DPD measurement Error

[cschnurr]

I would like to shock for Mustard Algae.
I have an SWG and my CYA is 70 per the pool calculator I should shock to 34.

I am using the Taylor K-2005 DPD test to check chlorine levels which requires dilution to perform measurements.
My understanding is that my error in measurement is essentially the error of the undiluted sample * the dilution.
Ex If I were to dilute be a factor of 9:1, then my diluted error would be 10x the error of the original measurement.

Is this correct?

If so, it got me to thinking that the Scale of the K-2005 kit is graduated 5,3,2,1.5,1.0,0.5.
I assume that this represents the increment of discernible measurement (test error).
So if I measure 4, it means it is between 3 and 5.

In order to get the lowest error, I can adjust the dilution ratio to fit the scale increments.
Example
If I dilute by a factor of 6:1 and measure 4 (+/-1), my value is 28 +/- 7.
However If I dilute by a factor of 15:1 and measure 1.75 (+/-.25) my value is 28 +/-4.
In all cases it gives me the lowest error when dilute as required to measure a value in the band of 1.5-2.
See attached spread sheet

For practicality reasons, I would only suggest this when measuring high CL levels.

Does this make sense?

[attachment=0:2oubac3v]DPD Test Error.xls[/attachment:2oubac3v]

 

[JasonLion]

You have correctly understood one of the sources of error. There are, however, other sources of error. For example a small measurement error could make a 15 to 1 dilution into a 16 to 1, or even 20 to 1, dilution fairly easily. Dilution measurement errors tend to be larger when doing higher dilutions, which will cancel out some, or all, of the gain in precision you calculated for high dilutions.

I don't generally recommend dilutions of greater than 3 to 1, though they can have some value in specific situations. Usually it is a better idea to get a FAS-DPD chlorine test and use that instead.