Swimming Pool Water Testing, Equipment and Methods
Water Testing Equipment and Methods
Electrochemical methods (Redox and Amperometric)
TDS (Total Dissolved Solids)
Langelier Balanced Water Index
Water Testing Equipment and methods
Notes on Sampling
It is very important that a representative sample of the pool or spa water is taken for analysis so
â€˘ always take the sample from the same place. This is generally best at the furthest point from the return to the pool of the freshly disinfected, filtered water.
â€˘ take the sample from approximately 300 mm below the surface and slightly away from the side of the pool.
â€˘ always rinse the sample container several times with the water before taking the sample for testing.
This method of water analysis is by far the oldest and simplest way of determining the concentrations of anions and cations in water.
In itâ€™s applications to swimming pool and spa water analysis it was established well over half a century ago that by adding a selective reagent to a sample of water a colour was produced, the intensity of which was proportional to the concentration of chlorine in the water.
Calibrated colour standards were produced, some in transparent plastic material, others in glass, against which the coloured test solution was matched to give the result of the analysis.
These two systems are still in use today with the plastic colour standards usually in the cheaper, less robust, test kits, while the glass colour standards tend to be in the better designed, professional kits developed for public pools and leisure complexes.
It is with this latter type, the chemically produced glass standards, which we are primarily concerned and have been produced for the pool market for over 60 years. These are designed for accurate visual analysis :
â€˘ accurately calibrated glass colour standards, guaranteed non- fading even under extreme conditions.
â€˘ a Comparator unit which accepts discs, cassette-style, ensuring very perfect location every time and which has an in-built optical prism unit to bring the two fields of view â€“ test solution and glass colour standard â€“ adjacent to each other for accurate colour matching.
â€˘ a complete reagent system based on tablets. These have been shown to be the most reliable and consistent in reagent preparations.
An important point worth mentioning and often overlooked by operators is that for accurate results to be obtained when using this type of equipment, the correct lighting conditions should be used.
Modern technology has brought the field of electronics to pool water testing and the use of portable photometers is becoming widespread.
These are instruments which pass a beam of light through the coloured test solution
onto a photodetector. The intensity of the light collected is then converted, by the microelectronics, into a value of concentration which appears in a digital display.
Hence, in this method of analysis, there is no colour matching for the operator to do, and providing the test procedure is followed exactly and some very important factors are noted, photometers can give very accurate test results.
These factors are :
â€˘ the correct type of reagent is used â€“ special photometer grades of tablets are often specified.
â€˘ the reagent must be completely dissolved with no particles floating in the test cell.
â€˘ there must be no bubbles adhering to the inside walls of the test cell.
â€˘ the test cell must be dry on the outside with no finger marks on the glass.
â€˘ the cell compartment itself must be kept clean and dry.
Redox Potential (ORP)
Redox is a term for an electronic measurement to assess the state of balance between the oxidised and reduced states of a substance. In a pool, this is usually chlorine and the measurement is in millivolts (mV). The mV reading is known as the redox potential. It is not a measure of the concentration of the oxidised or reduced forms of the chlorine but indicates the state of the reaction.
An increase in the mV reading indicates an increase in the concentration of the oxidising substance (free chlorine) but the reading is not related directly to the free chlorine concentration.
Redox controllers are used only to give a qualitative estimate of free chlorine â€“ a reading of around 700 mV would indicate approximately 1 mg/l â€“ and so give a useful indication of how water quality is affected by bather load.
Redox response is not linear however and the response levels off rapidly above about 1.5 mg/l of free chlorine
Redox response is very sensitive to pH changes and unless the pH is finely controlled redox is very imprecise in control applications. The response of redox electrodes is relatively slow after start-up â€“ often in the region of 20 minutes, so time must be allowed for the reading to stabilise.
When redox units are in operation, it is important that the electrodes are maintained
and cleaned on a regular basis. To do this the electrode is taken out and impurities removed mechanically. The electrode must then be recalibrated using a special redox calibration solution, rinsed then replaced. Manual testing of the pool water for free chlorine remains important.
This is form of electrochemical measurement and is concerned with the determination
of the concentration of free chlorine as hypochlorous acid â€“ as mentioned before this is the â€ťactiveâ€ť form of free chlorine. Other methods measure both hypochlorous acid HOCl and hypochlorite ion OCl-.
Amperometric analysers form the basis of automatic controllers in large pools as they can accurately control the level of hypochlorous acid in the water. Further more the response time of the sensors is rapid ensuring a minimum of delay in adjusting the level of disinfectant.
