Testing a Hanna Phosphorus ULR Checker - Am I Missing Something?

chipmunkofdoom2

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I have a Hanna Phosphorus ULR Checker for measuring phosphates. I stopped using it a long time ago because I got frustrated with it always reading zero. I was pretty sure there was some phosphate in my tank, but it never registered anything. I started testing phosphate again recently and had a similar experience. So, I decided to make some samples and see how the Checker read them. The results were way off what I expected. Before getting in touch with Hanna, I want to make sure I'm not overlooking anything that would explain these results. Here's the procedure I followed:

1. Create a phosphate solution with Na3PO4. I added 7.12 grams to one gallon of deionized water. Since Na3PO4 is 58% phosphate by weight, this produces a solution with a concentration of roughly 1,090 ppm phosphate.

2. Fill a 5 gallon bucket with fresh artificial seawater. Add a small pump for circulation

3. Test the seawater in the bucket and record the result as a baseline.

4. Dose 0.87 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.05 ppm over baseline. Test and record result.

5. Dose 0.87 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.10 ppm over baseline. Test and record result.

6. Dose 2.6 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.25 ppm over baseline. Test and record result

7. Dose 4.34 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.50 ppm over baseline. Test and record result.

Notes:
- I would have liked to scale the test up more to avoid measurement error, but I didn't have enough equipment to scale the test any further. This scale should have been adequate to answer the question at hand.
- The Checker's battery was replaced before the tests
- The Checker reagents are new and were ordered 2 weeks ago (lot H100)
- Procedure from the Checker instructions were followed exactly with all tests.
- The same cuvette was used for all tests.
- The same cuvette was used for both the C1 and the C2 test.
- The cuvette was inspected for any scratches or blemishes before the tests began. There were none.
- Before the tests, the cuvette was cleaned with vinegar then deionized water.
- Between each test, the cuvette was rinsed with tap water, then deionized water.
- The cuvette was wiped with a clean, dry cloth before it was placed in the Checker.
- During step C2, the vial was mixed for exactly two minutes as measured by a timer.
- The reagent packets were opened and prepared before the test began to ensure that there was ample time to allow two full minutes of mixing
- The Na3PO4 used was a 99.9% pure food-grade supplement
- The scale used to measure the Na3PO4 has a maximum capacity of 500g and reads in increments of 0.01g
- The container used for the Na3PO4 solution in step 1 was a clean 1 gallon plastic jug that previously contained distilled water. It was rinsed with tap water, then deionized water
- The Na3PO4 solution in step 1 was mixed for several minutes and allowed to settle overnight. It was mixed again before the tests.
- The seawater used in this test was Instant Ocean mixed to 52.5 mS (about 35 ppt). It had been mixing for just over 2 days prior to the tests.
- The 5 gallon bucket from step 2 was rinsed thoroughly with deionized water
- The circulation pump from step 2 was soaked in deionized water overnight
- The doses from steps 4 through 7 were dosed with a clean, new 1 mL syringe
- The samples for the each test were drawn about 6 - 7 minutes after the last dose of PO4 was added.
- To determine the dose of Na3PO4 solution to add, I used the following equation. I refactored a common dosing calculation to allow me to solve for the amount of solution to add:

equation.PNG

Where:
Tv = tank volume
Tc = tank concentration
Sv = volume of solution to dose
Sc = solution concentration
Dc = desired concentration
Dc - Sc ≠ 0

Results

TestExpected Reading (PPM)Actual Reading (PPM)
1​
0​
0​
2​
0.05​
0.006​
3​
0.1​
0​
4​
0.25​
0.077​
5​
0.5​
0.15​

ppm.png


Is there something wrong with my testing procedure? Is there an error in one of my calculations, either for the strength of the Na3PO4 solution or the amount I should have dosed? Any input would be appreciated.
 

