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

[CalmSeasQuest]

When using the HI 736 ULR Phophate checker, I note considerable variations between the initial test done using the 3 minute timed method and subsequent, immediate retests made using the same sample via the instant method.

Tests are made according to the manual, all cuvettes are wiped clean with a microfiber cloth prior to each test, each cuvette is rinsed with tank water prior to testing. Care is take to make sure all the powder agent is introduced into the cuvette. The instant re-tests are done using the second cuvette for the C1 standard sample.

Over the last two days I have obtained the following results...

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 understand the use of a second cuvette for the C1 tank water standard for the instant tests might introduce some variation, but I have alternated the the cuvettes used and still obtain similar variations.

Has anyone else experienced this issue? I've posted this in Hanna's forum with the hope they can explain why initial testing might produce results much higher than all subsequent testing? FWIW, based on my tank trends, I believe the instant retest values to be more accurate.

[CalmSeasQuest]

Today I used 2 cuvettes for all the tests to minimize any variable caused by the use of the second cuvette on the retests. Care is taken to use the same alignment when inserting into the checker (10ML symbol facing front) and the checker was turned off for ~10 seconds between retests. The results were

4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3

*I conducted the retest two additional time as I was getting results of 0 ppb.

For comparison, here are are the daily results side-by-side

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 believe the trend of phosphates being lowered is correct as the GFO reactor was recently changed, but I am not confident in which value range to trust. I am puzzled by the difference and the fact that the retests always produce much lower results than the initial tests - seemingly well beyond the stated error rate for the device. Also curious is that all the retests provide very consistent results within the stated accuracy range.

I wonder if any other hobbyists with a 736 could replicate these tests to compare results? It only takes a few minutes as the retests are almost instant and require no additional reagents. I'd be happy to PM or speak with anyone interested to make sure the processes are identical.

I'm still hopeful Hanna will respond with some insight.

[Sir Patrick]

I will do some testing tonight and post my results.

[CalmSeasQuest]

Originally Posted by Sir Patrick

I will do some testing tonight and post my results.

Great - Thanks Chris. I'm curious to see if you come up with similar results.

[CalmSeasQuest]

I repeated the exact same tests and process today with nearly identical results. I am becoming convinced there is an issue with my initial test result values. Here are all the results side-by-side...

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 believe I might have found the problem, and it may be partially my methodology. When mixing the reagent, I have not been mixing it for a full 2 minutes as contained in the directions. I shorten this time so as to not have the checker power off as, again per the directions, the unit turns itself off after 2 minutes on "non-use". There is still a tiny amount of undissolved reagent visible in the cuvette when the vial in placed in the checker for the initial test. Although the reagent is completely dissolved 3 minutes later when the checker takes the reading, I'm wondering if perhaps during the 3 minutes the vial sits, unagitated that variations or striations in reagant concentration are created in the cuvette thereby altering the test results.

In an attempt to confirm this I'll alter my procedures tomorrow. I will again use 2 cuvettes, but manually mix the C2 sample for the instructed full 2 minutes (hoping the unit doesn't shut itself off), then repeat the follow-up instant tests. If this is the cause of the variation in results, it might mean the the follow-up instant test results are more accurate.

I've still not had any reply from Hanna - I'm hoping they can provide some guidance, especially regarding the stated 2 minute auto-off policy when the user is instructed to mix the reagent for 2 minutes. Considering the time required to remove the cuvette, add the reagent powder and mix - it's not possible to complete those actions in the allotted 2 minute time frame.

For any other 736 users, BeanAnimal put together a very handy chart for converting PPB Phosphorus to PPM Phosphates...

[CalmSeasQuest]

So much for that theory...

Today I made sure to mix C2 for the full 2 minutes. The good news was despite the 2 minute auto time-off period detailed in the instructions, the checker did not turn off. At the end of the 2 minute period, virtually all of the reagent was dissolved. The test results were virtually identical to the previous,

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

This coupled with the following response from Hanna provided in another thread seems to do away with my theory that undissolved reagent allowed to sit in the still cuvette for 3 minutes might have caused the variance...If the powder still isn't completely dissolved by the end of the 1 minute 45 second shaking, it will not affect the reading as long as you see that it is dissolved when you remove the vial after the three minute countdown and reading. If there is still undissolved powder at that point, it is likely that you will get a false low, but it is impossible to quantify.

I'm at a total loss in understanding why these results vary so widely. I believe my test methods have been sound including alternating the cuvettes used for C1 and C2 on subsequent days to account for any optical variance in the glass - especially since the results have been consistent and reproducible. I'm disappointed that despite responding to other topcs Hanna has seemingly ignored this one.

[CalmSeasQuest]

I did the same tests one last time with virtually the same results...

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

To me at least, the differences are not insignificant. If the initial test results are to be believed, my PO4 levels are still higher than desired. If the retests are more accurate, then I've likely achieved my desired levels (<10 ppb Phosphorus / 0.03 ppm Phosphate.)

The most disappointing part of this is despite responding to others thread and questions since this was posted, Hanna has "avoided" responding to this issue. All that has been asked for is help determining which values are more accurate and why. It seems Hanna would be able to easily verify this and provide guidance. Unfortunately, without a response form Hanna, rather than providing a solution to the problem, this thread might end-up serving as more of a warning for those considering a Hanna checker.

[Badfish]

Thanks a lot for posting this CSQ, I've been think of buying one of these testers myself but after reading about the initial and retesting inconsistency, I think I'm going to hold off. Please keep us posted if you hear anything more from Hanna.

[Flynnstone]

have you changed the batteries in the tester?

[CalmSeasQuest]

Originally Posted by Flynnstone

have you changed the batteries in the tester?

Yes, batteries were swapped on the 4/25 (despite being brand) new just to eliminate that variable. The result pattern was unchanged.