Amperometric controllers are much more sensitive and prone to external interference
than redox controllers. They actually measure a small current flow which is proportional to the number of chlorine atoms discharged at the operating electrode in the cell. Any changes in the conductivity of the water will affect this and in a spa pool particularly unless the important parameters of alkalinity and pH are controlled along with some form of continuous dilution, an amperometric system can become very unbalanced and inaccurate.
Ortho Tolidine (OTO) has been used as a colorimetric reagent for chlorine for over 70 years. It is simple to use and produces an instant yellow colour with chlorine. However, as knowledge was gained on the mechanism of the chlorine disinfection
process and the fact that there is more than one kind of chlorine residual, the chemistry of OTO was examined and important factors were found which could adversely affect the test. These included:
â€˘ the acidity of the reagent solution
â€˘ the method of adding the reagent
â€˘ the effect of time on colour development
â€˘ the need for a re-evaluation of the test for chlorine levels greater than 1 mg/l
Research suggested that when testing water samples at normal pool temperatures the result indicated by OTO was that for total residual chlorine i.e. free available chlorine plus combined chlorine (chlorine which has combined with ammonia to form chloramines). To obtain the free available chlorine separately the sample should first be chilled to 1Â°C.
Recently however, by far the most damming feature of OTO is its toxicity. In Europe in the 1970â€™s its use became restricted when it was categorised within a group of aromatic amines which are suspected of being carcenogenic (causing cancer). Workers handling them since then have been subject to regular medical inspections and the use of OTO in the swimming pool industry has been actively discouraged â€“ its use in many countries is prohibited. Despite this, OTO test kits are still widely available across the world, mainly due to their low price.
Thankfully a satisfactory, safe alternative to OTO was introduced in the late 1950â€™s by Dr.A.T.Palin in England who found that the reagent NN Diethyl-p-Phenylene Diamine Sulphate could be used to selectively produce a colour with free available chlorine. We all know this reagent today as DPD and it has become accepted in many National and International Standards for drinking water analysis. It follows then that it is eminently suitable for monitoring swimming pool water.
The reagent itself is commonly available in two different forms: as a liquid and as a tablet.
With DPD in solution, care must be taken in storage as it deteriorates on exposure
to light. In addition, it is only stable in acid solution, so a separate reagent consisting of an alkaline buffer solution must be used with it to ensure that the correct pH is produced in the test solution (about 6.3) for the full intensity of colour (red) to be developed.
The most popular form of the reagent with pool operators is when it is supplied in the form of tablets. These are packed in aluminium foil giving them a long shelf life, and this, together with production techniques ensuring accuracy in formulation, makes the product a most reliable and consistent test reagent.
Advantages of Tablet Reagents over liquids are:
â€˘ ease of handling
â€˘ ease of dosing â€“ one per test
â€˘ long shelf life
â€˘ no storage problems
Mechanism of the DPD test
There are two DPD tablets which are regularly used in swimming pool and spa water analysis:
DPD No.1 â€“ measures free available chlorine,
DPD No.3 â€“ used in conjunction with the No. 1 tablet and measures the total
Residual Chlorine from which the combined chlorine is calculated.
Free Available Chlorine
The DPD No. 1 tablet, which contains the NN Diethyl p Phenylene Diamine Sulphate,
gives a colour which is specific for free chlorine, and this colour is measured either colorimetrically or photometrically:
A clean test cell is rinsed with the water to be tested and is left empty.
A DPD No. 1 tablet is added and crushed with a clean stirring rod. The water sample is then added and the cell is filled to the 10 ml mark.
The solution is mixed well with the stirring rod until the tablet has completely dissolved.
The lid is put on the cell.
The colour must then be measured immediately to determine the free chlorine content of the water in mg/l (ppm).
Combined Chlorine (Chloramines)
This is the general name given to the derivatives of chlorine which are produced when free chlorine reacts with nitrogen compounds like ammonia and urea from bathers.
HOCl + NH3 = NH2Cl + H2O
Hypochlorous Ammonia Monochloramine Water Acid
HOCl + NH2Cl = NHCl2 + H2O Dichloramine
HOCl + NHCl2 = NCl3 + H2O Trichloramine
These are the products of the chlorine reactions which are responsible for most of the complaints form bathers of skin and eye irritation.
Trichloramine, more commonly called nitrogen trichloride, produced in the last reaction, is an unstable compound and being volatile comes off the surface of a pool as a gas with a nauseous odour. In addition, it is an extremely severe eye irritant. The chemical reaction does not proceed to completion at pH values above 5 so usually only very small amounts, if any are produced.
