Pool and Hot Tub testing explained

Published: 08 September 2022

The What, Why and How of Pool and Hot Tub testing

Whether you are a new pool or hot tub owner or an experienced pro, we hope you will find information here to help you maintain pool & hot tub water in optimal condition.

We’ve put together this short guide which should provide you with the What, Why and How of testing pool and hot tub water.


The What?

First off, what do you need to test for? Well, the main parameters for pool and hot tub water are shown below, but don’t worry if it looks a long list, you won’t have to test all of these, all of the time!



Cyanuric acid

Conductivity in µS/cm and TDS in ppm

Total alkalinity 

Total hardness (Calcium hardness)

Cyanuric acid

Nitrate & Phosphate 



The Why?

Correct chemical control of your pool or hot tub water is essential to ensure bather safety and to maximise life of the mechanical and electrical parts of your system. Here we try and explain why each of the parameters you measure in your pool is important and provide guidance on what the optimal levels should be.


Ideal range between 7.2pH and 7.6pH

pH uses a scale of 0 to 14 and is a measure of how acidic or alkaline the water is.

pH values above 7 pH mean the water is alkaline, whereas pH values below 7pH mean the water is acidic. Therefore a pH reading of 8 pH would indicate an alkaline and a pH reading of 6pH would indicate an acidic reading. It is worth noting that pH increases ten times in strength for every unit of pH, therefore 5 pH is 10 times more acidic than 6 pH!

pH is the first measurement you should make because pH influences all the other parameters, so it’s very important to have the correct pH value and measuring it correctly before doing anything else!

Top tip: Always measure your pH when the water is at the right temperature! This is because at different temperatures the pH of the water will change, so if you measure the pH when the water is cold and then heat it up you’ll find the pH value has altered.



This is the most commonly used sanitiser in pools and hot tubs. Provided the pH of your water is in the correct range, then it is an extremely effective chemical.

In order to ascertain the level of chlorine in the water which is available to act as a disinfectant we measure free chlorine, in effect this is the measure of the active ‘part’ of the chlorine in the water.

We also measure something called combined chlorine, which is the measure of the chlorine that has done its job and disinfected the water. Depending on region and local authority requirements, for pools and hot-tubs the accepted range for combined chlorine is generally between 0 to 0.6mg/L.

The ideal range is a little harder to recommend as it depends a little on bather load, frequency of pool use, pool or hot tub location, but as a general guide:

Free chlorine ideal range for indoor pools 1 to 1.5mg/L

Free chlorine ideal range for outdoor pools 1.5mg/L to 3mg/L

Free chlorine ideal range for hot tubs 3 to 5mg/L

Top tip: You’ll also hear the term ‘total chlorine’ and might be wondering what it is. Well, it is simply the total. Total chlorine = free chlorine + combined chlorine.


Cyanuric Acid

Ideal range 20mg/L to 70mg/L (depends on local standards and if pool is outside)

Cyanuric acid is added to pools to protect the chlorine from degradation by ultra-violet light, which is present in sunlight. Therefore, it is a useful additive for outdoor pools and hot tubs. However, it does have a couple of drawbacks which are worth noting.

If you’ve ever heard the expression “too much of a good thing”, then please bear that in mind with cyanuric acid. If more than 70 mg/l of cyanuric acid is present, the free chlorine will become too stable and the active part of the free chlorine will substantially decrease.

What is even more frustrating is that the usual method employed to measure free chlorine in a pool (known as the DPD method) doesn’t ‘see’ the difference of free chlorine with cyanuric acid or without!

What happens when your cyanuric acid level is too high? Sadly, the only reliable method to remove cyanuric acid from swimming pools is through draining and/or dilution. As cyanuric acid doesn’t get ‘used up’, this is a frequent issue we come across. There is nothing as frustrating or time consuming, (not to mention the cost), of draining a pool of water just because the cyanuric acid levels had got too high for the chlorine to do its job.

✔ Top tip: Stabilised chlorine contains cyanuric acid. Whether you buy stabilised chlorine as tablets or in powder/granule form, it’s worth remembering that the term stabilised means it has cyanuric acid present.