Crabs McJones

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I only ask because i've ran into this before, when you're shaking the cuvette with the reagent for 2 minutes, does the tester turn off or are you able to get it into the hannah before it shuts down and run the 3 minute test.
 
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chipmunkofdoom2

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I only ask because i've ran into this before, when you're shaking the cuvette with the reagent for 2 minutes, does the tester turn off or are you able to get it into the hannah before it shuts down and run the 3 minute test.

Yes, I actually used two timers. One was set for three minutes and started right when the Checker started flashing C2. This let me know how much time I had before the Checker timed out. The other timer was set for two minutes so I knew how long I was mixing the reagent. Between this and preparing the reagent before I started the test, I didn't have any trouble getting the sample ready before the Checker timed out.

This timeout is one of the many reasons I hate this Checker so much. The timeout during the C2 step is unacceptably short. The manual brags that a single battery will last for five thousand tests. If you measure phosphate every single day, that's nearly 14 years of tests. The battery will go bad long before that. Even if you double the timeout to 6 minutes and assume that cuts the test count in half, that's still almost 7 years of daily tests on a single battery. Again, most batteries will not last this long anyway. I can't think of a single good reason why the timeout on this Checker should be so short.
 
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chipmunkofdoom2

chipmunkofdoom2

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did u multiply the number by
3.099
then / 1000

Per @Crabs McJones it is x 3.066 / 1000. I actually didn't do any math though. I took the PPB reading from the Checker and cross-referenced it to Hanna's official conversion chart. Here are the PPB results for the sake of completeness. These are the actual results shown by the Checker:

TestExpected Reading (PPB)Actual Reading (PPB)
1​
0​
0​
2​
16​
2​
3​
33​
0​
4​
82​
25​
5​
163​
49​

ppb.png
 

Potatohead

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Hanna makes a calibrated solution you can buy for a few bucks and it comes with an extra cuvette, I would probably cross reference against that. Maybe you are having some precipitation with your solution or something. A while back I was doubting my checker and the solution tested at 96 when it's supposed to be 100.
 
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chipmunkofdoom2

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Hanna makes a calibrated solution you can buy for a few bucks and it comes with an extra cuvette, I would probably cross reference against that. Maybe you are having some precipitation with your solution or something. A while back I was doubting my checker and the solution tested at 96 when it's supposed to be 100.

I've seen the standard, I just don't think that would make me feel any more confident in this Checker. Especially after the tests I did. If there's not something going on chemically that I don't understand with regards to adding NaPO4 to seawater, I should have gotten better results than I did with my tests. The tests seemed to be at reasonable scale and I feel that I took reasonable precautions to get accurate results.

In reality, all the standard would really do is tell me if I have a bad Checker. It's outside of warranty and the first time I encountered issues with the readings, Hanna support was breathtakingly unhelpful, so I don't expect anything meaningful from them. I'm done spending money on this thing. I already paid $50+ for the checker and another $10 for the reagents. I just want to know if my testing procedure was viable and that there's nothing going on chemically that I don't understand before I send a nasty email to Hanna and throw this thing in the garbage.
 

Potatohead

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I've seen the standard, I just don't think that would make me feel any more confident in this Checker. Especially after the tests I did. If there's not something going on chemically that I don't understand with regards to adding NaPO4 to seawater, I should have gotten better results than I did with my tests. The tests seemed to be at reasonable scale and I feel that I took reasonable precautions to get accurate results.

In reality, all the standard would really do is tell me if I have a bad Checker. It's outside of warranty and the first time I encountered issues with the readings, Hanna support was breathtakingly unhelpful, so I don't expect anything meaningful from them. I'm done spending money on this thing. I already paid $50+ for the checker and another $10 for the reagents. I just want to know if my testing procedure was viable and that there's nothing going on chemically that I don't understand before I send a nasty email to Hanna and throw this thing in the garbage.