The two chloramines we are most concerned with are monochloramine and dichloramine. In the DPD test they are usually determined together using the DPD No.3 tablet :
The cell containing the dissolved DPD No.1 tablet â€“ from the free chlorine test â€“ is removed from the instrument and a DPD No. 3 tablet is added to it and mixed to dissolve with the stirring rod. The cell is allowed to stand for 2 minutes for complete reaction of the combined chlorine (monochloramine and dichloramine).
The cell is then replaced in the instrument and the colour is measured again. The result is total chlorine in mg/l.
To obtain the result for combined chlorine apply the following equation :
Combined Chlorine = Total Chlorine - Free Chlorine
Important Note : The DPD No.3 tablet contains potassium iodide which even in minute traces will cause a reaction from the combined chlorine present in the sample. It is essential that test cells and lids are thoroughly rinsed after using this tablet before another free chlorine test is carried out otherwise a false reading for free chlorine will be obtained.
To eliminate this problem some operators prefer to use separate cells for the free chlorine test and the total chlorine test, transferring the liquid from the free chlorine cell into another clean cell to which the DPD No.3 tablet is added, thus avoiding any Iodide contamination of the first cell.
Interpretation of Results
The reading for free chlorine (HOCl and OCl -) is the most important of all the pool tests. The general recommendation is for at least 1 mg/l of free chlorine to be present at all times in the water (more, if Cyaneric Acid is used: see page 17).
In addition it is also important that the ratio of the concentration of free chlorine to combined chlorine is at least 2:1. For example if the free chlorine concentration is 1.5 mg/l the combined chlorine concentration should be 0.75 mg/l or less.
In spa pools, with elevated temperatures, high turbulence and possible high organic loading from heavy usage, a free chlorine residual of 3 â€“ 5 mg/l should be maintained.
In all cases it is desirable that the combined chlorine concentration should be below 1 mg/l if practically possible.
The test for bromine is very similar to that for chlorine in that it uses the DPD
No. 1 tablet.
Where it differs is that in the bromine test, the tablet not only responds to free bromine but also to any combined bromine â€“ bromamines â€“ which may be present. As discussed before, these compounds are good disinfectants in their own right, unlike the chloramines which have very little disinfecting power.
We therefore say that the DPD No.1 test is measuring total bromine or active bromine.
Levels of bromine as measured by the DPD No.1 tablet should be between 4 and 6 mg/l. This applies in pools as well as spas. The test procedure is as follows:
A clean test cell is rinsed with the water to be tested and is left empty.
A DPD No.1 tablet is added and crushed with a clean stirring rod. The water sample is then added and the cell is filled to the 10 ml mark.
The solution is mixed well with the stirring rod until the tablet has completely dissolved
and the lid is then put on the cell.
The colour produced is then measured to determine the concentration of total bromine in mg/l.
Although it is not strictly necessary to monitor the build-up of combined bromine on a day to day basis, it is a good idea to occasionally separate the total bromine into free and combined as, like with chlorine. It is desirable to have a ratio of free to combined of at least 2:1. This is carried out with the use of a DPD Nitrite tablet:
Prepare a clean cell and crush a DPD No.1 tablet in the bottom of it â€“ leave it empty of water.
Rinse out another cell, then fill to the 10 ml mark with water sample, and add a DPD Nitrite tablet.
Crush and mix to dissolve with a clean stirring rod.
Add the contents of the cell to the empty cell containing the crushed DPD No.1 tablet. Mix well to dissolve the tablet particles.
Measure the colour produced which gives the concentration of combined bromine
To obtain the concentration of free bromine subtract the result for combined bromine from that for total bromine.
As we have seen pH measurement and control is essential in any pool or spa to maintain the value within the desired range. For heavily used pools, the pH value should be measured continuously and adjusted automatically, for other pools it is sufficient to measure the pH value regularly and adjust it if necessary .
pH measurements in these cases are by colorimetric indicator and the one which is used world-wide is Phenolred.
This has a good colour change from yellow to red over the pH range 6.8 â€“ 8.4 which makes it ideal for the monitoring of pool and spa water, which should be around the middle of this range.
Testing can be carried out with phenol red tablets or liquid, but in the case of the latter it is necessary to use a separate dechlorinating/debrominating liquid to prevent the disinfectant reacting with the indicator and changing itâ€™s colour. The tabletted reagent has this dechlorinator/debrominator as an ingredient in the formulation. Phenol red in tablet form is also much more stable than the liquid and is easier to use:
A freshly rinsed cell is filled to the 10 ml mark and a phenol red tablet is added.