✔ If you’ve ever had a pool and watched it gradually go green with algae, even though your meter tells you there is plenty of chlorine present, then it’s most likely a result of too high a level of cyanuric acid even if you think you haven’t been adding any!



Is a measure of the ability of a material in the water to conduct an electric current and is conventionally expressed in µS/cm (microsiemens/cm) or mS/cm (millisiemens).

For anyone with a salt controlled pool (using a salt electrolyse system) then it is a useful measurement to determine when to back-wash the pool.

Back-wash if the conductivity reading is greater than 2100 µS/cm.

Top tip: Accurate conductivity readings are very dependent on temperature, so it is important to allow your conductivity meter time to reach the temperature of the water you are measuring otherwise an incorrect result will be obtained. Luckily most conductivity meters have a thermometer built-in, so it’s just a case of waiting until the temperature readout is stable before taking the conductivity reading.


Total Dissolved Solids

TDS or Total Dissolved Solids is a measure of the total ions in solution and in terms of a pool or hot tub, it is a measure of the concentration of the dissolved compounds in the water which could be from the chemicals you’ve added, but also from what the bathers introduce to the water.

Ideal range: TDS should be no more than 1000 mg/l higher than the incoming fill water


Total Alkalinity

Correct levels of alkalinity prevent large swings in pH thus making it much easier to control the pH of your pool or hot-tub water. It is primarily a measurement of the bicarbonates in the water and is expressed in mg/L CaC03 (calcium carbonate).

Ideal range: 75 to 120mg/L CaCO3

Top tip: If you find it’s difficult to control the pH of your pool water and every time you add pH plus or pH minus the pH values seem to run away with you, then it’s worth checking your total alkalinity and ensuring you’ve got the right amount in your pool to buffer and protect against large pH swings.


Total Hardness

Total hardness is the sum total of the calcium and magnesium hardness of the water, although the vast majority will be calcium and therefore, a reading expressed as mg/l CaCO3.

It is a useful measurement as too much calcium hardness in your water will cause scaling of heaters, pumps, jets and pipework.

If hardness is too low, corrosion is possible because of the lack of a thin layer of calcium to protect against pitting corrosion of metal parts allowing the acid in the pool to attack the components.

Ideal range: 80 to 200 mg/l CaCO3

Top tip: If your water appears cloudy, it may well be an indication of too high a value of calcium hardness.


Nitrate and Phosphate

Both these compounds can cause eutrophication and a clear sign of them present in the water is if the pool water suddenly goes green due to rapid algae growth as they are both nutrients.

Both are naturally present in water although, generally, in such low levels as to not present a problem.

Ideal range for Nitrate is less than 50 mg/l NO3

Ideal range for Phosphate is less than 2.5 mg/l PO4

Top tip: If you find you’re using a lot of chlorine and your pool water is still green, then you’re nitrate or phosphate levels are probably higher than they ought to be!



Turbidity is a measure of how cloudy water is and an indicator of pool water chemistry or filter issues. From a safety perspective, it is an important measurement in pools as water with a high turbidity value might make it difficult to see someone in trouble at the bottom of a pool.

Ideal range should be less than 0.5NTU


The How?


There are a wide variety of methods to measure pool and hot tub chemicals, ranging from test strips, liquid chemical test kits, electronic photometers which utilise tablet/liquid or powder reagents and direct reading digital meters. The accuracy and ease of use of the various methods are reviewed below, but first we need to cover taking a water sample.

Taking a water sample

In order to maximise the accuracy of your reading it is important to try to obtain a representative sample of the water in your pool or hot tub. Where you sample from is important:

✔ Always use a plastic container and NOT glass. Broken glass in a pool or hot tub is near invisible and thus a real safety issue.

✔ Always use a clean plastic bottle or sampling container, we find used plastic drinking water bottles idea.

✔ Hold it upside down so the opening is facing the floor of the pool or hot-tub

✔ Insert the bottle into the water at least to the depth of your elbow and only then turn it the right side up to collect the water sample from below the immediate surface water.

