Salifert CA test kit discrepancy

laverda

Well-known member
I get a discrepancy in calcium levels when using the Ca test kit and strontium test kits from Salifert of about 25. While I realize it is not a big discrepancy, I would expect then to be the same. I am guessing the lower reading of the strontium kit is more accurate as the test sample is 2.5 times the size. I have had several of each kit with the same results. Does anyone else get the same results?
 
I don’t. The first step of the strontium test is a Ca test. Then you add some powder and more drops to get the results for strontium.
 
I get a discrepancy in calcium levels when using the Ca test kit and strontium test kits from Salifert of about 25. While I realize it is not a big discrepancy, I would expect then to be the same. I am guessing the lower reading of the strontium kit is more accurate as the test sample is 2.5 times the size. I have had several of each kit with the same results. Does anyone else get the same results?

Your intuition is correct. A 25 ppm difference is not large and probably not statistically significant. If you would run the calcium test at least three times for each kit on the same sample of water, you would very likely see variations in all measurements. Those variations which determine the standard deviation would indicate whether a 25 ppm difference was real or just noise.
 
I had similar limitations with those two test kits in the past. Maybe if you're very careful about giving plenty of time for each drop of titrant you might be able to be more accurate, but I wouldn't count on it.
 
Thanks for the input. I don't think it is the way or condistance as I have run both test multiple times when I first noticed the discrepancy. The discrepancy has been pretty consistent over multipul tests and kits.
 
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Hmm, that's odd. I had somewhat inconsistent results, with the levels matching very closely or being up to 30 ppm off, or something in that range. It's been a very long time.
 
Mine a are 20-25 diferent so not a lot. It just seems like two test kits from.the same company should be more consistent. I am probably expecting too much from hobby test kits.
 
I think that the calcium kit is good to ±8 ppm or so in theory. The strontium kit probably is more touchy, since it's measuring calcium while attempting to ignore strontium. I think that 20 ppm is a bit more than I'd like, but I am picky, and for hobbyist purposes, it's close enough.
 
I think that the calcium kit is good to ±8 ppm or so in theory. The strontium kit probably is more touchy, since it's measuring calcium while attempting to ignore strontium. I think that 20 ppm is a bit more than I'd like, but I am picky, and for hobbyist purposes, it's close enough.

Just to be a pain...and possibly not quite right...if one standard deviation is 8 ppm, than you would be 95% sure that the test result would have a range of ± 16 ppm, making a 20 ppm difference on the verge of being interesting.

Test kit vendors, except Hach and Hanna, are mysteriously quiet about the inherent variability of their kits.

By the way, where did you get the "œÂ±"
 
You'd need to take into account the variation of both tests, depending on the number of data points available, but I agree that it's a bit large. I don't know the stated limit of the strontium kit, though.

Shift-Alt/option-= produces ± on my Mac.
 
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