This is crushed and mixed thoroughly to dissolve, using clean stirring rod.
The colour produced is matched either visually or in a photometer to give the pH value of the sample.
Note. If the colour produced is purple when testing a brominated water sample this is usually an indication that the bromine concentration is above 10 mg/l
For disinfectants based on chlorine to work properly and efficiently the pH value of the pool or spa water is critical. The normal recommendation is that the pH value should be maintained at between 7.2 and 7.6 with the target being 7.3 to 7.5, as disinfection will be more effective.
For pools and spas using bromine-based disinfectants a wider pH range is acceptable
â€“ 7.2 to 7.8. This is due to the fact that the efficiency of the disinfection is maintained over this range.
Alkalinity at levels below 50 mg/l may cause â€ťpH bounceâ€ť which means large changes in pH value in response to changes in dosing levels of disinfectant and/or pH correction chemicals.
To prevent this, the level of alkalinity in a pool or spa should be based on the type of disinfectant in use :
Chlorine gas disinfection 180 â€“ 200 mg/l
Sodium hypochlorite disinfection 120 â€“ 150 mg/l
Calcium hypochlorite disinfection 80 â€“ 120 mg/l
To raise total alkalinity the addition of sodium bicarbonate is required â€“ 1.5 kg per 50 m3 (11.000 gall) will raise TA by 15 mg/l..
If too high â€“ over 200 mg/l - use sodium bisulphate â€“ 2.4 kg per 50 m3 (11.000 gall) reduces TA by 20 mg/l.
10 litres of 15% hydrochloric acid (muriatic acid ) reduces TA by 20 mg/l.
The test for TA is quite straightforward using either liquid reagents in a drop test method or with tablet reagents in a tablet-count method.
In the liquid method a few drops of a colorimetric indicator is added to a measured volume of sample. Titrant is now added dropwise, until the indicator changes colour. The number of drops of titrant used are counted and a simple calculation gives the total alkalinity in mg/l as CaCO3.
If the Calcium Hardness in a pool is below say 70 mg/l as CaCO3 the water is likely to be corrosive to the pool structure and have a â€ťcalcium demandâ€ť. Ideally the level should be raised to at least 200 mg/l by the addition of calcium chloride flake â€“ 1.5 kg added to each 50 m3 (11.000 gall ) of pool water will raise the calcium hardness by 20mg/l.
The test for the level of calcium hardness can be photometric but is more usually carried out by the tablet-count method :
Calcium Hardness tablets are added one at a time to a 50 ml volume of the pool or spa water. The colour produced initially is pink which changes to purple at the end-point. The number of tablets used is counted and the following formula is applied:
(No of tablets x 40) â€“ 20 = Calcium Hardness in mg/l CaCO3
Ozone is a toxic gas and consequently in larger installations in particular, it must be removed form the water before it is returned to the pool after treatment.
However in spa pools, small amounts are generated to combat the oxidisable products which are produced â€“ combined chlorines etc. which means that ozone rarely gets back into the spa itself. In any case the concentration of ozone in the atmosphere above spa pool water should not exceed 0.1 ppm.
The test for ozone in water can be carried out using the DPD method, but recently
a new reagent based on Indigo Trisulphonate has been developed which is more selective, as the DPD method suffers from interference from any chlorine or bromine which may also be present.
DPD Method for Ozone
â€˘ Ozone in the absence of residual chlorine or bromine:
Rinse a cell with sample and leave empty.
Add either [one DPD No.1 tablet and one DPD No.3 tablet], or (one DPD No. 4 tablet) and crush with a clean stirring rod.
Add the water sample to the 10 ml mark and mix gently with the stirring rod to dissolve the tablet(s).
Match the colour produced either colorimetrically or photometrically and record the reading as residual ozone in mg/l â€“ call this reading A.
â€˘ Ozone in the presence of residual chlorine or bromine.
The above procedure is followed and the reading now corresponds to ozone plus total residual chlorine or bromine.
The second procedure is as follows;
Rinse the test cell thoroughly then fill to the 10 ml mark.
Add one DPD Glycine tablet, crush and mix to dissolve with the clean stirring rod.
Rinse a second cell with sample and then leave empty.
Add either [one DPD No.1 tablet and one DPD No.3 tablet], or (one DPD No. 4 tablet) and crush with the stirring rod.