You can spend $11 to have the answer to all those questions

 
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chipmunkofdoom2

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You can spend $11 to have the answer to all those questions


It's not just $11. I don't need anything from BRS right now so it would be $5 shipping plus sales tax. Even if I paid the almost $20 or waited til I needed to order something, to what end am I spending money to test this Checker? If I test the standard and it reads incorrectly, then it just confirms that the Checker is junk. I already believe it to be junk. With the lousy customer service I got from Hanna the first time I asked for their help, I'm not buying another one.

Even if the Checker did read the standard correctly, I'm still not convinced that it's any good. I put phosphate in seawater and the meter couldn't read it with any kind of accuracy. I've never heard of any mechanism by which seawater sucks PO4 out of the water column, so I don't understand how that's possible. Unless I could explain why my process above didn't work, I wouldn't want to use the Checker even if it read the standard correctly.

I'm probably just going to dose phosphate every day and stop if and when my acros look unhappy. Some Checkers might be good, but mine is clearly not. I'm still open to any perceived issues with my testing procedure.
 

jccaclimber

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Assuming you’re covered for this, but I’ve had issued with micro bubbles sticking to the walls of the cuvette. In the C2 step it gives a high read, in the C1 step I would expect a low read.
It sounds like your procedure is pretty good.
I didn’t see mention of ensuring the inside of the meter is clean. Typically not an issue, but worth checking.
 
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chipmunkofdoom2

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Assuming you’re covered for this, but I’ve had issued with micro bubbles sticking to the walls of the cuvette. In the C2 step it gives a high read, in the C1 step I would expect a low read.
It sounds like your procedure is pretty good.
I didn’t see mention of ensuring the inside of the meter is clean. Typically not an issue, but worth checking.

I don't shake the cuvette in step C2. It causes way too many bubbles. I simply invert the cuvette continuously for two minutes. This is plenty of time for the reagent to fully dissolve and there are no bubbles.

I didn't check the inside of the Checker until now, but there is no dirt, dust or any dried saltwater.

In another thread discussing this Checker, several users outlined their testing procedure. Some hobbyists use procedures that were outright against the instructions or should lead to inaccuracies. If these hobbyists are getting accurate readings by not following the directions, I should be by following the directions to the letter.
 

jccaclimber

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I must be inverting too quickly or something. I don’t shake the vials either.
Regardless if it’s clean then you should be getting good results and it seems you are not.
Na3PO4 can be anywhere from anhydrous to a dodecahydrate, which would decrease the concentration to (late night math handicap warning) 43% of its original strength on a mass basis. Are you doing anything to ensure you had no hydration in your source Na3PO4?
Your error seems to be greater than that, but it’s worth investing. I don’t know off hand what is needed to ensure the source is anhydrous.
Some compounds give off their moisture easily with heat, others less so. Randy may be able to chime in on that (and the rest of this too).
A bad reagent batch is possible as well.
 

saltyhog

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I'm really surprised at this. I've been using this checker for about 5 years and every time I've done an ICP test the value from the lab is very close to what my ULR Phosphorous checker gives me. Last one was March...ICP was 0.09, Hanna 0.08. Some checks have been exactly the same. Weird!
 

Uncle99

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I just tried my Hanna 736 ULR yesterday.
I have been using Salifert for years, phosphate always 0.03ppm.
Fired up the Hanna and got 27ppb, or 0.027ppm.
That’s close enough and within their stated margin of error.
 
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chipmunkofdoom2

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I just tried my Hanna 736 ULR yesterday.
I have been using Salifert for years, phosphate always 0.03ppm.
Fired up the Hanna and got 27ppb, or 0.027ppm.
That’s close enough and within their stated margin of error.

The HI736 doesn't measure phosphates directly, it measures phosphorus. To convert the reading from the Hanna Phoshporus ULR Checker to phosphates, either use this chart or multiply by 3.066 and divide by 1000. A reading of 27 ppb means 0.083 ppm phosphate, not 0.027 ppm phosphate.
 
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