Shot glass method doesn't work with DPD?

[polyvue]

Why do folks here say that the shot glass method only works with OTO?

I'd think that by combining equal parts distilled water and pool water and proceeding with the DPD test (doubling the result) one would arrive at a rough estimate of chlorine. Taylor says this about dilution of the DPD chlorine test:




* If color is off-scale: Repeat test using 4.5 mL sample
diluted to 9 mL mark with tap water. Multiply reading by
2 to obtain approximate sanitizer level. If color is still
off-scale: Repeat test using 1.8 mL sample diluted to 9 mL
mark with tap water. Multiply reading by 5 to obtain
approximate sanitizer level.
Source: http://www.taylortechnologies.com/products_instructions.asp?Type=Number&Number=5136

Enlightenment appreciated.

[Poconos]

Dilution works with any chlorine test. The OTO test has an upper limit so maybe that's why it was stated that way. DPD goes a lot higher. I do take exception with the tap water comment. Many municipalities chlorinate their water and that can mess up the results. Should use distilled water.
AL

[waterbear]

Also, most inexpensive test kits are OTO and that is what 'newbies' usually have at their disposal
FWIW, if you are going to spend the money on a DPD test kit like the K-2005 you might as well spend a bit more and get an FAS-DPD kit such as the K-2006.
I have never come across anyone who said you cannot do dilutions with DPD and, in fact, have pointed out myself many times that dilution marks for 2:1 AND 4:1 are molded into the comparator that comes in the K-2005 and that the instructions for doing the dilutions are on the inside of the lid.

[polyvue]

Dilution works with any chlorine test. The OTO test has an upper limit so maybe that's why it was stated that way. DPD goes a lot higher. I do take exception with the tap water comment. Many municipalities chlorinate their water and that can mess up the results. Should use distilled water.
Thanks. I also wondered why Taylor suggests tap water, though the dilutions they suggest would only add a portion of the measurable Free Chlorine, skewing the test upward by perhaps .5 to 1 PPM FC. That's not too much if someone is diluting because chlorine exceeds the test's 5 PPM upper limit, but it does introduce error.


FWIW, if you are going to spend the money on a DPD test kit like the K-2005 you might as well spend a bit more and get an FAS-DPD kit such as the K-2006.
No argument from me! I was thinking of the shot glass / dilution method in the context of a pool owner's ad hoc attempt to measure high chlorine without immediate access to the FAS-DPD test, say, when fighting algae and waiting for their kit to be delivered.



I have never come across anyone who said you cannot do dilutions with DPD [...]
Thanks for the clarification. I just wanted to be sure. :)



Dilution works with any chlorine test.

[Ohm_Boy]

Dilution does work regardless of test. If you think about it, all of our chlorine samples are diluted - the test tells us how much. That's how it works.
Naturally, further diluting the sample increases the scale and reduces the accuracy by the same factor.