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Re: Nitrate test kits

Eric wrote,

Regarding the comments about the discrepancy between the measurements:
About a year ago I spoke with a Hach field rep and he said that the
particular kit I had was subject to error induced by the characteristics 
ofthe tester. That is to say, the test results were variable to a 
significantdegree by interpretation of the instructions by the tester. 
Theinstructions say add the reagent, cap the test tube, and shake 
vigorouslyfor exactly one minute. Well, how vigorous is vigorous and from 
what pointin the process do you start the "exactly one minute" count as it 
takes afew seconds to empty the reagent packet into the test tube and cap 
the tubebefore you can shake it. I never got a verification from him about 
theeffects of my interpretation and testing procedure because I didn't 
havethe test kit with me when I saw him and I wasn't about to pay for a 
call.  I have not read the article in the magazine_PracticalFishkeeping_ 
that Roy alludes to but this may be the reason for the discrepancy 
mentioned; I'll read it if I can find it. Roy, can you elaborate on the 

This was exactly the gist of what they were saying. Different testers 
interpreted the results to give different readings. It appears it was not so 
much the way that they carried out the tests, but the comparison of the 
result with the colour charts. Results were better if the actual 
concentration was close to one of values shown on the colour chart but if it 
fell in-between then there was much more variation. Nontheless, in many 
cases all, or the majority, of the testers interpreted the results to mean 
much higher or lower levels than it actually was. This was not just a matter 
of being a feww ppm off but several fold off. In our situation people using 
these kits could well be adding nitrate to tanks that already exceeded the 
recommended range, or, conversely, not adding nitrate in the belief that 
there is sufficient present in their water.
How you interpret this is up to you. As an individual testing in a 
consistent way you can probably at least detect trends in the nitrate 
levels. However, if you want to be sure of the actual levels you will need 
kore acurate tests than the average Hobbyist kit. On the other hand it may 
be an argument for relying on observation more than testing.

Regards, Roy.

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