When and
How to Test Your Spa Hot Tubs Water
To ensure that pool water is safe for bathers to swim in and
to allow the creation of the best environment to protect plant and equipment several tests
need to be carried out. These tests are:
- Free and total chlorine
- pH
- Alkalinity
- Calcium Hardness
- Total Dissolved Solids
NOTES ON WATER
SAMPLING
These general notes apply to all water tests:
- Wash and dry your hands before commencing water tests as any
chemicals on your hands could radically affect your results.
- Always take the sample from the same place; in a conventional
pool two thirds along the length towards the deep end or at a point determined by the
Baths Manager.
- Take the sample from approximately 300mm (12") below the
surface.
- Sufficient water should be taken to carry out all tests at one
time. The sample should be taken in a polythene bottle. Do not take glass bottles on to
the pool side.
- Rinse the bottle at least three times before taking the
sample.
- On completion of the tests empty and rinse bottle and test
tubes in fresh tap water..
- Chlorine and pH tests are best taken using non fading colour
standards and light cabinet which provides a uniform source of white light. Taking
readings under fluorescent light can distort the results. If a light cabinet is not
available do not hold the comparator up to an electric light or the sun; use a daylight
source, preferably north facing.
- Ensure the person taking the water tests is not colour blind.
FREE AND TOTAL
CHLORINE
Chlorine exists in the pool in two forms.
- Free Clorine is also known as hypochlorous acid. This is a
powerful disinfectant and will kill bacteria in seconds. As its name suggests, it is
chemically free from any other compounds.
- Combined Chlorine also known as chloramines. This is chlorine
that has already oxidised some pollution. It has combined with some organic compounds. It
is a very weak unsatisfactory sanitiser up to 100 times weaker than free chlorine.
Combined Chlorine exists in three forms:
- Monochloramine
- Dichloramine
- Trichloramine or Nitrogen Trichloride. This has an extremely
pungent odour and is a severe eye irritant.
Breakpoint Chlorination
This is the system employed in most pools in the UK. When
chlorine is added to a polluted pool, it combines with pollution and oxidation commences.
When and only when all the pollution has been oxidised by the continual addition of
further chlorine with chlorine existing only in the combined form, a breakpoinrt occurs
and the residual falls deespite the addition of further chlorine. At this point nearly all
the pollution has been oxodised and free chlorine can exist alongside combined chlorine.
To ensure that the pool operates on breakpoint, tests for free and total chlorine must be
taken. By subtracting the free residual from the total residual the combined residual in
all forms can be determined.
There is no single figure for free chlorine which ensures
breakpoint chlorination but a recommendation of a free chlorine residual twice that of the
combined chlorine residual should be aimed for.
|
Breakpoint |
Not Breakpoint |
| Free Chlorine |
1.5 |
1.5 |
| Combined Chlorine |
0.8 |
2.0 |
| Total Chlorine |
2.0 |
3.5 |
How to test for Chlorine
Read the notes on Water
Sampling.
- Make sure all equipment is clean.
- Fit correct disc to comparator.
- Use 2 x 10ml calibrated test tubes.
- Rinse test tubes in sample water three times.
- Fill the first test tube to the 10ml mark.
- Put a small quantity of water into the second test tube.
- Place a DPD No. 1 tablet into the second test tube. It is
essential that you do not touch the tablet with your fingers. Crush the tablet.
- Fill the second test tube up to the 10ml mark and mix. Use the
plastic cap and not your finger to hold the sample in if you invert the tube. Place the
test tube into the right hand side of the comparator and rotate the disc to obtain a
colour match. Take the reading as quickly as possible; delay could cause innaccuracy. Note
the reading.
- This is the Free Chlorine Residual in milligrammes
per litre/parts per million.
- Place a DPD No. 3 tablet into the same test tube. Dissolve the
tablet and allow at least two minutes for the full colour to develop. Taking the reading
too quickly will lead to innaccurate results.
- Place the tube back into the comparator and rotate the disc to
obtain a colour match. Note the reading.
- This is the Total Chlorine Residual in milligrammes
per litre/parts per million.
- Subtract the reading obtained in 9 from that obtained in 12.
The result is the Combined Chlorine Residual.
pH
pH is a logarithmic scale indicating whether the pool water
is acid or alkaline (basic). The scale runs from 0 (acid) to 14 (alkaline). Being
logarithmic, each unit higher is 10 times more alkaline and each unit lower is 10 times
more acid. Therefore, for example, pH8 is 100 times more alkaline than pH6.
It is obvious that a very small movement in pool water pH can
have a tremendous effect on the water condition.
Most of the chemicals added to pool water have an effect on
the pH. ie. sodium or calcium hypochlorite, alum, soda ash etc. It is however essential
that the pH is controlled in the range 7.2 - 7.6 but ideally between 7.3 - 7.5. There are
several reasons for this:
- The most important is the effect that pH has on chlorine
efficiency. The rate at which hypochlorous acid (free chlorine) breaks down into
hypochlorite ion and hydrogen is governed by the pH. Within acceptable limits the lower
the pH the more hypochlorous acid is retained.
- For Bather comfort it is important that the pool pH is close
to the natural pH of body fluids eg eyes, ears nose etc which is about 7.3.
- pH affects the floc formation of alum. The higher the pH the
less efficient the floc formation.
To raise the ph Level add Soda Ash
To lower the pH level add Acid eg. hydrochloric
How to test for pH
Read the notes on Water
Sampling.