Add to this cell the solution in the first cell and mix thoroughly to dissolve the tablet(s).
Match the colour produced either colorimetrically or photometrically and record the reading as total chlorine or bromine in terms of ozone in mg/l â€“ call this reading B.
To obtain the ozone concentration subtract reading B from reading A.
Ozone using Indigo Trisulphonate
In acidic solution, ozone rapidly decolourises indigo. The reduction in colour of a standard indigo solution is related to the amount of ozone present in the water.
Rinse out a suitable cell with the water sample then add one Ozone test tablet. Crush with a clean stirring rod then add the water sample carefully up to the fill line, avoiding air bubbles.
Mix gently to complete solution of the tablet, avoiding vigorous stirring.
When solution is complete, measure the colour produced colorimetrically or photometrically and record the reading as ozone in mg/l.
In pools running continuously on sodium hypochlorite disinfection, the build up of chloride can become a problem. In addition pH correction using hydrochloric acid (muriatic acid) will add chloride to the water.
High chloride levels can impart a salty taste to the water as well as giving poor colour and clarity. Levels are acceptable up to about 1000 mg/l and can be reduced
by regular backwashing of the filters and/or adding fresh water.
Obviously the level of chloride will be a lot higher than 1000 mg/l for a pool running
on salt for the electrolytic generation of chlorine. In this case the chloride concentration is about 2500 mg/l as Cl or 4000 mg/l as NaCl.
Testing for chloride is a simple procedure using Chloride tablets in a tablet-count method.
For a range of 0 â€“ 1000 mg/l as Cl measure a 10 ml sample of water into a clean container and add about 40 ml of chloride-free water (deionised).
Add one Chloride test tablet and shake to dissolve. The solution will go yellow.
Continue adding tablets one at a time until the colour finally goes brown. Count the total number of tablets used and apply the formula:
(No. of tablets x 100) â€“ 100 = chloride in mg/l Cl
For a range of 0 â€“ 5000 mg/l as Cl measure a 2 ml sample into a clean container and add about 40 ml of chloride-free water (deionised).
Add one Chloride test tablet and proceed as above. Finally apply the formula:
(No. of tablets x 500) â€“ 500 = chloride in mg/l Cl
To convert the result to mg/l sodium chloride NaCl multiply by 1.6.
It is becoming increasingly apparent that high sulphate levels can cause severe damage in concrete pools by attacking cement-based materials. In tiled pools sulphate attacks the tile grouting causing crumbling and expansion of the cement. This ultimately may result in tiles falling off the pool wall and floor.
Sulphate is introduced into the water from the use of sodium bisulphate (dry acid) for pH correction and from the use of aluminium sulphate as a flocculant. The problem is most likely to be found in heavily used pools using alkaline chlorine donors like calcium or sodium hypochlorite.
A maximum recommended level for sulphate in pool water is 360 mg/l. It can only be reduced by dumping some of the water and refilling with fresh.
A simple turbidimetric method is available for monitoring sulphate levels where a tablet reagent is added to the water sample in a special double-tube assembly.
Any sulphate present will produce a cloudy solution which is measured by moving the inner tube until a black spot printed on its base just disappears from view. The result is then read off from a scale on the side of the outer tube.
This tube assembly is also used for the turbidimetric Cyanuric Acid test.
The presence of cyanuric acid in pool water results from the use of chlorinated isocyanurates as disinfectants.
In the process of disinfection, the chlorine becomes used up but the cyanuric acid molecule remains, and over time can build up to such a concentration as to cause what is popularly known as Chlorine-lock in the pool.
Chlorine-lock usually occurs when the concentration of cyanuric acid in the pool water reaches levels of 150 mg/l and above. The water itself looks dull and lifeless and perhaps has a greenish tint and yet the DPD No.1 test for free chlorine still shows a good result â€“ the water has become "over-stabilised" and the chlorine is locked in.
Hot weather and extended periods of drought with perhaps water rationing promote elevated levels of cyanuric acid in pools treated with chlorinated isocyanurates.
A level of 30 â€“ 50 mg/l is satisfactory for stabilisation and should the level rise to above 100 mg/l it is advisable to reduce it by dumping some of the water and topping up with fresh.
It may be necessary to shock-dose the pool with free chlorine to kill any algae growth which may have appeared. In this case it is important to use either Sodium or Calcium Hypochlorite not more of the stabilised chlorine (Di-Chlor orTri-Chlor).
