FAS-DPD Test Question

[s1njin]

I've reviewed this page:

http://www.troublefreepool.com/content/215-FAS-DPD-Chlorine-Test

I have two questions.

Is it necessary to swirl the entire time if you know your chlorine level is high? For instance a SHOCK chlorine level of 30 requires 60 drops. When undergoing my FAS-DPD test in the morning (performing a OCLT) I will usually just dump 50 drops in the tube, swirl it, and than go one at a time. Am I really affecting the results this way? I can see the need to swirl the entire time if your chlorine is low and you can clear your solution in 4-6 drops easily.

Also, can somebody clarify this line under notes?

• The precision of the measurement is plus or minus one drop when up to 10 drops of titrant are used, or plus or minus 10% of the final reading, when more than 10 drops of titrant are used.

If I'm putting 60 drops in, I could be off by up to 6 drops? If I'm reading this right, how can you do a OCLT w/ high level of chlorine if you have a 10% margin of error? 10% of 30ppm is 3ppm, which is 3 times the level of the 1ppm or less chlorine drop which signifies a successful OCLT.

????

Thanks much.

 

[duraleigh]

Here is my take and it may not jive completely with the official position. If you know you are going to use many drops, you can certainly "gang drop" in 10's or 20's until you get close to your target. The problem with that is there is a tendency to force the drops into the cylinder instead of letting them fall.....be careful.

Secondly, I have tested FC up to 74 ppm and saw no discernible difference in the rate of color change. It was very crisp and clear.

Naturally, you cannot be as precise as you can at 3-4 drops but it doesn't matter. You are looking for repeatability. Will the color change at 64 drops every time? The answer from my experience is yes. You may not have exactly 32 ppm FC but your repeatability is quite accurate.

 

[mikemass]

You can do a lot of drops at once if you know you have a very high FC reading. But when you get to the point that you are going to slow them down, make sure you swirl real good. (You sound like a good candidate for a speed stirrer)

As far as that quote. I don't think you need clarification at all. You read on it is totally accurate. To lessen the degree of inaccuracy do your OCLT at around 10ppm. It is because of these margin of errors on these these that I opt to always stay on the upper end of my FC range during maintenance operation.

 

[s1njin]

Okay, I will drop in batches of 10. That seems reasonable to me. I've been dropping at the rate of roughly 1/sec. 1, 2, 3, 4, 5, you get the idea. I have the bottle inverted the whole time. Is that forcing the drops?

As to the repeatability, thank you. That makes perfect sense. The reading might be 10% off but the reaction from pink (or in my case red) to clear should be repeatable.