HI 736 ULR - Test / Same-Sample Retest Result Variations

April 30-
I have the model HI 713. Not sure if it makes a difference though.

In PPM-

Tank 1-

9:30pm- .06 .03 .00

Tank2- The small high nutrient tank-

9:30 pm- .03 .00

And I am out of regent.....

Looks like the variation on my first night of test results is still on from the precision rating of

Interesting Chris - the 713 measures in ppM, so in comparison, your variance is much large than I'm experiencing with the 736 measuring in ppB.

I provided one more days test data on the outside chance that Hanna's failure to reply thus far is result of their chemistry department taking the time to duplicate these tests to formulate a response.

5/02/12 7:00AM Initial - 12 ppb Immediate follow up tests - 3, 0, 0, 0, 0
4/30/12 7:00AM Initial - 25 ppb. Immediate follow up tests - 3, 12, 10, 4
4/27/12 7:00AM Initial - 17 ppb. Immediate follow-up tests - 4, 2, 7, 0, 6
4/26/12 7:00AM Initial - 16 ppb. Immediate follow up tests - 0, 4, 6, 5, 4
4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3
4/24/12 7:00AM Initial - 29 ppb. Immediate follow up tests - 11, 9, 8, 12
4/23/12 7:00AM Initial - 43 ppb. Immediate follow up tests - 5, 8, 9, 13, 8

The results are so consistent and repeatable, that It's hard to come up with any explanation. I have posed 2 questions to Hanna,

1) Which set of values is to be believed?

2) Why?

This gets even stranger...

Another HI 736 user indicated he was able to get far more consistent results by allowing the C2 sample to sit for 7 minutes prior to testing. While I still don't understand the mechanism, in addition to my regular testing, I followed up with a timed test conducted after allowing the cuvette to sit still for 7 minutes. The results were...

5/03/12 7:25AM 7 min - 04 ppb. Immediate follow up tests - 3, 1, 2, 6, 1
5/03/12 7:00AM Initial - 12 ppb. Immediate Follow up tests - 1, 0, 1, 3, 1
5/02/12 7:00AM Initial - 12 ppb. Immediate follow up tests - 3, 0, 0, 0, 0 (fresh GFO)
4/30/12 7:00AM Initial - 25 ppb. Immediate follow up tests - 3, 12, 10, 4
4/27/12 7:00AM Initial - 17 ppb. Immediate follow-up tests - 4, 2, 7, 0, 6
4/26/12 7:00AM Initial - 16 ppb. Immediate follow up tests - 0, 4, 6, 5, 4
4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3
4/24/12 7:00AM Initial - 29 ppb. Immediate follow up tests - 11, 9, 8, 12
4/23/12 7:00AM Initial - 43 ppb. Immediate follow up tests - 5, 8, 9, 13, 8

So my 7 minute results were much like those reported, although I'm still puzzled as to why. My C2 sample was about 25 minutes old when I conducted the 7 minute test - I wonder if the reagent in solution might be breaking down and altering the results? But even that does not explain the variance between the initial and follow up tests. Today's tests were also made using a new box of reagents (lot HO13). Assuming the reagent is completely dissolved (which I believe it is) the results are hard to explain.

I would love to trust the follow up and 7 minute values as they provide more favorable results - It sure would be nice if Hanna provided some type of input.

Although I cannot see any micro-bubbles present, even when using a 10X jewelers loupe, at this point it is my best guess as to the cause of the issues as it's the only thing that could seemingly explain the results obtained.

Today I altered the process. Instead of inserting C2 into the checker when the 3 minute timer starts, I continued to gently mix the sample until ~2:45. Then rolled it horizontally in an effort to remove any micro-bubbles that night be adhering to the sides of the cuvette. The results appeared more consistent but unfortunately (or fortunately depending on the perspective) my PO4 levels have dropped to the point virtually all of the results now fall within the stated accuracy claims of the device....

5/04/12 7:00AM Initial - 07 ppb. Immediate follow up tests - 0, 4, 4, 3, 0 (agitated/rolled during 3 minute timer)
5/03/12 7:25AM 7 min - 04 ppb. Immediate follow up tests - 3, 1, 2, 6, 1
5/03/12 7:00AM Initial - 12 ppb. Immediate Follow up tests - 1, 0, 1, 3, 1
5/02/12 7:00AM Initial - 12 ppb. Immediate follow up tests - 3, 0, 0, 0, 0 (fresh GFO)
4/30/12 7:00AM Initial - 25 ppb. Immediate follow up tests - 3, 12, 10, 4
4/27/12 7:00AM Initial - 17 ppb. Immediate follow-up tests - 4, 2, 7, 0, 6
4/26/12 7:00AM Initial - 16 ppb. Immediate follow up tests - 0, 4, 6, 5, 4
4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3
4/24/12 7:00AM Initial - 29 ppb. Immediate follow up tests - 11, 9, 8, 12
4/23/12 7:00AM Initial - 43 ppb. Immediate follow up tests - 5, 8, 9, 13, 8

I'll have to wait until the current batch of GFO exhausts, then repeat a few tests to confirm.