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Re: Nitrate test kits
One of the issues I've always struggled with is exactly where do you hold
the sample with respect to the white background on the colour chart.
The main options that make a difference are:
1) Directly against the chart, so the white background is lit by light
passing through the sample (& focussed onto the background, as if the
liquid was a lens);
2) Directly against the chart, so the white background is lit by light
passing through the sample (but not focussed onto the background - tends to
make the background quite dark);
3) Slightly away from the chart so the white background is lit from light
entering at the side & not through the sample.
This usually changes the reading by quite a large amount depending on which
method is used.
Presumably, when the charts are designed, one or other of the above options
is the one that is supposed to be used (I assume it's #3)?
It never tells you on the instruction leaflet - & this is true for most
colour comparison based tests I've used.
Is there anyone on the list who works for a test kit company & has the
definitive answer (& the answer to the below question regarding Nitrate
testers - How long is a minute? Is more than a minute OK?).
I've read the article referred to below - it doesn't answer these
BTW, I've tried Interpet, API, Cyprio (for ponds but quite a good design &
works OK for aquaria) & Tetra Nitrate kits over the last few years & the
API kit seems to be the most clear & consistant from my point of view
(subject to the above problem). It discriminates well between "No Nitrate"
(pale yellow), "Some Nitrate" (brown) & "Too much Nitrate" (Red) & is the
one I always go back to; although they're all OK if you don't take it too
(YMMV - Please don't sue me).
Date: Fri, 24 Jan 2003 10:59:09 +0000
From: "Roy Wadey" <roy_wadey at hotmail_com>
Subject: Re: Nitrate test kits
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
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
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
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.