A simple turbidimetric method is available for monitoring cyanuric acid levels; alternatively a low cost photometer is available which carries the test along with chlorine and pH.
The turbidimetric test is as follows;
The tablet reagent is added to the water sample in a special double-tube assembly. Any cyanuric acid present will produce a cloudy solution which is measured by moving the inner tube until a black spot printed on its base just disappears from view. The result is then read off from a scale on the side of the outer tube.
Total Dissolved Solids (TDS)
The Total Dissolved Solids content of pool and spa water is a measure of the total quantity of solid material dissolved in it.
In mains water this comprises of hardness and other natural salts and the level will depend on the source of supply, but is within the region of 50 â€“ 500 mg/l
The TDS level will gradually increase in a pool due to evaporation and concentra42
tion of hardness salts, impurities introduced by the natural elements, wind and rain, and the chemicals added to the water as part of the treatment process â€“ chlorides and sulphates for example.
The real value of TDS measurement is that it can indicate whether too many chemicals have been added as a result of heavy bather load or lack of dilution and the water is becoming â€ťstaleâ€ť.
It should be monitored by comparison between the pool and the mains feed water to the pool. TDS should ideally not be allowed to rise more than 1000 mg/l above the feed water, up to a maximum of 3000 mg/l
Should it become necessary to reduce the TDS level, this is carried out by replacing
some of the water in the pool with fresh water. In some pools a satisfactory TDS level can be maintained by regular backwashing of the filters.
Measurement is by electronic meter which is really taking a conductivity reading of the water and applying an internal factor to display the TDS in mg/l.
Balanced Water (Langelier Index)
When a water is in balance, it is said to be neither corrosive nor scale-forming. In other words, it will not deposit a layer of calcium scale neither will it dissolve an existing layer of scale.
For most well run pools, the water will be in balance if the pH value is kept within the recommended range, but other factors should be taken into account which can affect the condition of the water. These are the total alkalinity, the calcium hardness,
the TDS content and lastly, the temperature of the water. The concentration of chlorine or bromine do not appear in the Balanced Water Calculation.
The formula for determining whether he water is balanced was developed by Langelier in the 1930â€™s, hence the result after applying this is often called the Langelier Index or the Langelier Saturation Index or simply the Balanced Water Calculation.
Why is the balance so important? Because if it is not correct, corrosion and erosion
There are 3 main causes of corrosion and erosion;
â€˘ Galvanic attack
â€˘ Aggressive water
â€˘ Low calcium hardness.
Galvanic attack occurs when two or more dissimilar metals are in close proximity in a water environment (pool or spa ) which contains high levels of chemical salts or TDS. The presence of chlorides encourages the water to be more conductive. To prevent this the TDS can be reduced or the level of calcium hardness raised so that a thin layer of scale is laid down to inhibit the metalâ€™s efficiency as an electrode. Lower levels of chlorides will prevent the water from acting as an electrolyte.
Low calcium hardness will often result in the loss of grout around the tiles, as the water tries to satisfy its need for calcium.
It is necessary therefore to maintain the TDS at sensible levels (ideally no more than 1000 mg/l above the feed water ) and yet maintain an adequate level of calcium hardness in the water (around 200 mg/l minimum ).
The formula for calculating the Langelier Index is as follows :
pH + Temperature factor + Alkalinity factor + Calcium Hardness factor - TDS factor
In practical terms an Index Value in the region of zero to + 0.3 is considered satisfactory i.e. a low positive result, which indicates that the water can lay down a thin layer of protective scale.
Example: Langelier calculation:
pH = 7.5 7.5
Temperature = 840 F (29 C0) f 0.7
Total alkalinity = 100 f 2.0
Calcium hardness = 300 f 2.1
TDS = 1100 subtract: f 12.1
Total = + 0.2
A high Total Alkalinity is no compensation for a low Calcium Hardness. Each parameter should bi within its recommended range.
Additional Water Balance Considerations
â€˘ In soft water areas where the constant addition of calcium is necessary to maintain a calcium level above the minimum, it could be advantageous to use calcium hypochlorite as the chlorine donor in order to obtain the calcium in addition to the chlorine from this product.
Also where the natural total alkalinity is low, the use of carbon dioxide gas for pH correction with calcium hypochlorite would be advantageous to produce an increase in the total alkalinity.
â€˘ In hard water areas where it may be difficult to reduce total alkalinity and pH to the recommended range, the use of hydrochloric acid (muriatic acid )may be necessary and it may be appropriate to operate with a total alkalinity of around 140 â€“ 150 mg/l.