Hanna 736 100ppb standard test

[chercm]

i think many of us are having the same problems:furious:

Ok, just for kicks I ran 5 consecutive tests on my tank water. Actually 7, as the first gave a vial inverted error (although it wasn't) the second I messed up, by not holding down the button long enough after adding the reagent.

For the 3rd through 7th, here are the results. According to the documentation it is accurate to 5ppb phosphorous +/- 5% of the reading. Although they claimed +/- 10ppb +/- 5% of the reading and latter lowered the estimate.

This test was more consistent with +/- 10ppb, as the tests ranged from 0 to 20. Of course, this is more of a real world test, not so controlled like the standard test I did before. Therefore I can't rule out testing error, bubble, particulate material etc... And, of course this only takes into account random error, not bias. So, I can't draw any independent conclusions about the accuracy without a better standard. Randy's point is taken, the color standard test is of limited value. I'm still looking into some standard
options.

However, Randy, I take the silence to mean you don't think my alternatives to the seawater standard are a reasonable idea?


Anyways, the results, in ppb P were 0, 11, 1, 20, 12. Converting to ppm phosphate, this would be 0, .0337, .003, .061, .037. Also, my camera died after the 3rd test LOL, so I had to grab my gfs cell phone. I have to say that the new iphone camera is pretty good, but that's a another test

0ppb

11ppb

1ppb

20ppb

12ppb

 

[m2434]

i think many of us are having the same problems:furious:

Oh, I thought the variation was reasonable LOL, although it depends on the bias. If there is not much bias, then it is probably as good or better than most of the kits on the market. Really the 20 reading could have been due to say a tiny food particle that got into the sample or something. It dosn't take much to raise the sample a few ppb. Or a bubble could have thrown off the reading etc... I have no doubt their estimate is based on ideal lab conditions, which our tanks are not.

However, most tests you match a color, which at least my eyes are not good at doing and the colors represent a range, not a point value. So, when you translate that color to a point value, you are going to start seeing the variation. So, that in itself does not bother me.

If you take that out, it was close to +/- .05, at least within the observed range and assuming there is no bias (which is unlikely). If you leave it in, it was still better than the 713 spec if there is no bias. So, the next question is bias, but this is tough to test. From the color sample, I didn't run enough tests to get an accurate estimate, but the average was +3.6. However, the sample is only guaranteed to represent 100ppb +/- 10ppb. And that is at just one point in the range. And you have to take the manufactures word that that is the color you would observe etc...

I think if the bias was +3.6 and std dev 8.3 as observed in the tank water test, then you could expect that 95% of the time your estimate would be off by no more than about .06ppm phosphate. For a hobby test kit, this is actually still pretty good. If you remove the 20 value as an outlier, then the standard deviation was 6.4 and you with a bias of 3.6, you'd expect 95% of the time the estimate would be off by no more than .038 ppm phosphate. Which for a $50 hobby test kit, in real world conditions, would be really good. Also, the variance is random error, so, you can always improve your estimate by taking multiple reading.


So, need to figure out the bias and maybe run a lot more tests to get some better point estimates.

 

[110galreef]

on repeated tests of my tank water i never had such a variation in results. often times they
would come out exactly the same... like 8 ppb and then 8 ppb when running a test twice.

Yeah same for me, I would get no more than 1 ppb variance of test?

 

[Randy Holmes-Farley]

Yeah same for me, I would get no more than 1 ppb variance of test?

Repeated testing here means a new water sample from the tank, new reagents added, and a new measurement?