentration is 20 to 50 mg/L.
If the level exceeds 70, your water may be over-stabilised. The stabiliser blocks the action of the chlorine. The only solution is then to drain some of the water from the pool
Too much chlorine renders it ineffective, leading to the proliferation of bacteria, algae and fungi.
Why should you regularly analyse your pool water?
Regular analysis of swimming pool water is essential to protect the health of bathers, guarantee crystal-clear water and ensure the longevity of equipment.
Protect the health of bathers: detect any imbalances (pH too high or too low, lack of disinfectant, etc.) and prevent the proliferation of harmful micro-organisms.
Maintain water quality: spot any problems with cloudy water, green water or algae growth early enough to take swift action.
Protect the equipment: the wrong balance (water that is too acidic or too chalky) can damage the walls, the pump and the filtration system, which is why regular monitoring is so important.
How often should you test your pool water?
The water in your pool is constantly changing. Several factors influence water quality. The frequency of pool water testing and maintenance depends on factors such as the season, the number of users and the weather conditions. Indeed, in summer, with the heat and frequent swimming, the water deteriorates more quickly.
Spring/summer: frequency of pool water testing
To maintain the quality of your swimming pool water, it is essential to test it regularly for various parameters, the frequency of which varies.
During periods of heavy use or if the weather is particularly changeable (high heat, rain), it may be necessary to test the water more frequently to prevent any imbalance.
Water parameters to be measured at least once a week
In the high season, your pool water needs to be analyzed on a regular basis. This allows you to intervene quickly with the right treatment, so you can continue to enjoy your pool every day.
pH
ORP / Redox
The temperature
Parameters to be measured once a month
TAC
Le taux de sel :
The sensors in the salt chlorinator may become fouled or go out of calibration. If there is a discrepancy in the salt level readings between the salt chlorinator, the test strips or the ICO, this is a sign that the salt chlorinator cell needs cleaning or that the sensor needs calibrating.
Furthermore, if the salt level is too low (3.0 g/L), the salt chlorinator will be overworked, thereby reducing the lifespan of the chlorinator cell.
The stabiliser level: as stabiliser never evaporates, it builds up over time. Checking it once a month allows you to see the level rising and to take action in good time to prevent the chlorine from being completely ‘blocked’, which would mean having to change the water and drain a third of the pool.
Parameter to be analysed twice a year
TH:
This should be checked at the start of the season after the pool has been brought up to standard. The TH level may drop in the event of heavy rain or significant refilling
pour les piscines au sel : vérifier également le TH à la mi-saison pour protéger la cellule. En effet, en raison de la chaleur et d’un pH élevé, la cellule peut s’entartrer plus rapidement.
How to carry out reliable analyses of your pool water?
The different pool water analysis tools
To test the various water parameters, you can choose between the manual method with test strips and electronic testers, or the automatic method with connected water analyzers.
To test pH, for example, you can opt for a chemical reaction strip, a ph-meter, which is a hand-held probe, or a connected water analyser. See
Pool water testing: essential for keeping the water clean and clear
As we have seen, water quality testing is essential throughout the season. Indeed, there are several parameters that need to be monitored. The frequency of testing depends on the variability of each factor.
Therefore, the pH, chlorine levels and, ideally, the ORP should be checked at least twice a week. This frequency may also vary depending on weather conditions. During heatwaves, more frequent checks are necessary.
The TAC, stabiliser and salt level should be checked at least once a month, particularly to help protect your equipment.
Finally, the total hardness (TH) should be checked at the start of the season. For salt-treated pools, it is advisable to check the total hardness again mid-season to prevent limescale build-up in the electrolysis cell.
