DPD or OTO: Which Is Better to Test for Chlorine?  

by Pool Builders on 05-18-2014 in Articles

As an experienced professional who has tested and evaluated water in thousands of swimming pools, I can say without a doubt that DPD tablets are far more accurate than OTO drops for testing chlorine levels.

Both DPD and OTO are colormetric tests - which means that you compare the color against a color comparator block. This can be a source of errors, as some people have difficulty determining subtle variations of color intensity. However, if you have a good eye for color variations, this should not be a problem.

For those who are unfamiliar with these acronyms, I'll give the definitions along with some basic information. DPD stands for diethyl-p-phenylene diamine. DPD is a little more time consuming and complex, but when properly done you'll find this system of tests is the most accurate. OTO stands for orthotolidine. My biggest problem with OTO is that the reagents in the liquid have a shelf life - although no expiration date is ever printed on the bottle. As a result, you will not know if the reagent has gone bad - and that causes problems by giving you inaccurate results. On the other hand, you can usually tell if DPD tablets have gone bad because the color of the tablets will change. DPD tablets are normally yellow - so if you open the package and see that the tablets have lost their yellow coloring, it's probably time to buy some new ones. My personal recommendation for brands of DPD test kits is Jack's Magic - which offers kits for around $100.

There are other methods of chlorine testing on the market that are often heavily advertised. My advice is to stay away from these. One of the most common is an electronic meter with an ORP electrode. This is not only very expensive, it is also extremely unreliable. I advise you to go with DPD tablets for accurate chlorine testing that has been proven to be consistently reliable.

Many customers ask me what is the best pool filter for their needs. Pool filters come in three varieties: cartridge, sand and diatomaceous earth. The fact is that all of these filters do a decent job of filtering the water âEUR" but each has its own advantages and disadvantages.

A cartridge filter will only filter out large particles of dirt. For finer filtration, you need a sand filter âEUR" and for the finest filtration available, you should go with a diatomaceous earth filter.

One disadvantage of a cartridge filter is the hassle of having to physically remove the cartridge and hose down the filter. ManufacturersâEUR(TM) instructions have you soak the filter in trisodium phosphate (TSP). To follow those instructions, you would need to have two filters on hand âEUR" one to operate the pool and the other to be soaking in a garbage can filled with solution of diluted TSP. Very few cartridge filter owners are willing to go through this extra step.

The sand filter is actually the perfect compromise between ease of use and particle size filtration. This filter will trap a particle size smaller than the cartridge filter âEUR" but not quite as small as the diatomaceous earth filter. The main advantage of the sand filter is that there is no need to hose off a cartridge. You simply turn the multiport valve to backwash and flush the system. One very important note âEUR" and a common mistake that most customers make âEUR" is failing to turn off the pump before they begin to rotate the multiport valve. Attempting to change the position on the multiport valve with the pump running will impinge the gasket âEUR" and this will cause the valve to leak.

The diatomaceous earth filter will filter out the finest particles of dirt. The only hassle with using this filter is that at each backwash cycle you need to add extra DE powder via the skimmer.

I should also mention that there is an innovative new product called Zeobrite that improves the filtration and ease of use for sand filters. If you are purchasing a new sand filter, Zeobrite can be added while the filter is being assembled. It is much more difficult to add Zeobrite to an existing filter âEUR" so this is probably something best left to a service technician. However, the advantage of Zeobrite is it will last longer than sand and filter out particles as fine as if not finer than that of a diatomaceous earth filter.

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