diluting reagents to improve test accuracy

[e2015]

Our pool is irregularly shaped with a sunshelf and several swim-outs which makes accurately calculating the total pool volume difficult. I am gearing up to try the chemical method outlined at http://www.troublefreepool.com/threads/64047 that requires measuring TA to within 1-2 ppm.

The standard approach to increasing test accuracy is to increase the amount of pool water being tested. This results in each drop of R-0009 corresponding to 1 ppm instead of 10 ppm. Given that the volume calculation method requires testing TA twice, I was thinking of a way to avoid using that much R-0009 (~150 drops).

What if I used the standard volume of pool water but instead diluted the R-0009 prior to dropping it into the tube? In other words:

• start with 25 ml of pool water (per the standard instructions)
• add the standard 2 drops of R-0007 and 5 drops of R-0008
• dilute R-0009 1:10 by adding 10 drops (0.5 ml) into 4.5 ml of water (mix well)
• add the diluted R-0009 until the color change with each drop corresponding to 1 ppm

This could be made even faster by adding undiluted R-0009 until you are within 10 ppm of the result (i.e. if you know the TA is between 60-70, you could add 6 undiluted drops then switch to the diluted drops).

I can think of a few issues and would love to hear feedback from the experts here:

1. What to dilute the R-0009 in? I imagine that tap water would be fine since you are adding <5 ml of tap water into 25 ml of pool water. The alternative would be something like double-distilled water (ddH2O). Or is there a known diluant to use to further dilute R-0009?
2. What to use to drop the distilled reagent into the tube? Would a standard dropper work?
3. Is there too much error introduced by diluting R-0009 1:10? If so, the method could be modified with a less aggressive dilution.

Looking forward to hearing your thoughts.

 

[jblizzle]

IMO, Sounds overly complicated for something that is not very important. There is no reason to need to know the volume to high accuracy, no chemical maintenance requires it. And the chemicals themselves can vary in strength or volume which also adds uncertainty.

Just take a guess at the volume and maintain your pool for awhile. If you are consistently missing your target, adjust the volume in PoolMath.

 

[BuckeyeChris]

yep, when we filled the pool I recorded the house's water meter during the process, and came to 10,600 or so. But after a few weeks it was easier to change the poolmath to 11000. Add in water in the filter, pipes, etc and adding a few hundred to the calculator gave more reliable results.

A few days of test/dose/1hr/test, and if you are always above or below expectations, just add 500 gallons to poolmath. doing the TA test a few extra times will still use fewer drops that that precision test.

oh, geez, ~31k gallon pool? maybe add 1000 gallons.

 

[JoyfulNoise]

I would discourage this exercise. You can easily use Pool Math and chlorine additions to quickly zero in on a rough pool volume.

You are adding too many variables and your accuracy will be compromised trying to dilute the R-0009. If you must do this, then you should always use distilled water for dilutions. Tap water and bottled water will have alkalinity in them that will react with the R-0009 and reduce it's ability to neutralized the alkalinity in your test sample. Even store bought distilled water has some small amount of alkalinity in it (I've measured as much as 10 to 15ppm) and so dilution will definitely add some error. The Taylor dropper tip standard is 25 drops/mL for all of it's titrating bottles. You will need to match that droplet volume with reasonable accuracy to ensure that you are dispensing the correct amount of diluted R-0009.

 

[AUSpool]

I wouldn't dilute the reagent for the above reasons but if you do I would do it by weight into an original Taylor dropper bottle. But as mentioned you would need some lab grade distilled water. The dropper bottle would need to be thoroughly rinsed with the distilled water and dried before taring and diluting by weight.

I have have and odd shaped pool but with mostly straight edges and right angles it was easy to break it down to smaller pieces. I got it to the nearest 100 litres then rounded to the nearest 1000 litres for use in PoolMath. I have traditionally run with a lowish CH and with a bit of dilution from heavy rain needed to add almost 4kg of Calcium chloride. I used PoolMath to calculate my volume from my CH reading after the addition and both values were identical.