Hanna Phosphorus reading variability.

[KGV]

Hi all, when I measure phosphate with this checker (ULR HI736), I fill two vials and follow their procedure with only one of them. Once done measuring, I re-measure using the unused one as a reference. What I noticed is that when you measure 4-5 times in a row the results can swing quite a bit; for example 15 - 20 - 25 - 18, etc. Any idea what the source of this variability is? @taricha @Rick Mathew ?

 

[Dan_P]

Hi all, when I measure phosphate with this checker (ULR HI736), I fill two vials and follow their procedure with only one of them. Once done measuring, I re-measure using the unused one as a reference. What I noticed is that when you measure 4-5 times in a row the results can swing quite a bit; for example 15 - 20 - 25 - 18, etc. Any idea what the source of this variability is? @taricha @Rick Mathew ?

You are looking at normal variation, possibly within the electronics of the Checker. Then there is light scattering differences due imperfections in the glass if the reference and sample vials are not placed in the Checker exactly the same position every time and even then, there is likely to be a small amount of variation.

 

[mmorrison55]

I don’t typically test like you did, but as a coincidence, I did yesterday because after I took my 1st reading I noticed my lid on my checker was not fully closed. So I filled a second vial with sample water to zero it as usual it step c1, then put the test vial in, making sure I wiped the vial and oriented in the same way. (I like to use the white lettering on the vial as a reference). Mine went from 10 to 11. I chalked that up as the sample sat longer than the original timed test, but idk how long after the recommended waiting for results the test should still be accurate.

So long story short. Mine didn’t fluctuate quite as much as yours. But if you are wiping your vial and placing it in the same orientation each time, then you may want to reach out to hannah to see if your findings are acceptable. If it were the reagent age affecting, then I would expect the numbers to climb with each test not get lower on the last.

 

[KGV]

I don’t typically test like you did, but as a coincidence, I did yesterday because after I took my 1st reading I noticed my lid on my checker was not fully closed. So I filled a second vial with sample water to zero it as usual it step c1, then put the test vial in, making sure I wiped the vial and oriented in the same way. (I like to use the white lettering on the vial as a reference). Mine went from 10 to 11. I chalked that up as the sample sat longer than the original timed test, but idk how long after the recommended waiting for results the test should still be accurate.

So long story short. Mine didn’t fluctuate quite as much as yours. But if you are wiping your vial and placing it in the same orientation each time, then you may want to reach out to hannah to see if your findings are acceptable. If it were the reagent age affecting, then I would expect the numbers to climb with each test not get lower on the last.

Thanks for mentioning that. You only tested twice in a row?

 

[KGV]

Thanks for mentioning that. You only tested twice in a row?

 

[KGV]

You are looking at normal variation, possibly within the electronics of the Checker. Then there is light scattering differences due imperfections in the glass if the reference and sample vials are not placed in the Checker exactly the same position every time and even then, there is likely to be a small amount of variation.

I thought so too but I have tested glass imperfections by measuring using two unreacted sample vials. The outcome is always zero. Even when I randomly rotate each vial.

 

[rishma]

I think the accuracy is +/-5. I have noticed that swapping vials gives me a bit more variation than retesting back to back with new reagent. I cannot recall more than +/- 5 when doing tests back to back. And I’ve do so many Phosphorus ULR tests I think Hanna should send me a Christmas card.

Why does swapping vials matter? I’d guess with glass inconsistency. I am not sure testing without reagent and getting zero eliminates that possibility.

 

[Troylee]

I know Hannah doesn’t mention it but I had that problem when I held my checkers to get results! If you put the vials in and hold the button for counter to start and don’t touch them again it will read more clearly and accurate with back to back tests.

 

[rishma]

I know Hannah doesn’t mention it but I had that problem when I held my checkers to get results! If you put the vials in and hold the button for counter to start and don’t touch them again it will read more clearly and accurate with back to back tests.

I don’t hold mine…because I get distracted and do something else. What does holding it do?

 

[Troylee]

I don’t hold mine…because I get distracted and do something else. What does holding it do?

No idea but my thinking is.. sometimes the powder isn’t fully dissolved and as you move it around it gives false readings from the cloudy water.. I was getting crazy readings with the same test vials back to back.. once somebody told me to stop holding them it was then I was only .01 or .02 difference doing the test back to back.

