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 asked Hanna if 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.

 

[revhtree Verified Admin]

Bump!

 

[CalmSeasQuest]

Thanks Rev - Still no reply from Hanna. I did some additional testing and had similar results...

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 st 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.

Hopefully Hanna will respond with some insight as I'm at a loss to explain the differing 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.

 

[flyfish4trout]

Tom,

Have you thought about verifying your results with laboratory analysis. There is an environmental lab in your neck of the woods and several in the metro Detroit area. I believe that the test you would run would be for Total Phosphorous and the cost is around $30 per sample. Most labs will provide the appropriate container for you to collect the sample in. It may give you some piece of mind one direction or the other. One other issue to consider if you use the lab to verify your meter is the laboratory detection limit. Depending on the methodology, the reporting limit for P is on the range of 1 ppb to 10 ppm, and different methods may have different interferences resulting in a higher reporting limit for the constituent being tested. Just a thought for your consideration. Best of luck.

 

[CalmSeasQuest]

Thanks ff4t,

I considered having a lab provide testing and getting a comparative known value sample. I have a suspicion it has to do with undissolved reagent. I should be able to confirm it with tomorrows tests. I think I'm most troubled by the checkers automatic time-off settings not providing enough time to mix the reagent as directed - That and the fact that Hanna hasn't bothered to respond. I suspect that many (most) users of the 736 may be getting erroneously high readings.

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 topics Hanna has seemingly ignored this one.

 

[UFreefer]

There are many threads on RC where people are having the same issues. While hanna has good customer service, they don't have any answers or solutions. I won't be buying anymore hanna checkers.

 

[Mike&Terry]

We switched from the HI Phosphate Checker to the HI 96713 Photometer because we were also getting inconsistent results. While a bit more expensive, we find it easier to use and have been getting consistent results.

 

[CalmSeasQuest]

There are many threads on RC where people are having the same issues. While hanna has good customer service, they don't have any answers or solutions. I won't be buying anymore hanna checkers.

I reviewed most of the other complaints, this one is a bit different. I've documented significant (well beyond the stated 5 ppb error rate), repeatable variances using the same samples.

We switched from the HI Phosphate Checker to the HI 96713 Photometer because we were also getting inconsistent results. While a bit more expensive, we find it easier to use and have been getting consistent results.

I would have gladly invested more in a meter to gain the accuracy, but the 96713 seemed to be a low-range meter measuring in ppm rather a ultra-low range meter measuring in ppb?

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.

 

[CalmSeasQuest]

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 also replaced the batteries (even though the unit was brand-new) on 4/25 to eliminate the possibility of faulty batteries causing the variance.

I have posed 2 questions to Hanna,

1) Which set of values is to be believed?

2) Why?

 

[VegasRick]

All the follow up tests seem to be within the 4 ppb. So something between the first and follow ups are causing the initial to give a false high. Have you tried rinsing and or soaking the vial for a few minutes before the initial test?

 

[CalmSeasQuest]

All the follow up tests seem to be within the 4 ppb. So something between the first and follow ups are causing the initial to give a false high. Have you tried rinsing and or soaking the vial for a few minutes before the initial test?

The cuvettes and caps are rinsed and dried after each use and prior to storage. They are then rinsed in tank water just prior to the next tests. I simply can't explain why the follow up tests conducted seconds after the initial test, using the exact same water and reagent sample produce such differing results...Apparently neither can Hanna, at least thus far.

 

[VegasRick]

Maybe soaking the vial for a couple of minutes, you probably have tried all this and I was pretty sure you were rinsing them just had to mention it. Maybe residue from whatever you dried it with prior to storage is not getting rinsed out. The only difference I see from initial to follow up is that and a 3minute soak.

 

[CalmSeasQuest]

Maybe soaking the vial for a couple of minutes, you probably have tried all this and I was pretty sure you were rinsing them just had to mention it. Maybe residue from whatever you dried it with prior to storage is not getting rinsed out. The only difference I see from initial to follow up is that and a 3minute soak.

Thanks Rick. It would seem that if contamination was the cause, it would impact all the tests results, not just the initial. The exact same, unopened or altered samples are used for the retests which occur seconds later.

 

[CalmSeasQuest]

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.

 

[CalmSeasQuest]

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 :roll 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.

 

[Gregluvzsf]

I"m having the exact same problem and I am totally frustrated. I test the first sample and get a reading anywhere between 8 to 18 or higher then I repeat the test using the same exact procedure and get 0?? I'm called Hanna with no reply..

 

[mike werner]

come on hanna your better than this.give us an answer or at least reply in some way.

 

[Randy Holmes-Farley Verified Community Expert]

I"m having the exact same problem and I am totally frustrated. I test the first sample and get a reading anywhere between 8 to 18 or higher then I repeat the test using the same exact procedure and get 0?? I'm called Hanna with no reply..

Assuming you are talking about the Hanna 736, then 0 and 8 ppb are not different by their stated uncertainty of +/- 5 ppb. So don't try to read excessive accuracy into them. 18 and 8 are also not outside the stated range. 0 and 18 are, but not by that much. Instead of +/- 5 ppb, +/- 9 ppb would fit.

Note that there can be many errors that are introduced by less than perfect everything, from scratches on the cuvette to timing issues.