✔ Avoid taking a water sample near the return jets or skimmer openings if at all possible


Options for testing pool and hot tub water

Test Strips

A common way of measuring the parameters in your pool or hot tub is to use a chemical test strip - a small section of plastic impregnated with a series of chemicals which individually react with certain parameters in your water.

Thus, a chemical test strip is quick and easy to use, but in terms of accuracy, there are some significant issues when using them. They require the user to wait a certain number of seconds for the chemical reaction to take place, and then they require the user to match the developed colour with a colour chart supplied. As we all see colour slightly differently, and because most of us don’t wait the required time for the colour to fully develop on the test strip, they may be a good indication of what is going on in the pool water, but not a particularly accurate way of getting an absolute reading.


Low cost

Easy to use

Fast results

Can test more than one parameter at a time


× Not that accurate in use

× Need to allow correct time for chemical reaction and colour to develop

× Need to dip stick for correct amount of time in the water

× Relies on the human eye to deduce which colour it most closely matched on the chart

× Must be stored in a sealed container out of sunlight, results WILL vary and the test strips degrade if left out in a humid area (like a pool plant room for example!) or in direct sunlight, such as beside an outdoor pool in the summer!


Liquid Chemical Test Kits

These usually comprise a plastic cube which is colour graduated or a wheel with different shades of colour around the perimeter. The idea behind them is the same as the chemical test strips in that a colour develops in the water you are sampling after you have added the supplied chemical (called reagent). You then compare the developed colour with that of the printed colour on the plastic cube or wheel.

Like chemical test strips, these are only as accurate as the person using them and are susceptible to the same issues such as the importance of allowing the correct time to elapse before taking a reading and the issue of colour interpretation.


Relatively low cost

Easy to use

Fast results

If used correctly, the potential to give better accuracy than a chemical test strip



× Not as easy to use a chemical test strip

× Need to allow correct time for chemical reaction and colour to develop

× Still relies on the human eye to deduce which colour it most closely matches

× Liquid reagents must be store out of sunlight and tablets should be stored in an ambient environment and not in a hot and humid plant room.


Electronic Photometers and Colorimeters

A photometer and a colorimeter are one and the same thing, it is just some manufactures like to call them photometers and others refer to them as colorimeters. To save saying both names we will just use the word ‘photometers’ in this text.

Photometers work in the same way as a chemical test strip or chemical test kit; by electronically examining the shade of colour developed in a water sample and giving a result. They are able to operate at a specific wavelength rather than the full visible light spectrum and don’t rely on human eye ‘interpretation’ of colour. As a result they are significantly more accurate than either test strips or chemical test kits and give a direct readout of chemical parameter in mg/L.


High levels of accuracy (as good as +/-0.01mg/L)

Direct readout in mg/L of the parameter you are measuring

Pre-programed with the correct ‘waiting time’ for the colour to develop

No need to rely on colour interpretation by human eye

Available as single parameter or multi-parameter instruments


× More expensive as one has to purchase the photometer

× Liquid reagents must be stored out of sunlight and tablets should be stored in an ambient environment and not in a hot and humid plant room


Direct read digital meters

Although it is not possible to measure all the parameters in a pool without some form of chemical reagent being added to the water, there are some parameters which can be measured by direct read digital meters.

Of all of these, pH is really the one we’d recommend getting a direct read digital meter for. Phenol red tablets and liquid test kits are not that good at getting accurate pH results, and colour interpretation on a chemical test strip can be at best half a pH unit out from the true value due to the reasons already mentioned.

A digital pH pocket test instrument provides direct readout of the pH of the pool with up to +/-0.01 accuracy. They are easy to use and quick to provide results. Simply switch on, place the sensor in the water and get a direct readout of the pH.

Other direct read digital meters which are also available for the pool and hot tub market are pocket testers for Conductivity, TDS (Total Dissolved Solids), ORP (Oxidisation Reduction Potential) and, of course, temperature.


No need to add chemical reagents

Easy to use

Direct readout of the parameter you want on the digital display

High degree of accuracy



× Although not expensive, they do cost more than chemical test strips

× Some models require calibration and a degree of care or maintenance to ensure accuracy


Relevant Articles:

The importance of pH testing in pools

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