 

[KGV]

No idea but my thinking is.. sometimes the powder isn’t fully dissolved and as you move it around it gives false readings from the cloudy water.. I was getting crazy readings with the same test vials back to back.. once somebody told me to stop holding them it was then I was only .01 or .02 difference doing the test back to back.

My guess is that the absorbance of the indicator is so low that even a tiny bit of sediment that can settle, or microbubbles on the glass can influence that reading as well. I haven't experience this variability at all with for example the Hanna Ca2+ or KH checker.

 

[Tangdora]

The variance of the checkers it 5ppb plus’ an additional 5% of the reading. So at bare minimum. Your reading would fall into a 10ppb range. Your multiple test all fall into this Range which probably leans to your actual reading being right around 20

 

[mmorrison55]

Thanks for mentioning that. You only tested twice in a row?

Yes. And I typically only test once for mostly all Hannah test except the Hannah calcium which can be fickle to test, so I like to test that one twice to get an average.

But this time for phosphate i tested twice since I noticed my cap was not fully closed

 

[Dan_P]

I thought so too but I have tested glass imperfections by measuring using two unreacted sample vials. The outcome is always zero. Even when I randomly rotate each vial.

That’s good!

That leaves the electronics bouncing around. I don’t have a sense of how much the Checkers vary in variability.

 

[RelaxingWithTheReef]

Years ago I was frustrated with this test, and chased the poor repeatability. I came up with the following contributors:

-The reagent did not fully dissolve in solution, and there was always a small amount of solids suspended in the solution.

-Any film or scratches on the cuvette influenced the reading.

-You can test for these contributors by illuminating the bottom of your cuvette with a high intensity LED flashlight after shaking and waiting the specified time, I think 3 minutes. My cuvette would light up with all sorts of stuff. I was surprised the test worked as well as it did.

-The dye produced by the reaction is pretty strong, and can stain the cuvette. I think cleaning the cuvette can improve performance, but I’m not sure how to clean it properly without potentially scratching it.

-If you are using tap water to wash the cuvette, cap, and what you use to sample the tank, keep in mind the wash water may have phosphate in it. My tap water has 163 ppb P (0.500 ppm PO4). Even your DI water may have some residual phosphate. You can try distilled water, and rinsing everything with your seawater a couple times prior to the test.

Fundamentally, the design of the Hanna test is challenging. It’s designed for a pretty wide range of 0-200 ppb (0.613ppm PO4). In doing so the reagent is not the most sensitive. At lower levels of phosphate, there is simply not much color change. This means the signal to noise ratio of the overall photo detector system is somewhat poor, and it makes the device pretty sensitive to all these “noise contributors.”

Keeping all this in mind, you may be able to improve your repeatability. But also keep in mind that Hanna publishes an Accuracy of +- 5ppb "added to" +- 5% of the reading. While some reefers claim better performance, Hanna definitely means it.

 

[KGV]

Years ago I was frustrated with this test, and chased the poor repeatability. I came up with the following contributors:

-The reagent did not fully dissolve in solution, and there was always a small amount of solids suspended in the solution.

-Any film or scratches on the cuvette influenced the reading.

-You can test for these contributors by illuminating the bottom of your cuvette with a high intensity LED flashlight after shaking and waiting the specified time, I think 3 minutes. My cuvette would light up with all sorts of stuff. I was surprised the test worked as well as it did.

-The dye produced by the reaction is pretty strong, and can stain the cuvette. I think cleaning the cuvette can improve performance, but I’m not sure how to clean it properly without potentially scratching it.

-If you are using tap water to wash the cuvette, cap, and what you use to sample the tank, keep in mind the wash water may have phosphate in it. My tap water has 163 ppb P (0.500 ppm PO4). Even your DI water may have some residual phosphate. You can try distilled water, and rinsing everything with your seawater a couple times prior to the test.

Fundamentally, the design of the Hanna test is challenging. It’s designed for a pretty wide range of 0-200 ppb (0.613ppm PO4). In doing so the reagent is not the most sensitive. At lower levels of phosphate, there is simply not much color change. This means the signal to noise ratio of the overall photo detector system is somewhat poor, and it makes the device pretty sensitive to all these “noise contributors.”

Keeping all this in mind, you may be able to improve your repeatability. But also keep in mind that Hanna publishes an Accuracy of +- 5ppb "added to" +- 5% of the reading. While some reefers claim better performance, Hanna definitely means it.

Thanks!