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SPN OCT 2010 Pool&SpaIndustry


www.swimmingpoolnews.co.uk YOUR QUESTIONS ANSWERED


In every issue, we provide the answers to questions, issues and problems raised by SPATA and BISHTA members. Here Peter Lang, SPATA Technical Advisor and Chairman of the SPATA Technical Committee, Bob Judd, Member of the SPATA Technical Committee and Howard Gosling, Chairman of the BISHTA Technical Committee answer your questions


allows the desired pH set point to be maintained) for the carbon dioxide to ensure the gas is dissolved prior to entering the pool.


What are all the steps to repainting a gunite pool which has started to chip? Answer from Peter Lang and Howard Gosling


We have been using Ccarbon


dioxide (CO2) successfully for several years, but for no reason our consumption has increased and we are unable to maintain the desired pH level within our very busy indoor school pool Answer from Bob Judd at Aquatrol


Carbon dioxide, when added to water, forms carbonic acid, a mild but still very effective acid. In addition carbon dioxide in solution forms natural bicarbonate and how much goes on to form bicarbonate depends on both the pH and the total alkalinity of the water. Because carbon dioxide elevates the alkalinity and depresses the pH at the same time, and does so in relative amounts dependant on pH and alkalinity, the chemistry can be very confusing. Carbon dioxide remains in a carbonic acid state in greater proportion or greater percentage, the lower the pH. The


higher the pH of the pool water the more will go into the bicarbonate form.


However it is not just pH accounts for this shift, but also the concept of saturation. How much dissolved carbon dioxide is in the water, what is the bicarbonate level and so forth, so the ideal range for using carbon dioxide as a pH depressant will be in pools operating at low pH and of course low total alkalinities.


It sounds if you may be caught in the position of pH level increasing thus calling for more carbon dioxide which in turn is increasing the alkalinity thus reducing the effectiveness of the carbon dioxide as a pH depressant. Check the alkalinity level if higher than 160- 170mg/l reduce via dilution or by the addition of a stronger acid such as hydrochloric acid to around 125mg/l. It should be noted that the PWTAG book ‘Swimming Pool Water’ from the Pool Water


Treatment Advisory Group) does not recommend the use of carbon dioxide where the total alkalinity of the mains incoming water is above 150mg/l.


To obtain effective pH control with carbon dioxide, select a pH set of 7.2 to 7.4 the closer to 75mg/l alkalinity the better and use the lowest feed output setting (which


I assume the question is in regard to a marblite or plaster finish on the gunite. Repainting is a means of renovation and the paint can be water-based or solvent based (“rubber based”). The latter must not be put in a damp environment. The main drawback of repainting is that it needs to be re-done at regular intervals. A more durable finish is to tile the pool or line it with fibreglass. In all cases the pool would have to be emptied and this must be carried out by a professional pool company who employ divers who can check that the pool is protected by a hydro- static relief valve before the work is started, the internal surface thoroughly cleaned and made smooth if necessary. Any crack


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