- Make sure all equipment is clean.
- Fit correct disc to comparator.
- Use 2 x 10ml calibrated test tubes.
- Rinse test tubes in sample water three times.
- Fill the first test tube to the 10ml mark and place in the
left side of the comparator.
- Fill the other test tube to the 10ml mark and add one Phenol
Red test tablet.
- Place cap on test tube and shake to aid dissolving.. Do not
use your finger.
- Place the test tube into the right hand side of the comparator
and rotate the disc to obtain a colour match.
- This value is the pH.
ALKALINITY
There are several forms of Alkalinity but the only one which
is of interest is bicarbonate alkalinity. This exists between pH5 and pH9.
Alkalinity acts as a buffer in the water - a safety net. If
there was no alkalinity in the water, the chemicals added to the pool would have a direct
effect on the pH. As has been shown it is very important to maintain a stable pH. By
having a reasonable level of alkalinity in the water this absorbs the effects of other
chemicals leaving the pH stable.
The ideal alkalinity range depends on the sanitiser being
used:
| Calcium hypochlorite |
80 - 120 mg/l |
| Tri-Tabs |
80 - 120 mg/l |
| Aquabrome |
80 - 160 mg/l |
| Sodium hypochlorite |
120 - 150 mg/l |
| Chlorine gas |
180 - 200 mg/l |
If the alkalinity level is too low we get what is termed 'pH
bounce'. The chemicals added to the pool affect the pH making it very difficult to
stabilise.
If the alkalinity is too high we get what is termed 'pH
lock', in which case it is very difficult to alter the pH as the effects of the pH
correctants are absorbed by the alkalinity.
To raise alkalinity add sodium carbonate (soda ash).
To lower alkalinity dump acid. This procedure is
hazardous. Mix up a solution of one part hydrochloric acid and six parts of water
in a plastic bucket. Dump the solution in one spot in the deep end of the pool. This
must be done under strict supervision and only when the pool is empty of bathers. Suitable
protective clothing must be worn as hydrochloric acid MUST be handled with great care.
How to test for Alkalinity
There are two types of alkalinity test tablets. Alkalinity M
and Total Alkalinity. Check carefully which type you are using. If using Total Alkalinity
test tablets proceed as follows:
Read the notes on Water
Sampling.
- Rinse stoppered bottle three times in sample water.
- Fill bottle to 50ml mark.
- Remove one alkalinity test tablet from its container or foil
wrapper taking care not to touch it with your fingers. Place in bottle and shake until it
has dissolved.
- Check the colour. Has it turned bright pink and remained pink?
If not repeat 3 until it has.
- Multiply the number of tablets used by 40 and subtract 20.
- The result is the Alkalinity.
CALCIUM HARDNESS
To aid corrosion inhibition it is now generally accepted that
it is beneficial to have a high calcium hardness level.
When low calcium hardness levels are combined with high TDS
levels corrosion usually takes place.
Calcium is a natural mineral found in water; water will
either seek calcium or deposit it. When water is seeking calcium it will take it from
anywhere that is available. In a swimming pool there is a plentiful supply contained
within the grouting around the tiles.
If left unchecked the calcium hungry water will attack the
screed beneath the tiles.
There is a school of thought which says that very high
calcium hardness levels cause problems. Experience has shown that, provided the pH and
alkalinity levels are kept within acceptable limits, the calcium hardness level can be
allowed to rise. There are many pools in the UK with calcium hardness levels of 1000mg/l
or higher.
How to test for Calcium Hardness.
Read the notes on Water
Sampling.
- Rinse stoppered bottle three times in sample water.
- Fill bottle to 50ml mark.
- Remove one calcium hardness test tablet from its container or
foil wrapper taking care not to touch it with your fingers. Place in bottle and shake
until it has dissolved.
- Check the colour. Has it turned violet and remained violet? If
not repeat 3 until it has.
- Multiply the number of tablets used by 40 and subtract 20.
- The result is the Calcium Hardness level in mg/l.
TOTAL DISSOLVED
SOLIDS
Total dissolved solids or TDS as it is more frequently
referred to is the residue left in the water from the chemicals we add to our pools. Each
chemical added to the pool is added to do a specific job eg. sodium hypochlorite as a
sanitiser. As well as disinfecting the pool the sodium hypochlorite adds significantly to
the TDS level by deposirting considerable quantities of chlorides in the water. Each
chemical added has an effect on TDS to a larger or lesser degree.
There are two main contributores to the TDS level in most
pools. These are chlorides and sulphates. The usual source of chlorides is sodium
hypochlorite and sulphates is dry acid. Chlorides will make the water conductive and will
promote corrosion via galvanic attack.
Sulphates on the other hand will attack the grout and, in
severe cases, the tiling glaze, This could prove very costly if the pool has to be drained
to effect repairs.
TDS can be controlled in two ways. If your pool already has a
high TDS (over 2000mg/l) then the only option is to introduce large quantities of fresh
water. This can be achieved either by backwashing more frequently or partially draining
the pool and refilling with fresh water. With water and energy costs as high as they are,
this can prove an expensive exercise indeed.
The other alternative is to look carefully at the chemicals
currently in use and choose those which will have mininmal effect on TDS.
How to test for Dissolved Solids.
The usual method for testing for TDS is to use an electronic
meter. There are several types available. Follow the manufacturers instructions.
Some meters read conductivity in microsiemens or millivolts.
To convert the reading to TDS multiply by 0.7 for a sample at 25°C.
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