HI 736 ULR - Test / Same-Sample Retest Variations (compendium)

[clydog]

Thanks clydog - Have you tried the retests and if so, can you share your process and results? It's entirely possible my checker is defective, and I probably would have come to the same conclusion until rehypes post. A few things make wonder...

• I can't image a defect that would produce the results seen. Seemingly consistent and predicable initial and retest results that vary widely, despite being taken seconds apart with the same sample and...a problem that disappears by simply allowing the C2 sample to sit for 7 minutes.
  
• If I understood rehypes comment correctly, he also had similar findings, hence the development of his 7 minute process and lastly...
  
• If Hanna thought my unit defective, it seems to me they would have simply said so and offered to replace it - problem solved, rather than ignore the issue entirely.

Although I don't understand why, at this point I suspect the solution is to allow the C2 sample to rest for 7 minutes after mixing. If this is correct, I wonder if many using the HI 736 aren't getting incorrect (consistent, but higher than actual) PO4 readings when following the directions provided by Hanna?

my first test is usually one of the highest,i think its from the reagent not being fully dissolved....but even then its still within the 3-5 ppb range (rarely 5 ppb difference,usually 1-3 ppb difference)

 

[clydog]

do you have any scratches/fingerprints on the vials? i always hold the vials up to the window to make sure its completely clean....also are you swirling the vial to mix it and not shaking it.....you could have microbubbles messing with the first test

 

[djkms]

Here is how I test and I seem to get consistent results week in and week out:

Wash a cuvette and cap thoroughly with RO/DI water
Dunk the cuvette in tank water several times
Fill to 10ml mark and cap
Wipe cuvette down with a microfiber cloth
Hold cuvette up to light and turn slowly, making sure there is no dirt or fingerprints on it
Open a packet and empty it onto a business card folded in half
Set my stopwatch for 3 minutes
Place the cuvette into the tester lining up the "b" in ppb on the tester to the "m" on the cuvette
turn on and press C1
Start stopwatch
Remove cuvette, add reagent and shake for 2 1/2 minutes
Turn cuvette on its side and wipe it down again while turning to remove fingerprints on the outside and air bubbles stuck to the walls on the inside.
Hold cuvette up to light and turn slowly, making sure there is no dirt or fingerprints on it
I then place the cuvette back into the tester, lining up the letters again and hold down C2 and get my results 3 minutes later

Week in and week out I get readings between 4ppb-8ppb. I have retested in the past and my results were exactly the same as the previous test for that day.

I did have the phosphate tester at one point and got WILD readings with it. One test would be like .02 then .12 then .06, etc. I since then switched to the phosphorous test and couldn't be happier. Consistent results!

 

[clydog]

+1 very accurate........ i have never had a 0 reading on my 75 DT but i just tested my frag tank thats been up for 2 months and got a 0 reading multiple times....it also read 0 a month ago on that tank so i believe its a very good way to test phos

 

[CalmSeasQuest]

Thanks very much for all the replies...

my first test is usually one of the highest,i think its from the reagent not being fully dissolved....but even then its still within the 3-5 ppb range (rarely 5 ppb difference,usually 1-3 ppb difference)

According to Hanna, undissolved reagent would result in lower test results.

do you have any scratches/fingerprints on the vials? i always hold the vials up to the window to make sure its completely clean....also are you swirling the vial to mix it and not shaking it.....you could have micro-bubbles messing with the first test

The test kit is new and no defects are visible on either cuvette. They are wiped using a micro-fiber cloth prior to every insertion. The cuvettes are slowly tilted back and forth to mix per the instructions to avoid micro-bubbles.

While no micro-bubbles are visible, it might explain why the initial tests are so far off all subsequent tests as the agitation of removing and replacing the cuvette from the checker for the retests might dislodge any micro-bubbles. This would also explain why the 7 minute tests show the same results.

I will test that tomorrow by rotating the cuvette horizontally to dislodge any micro-bubbles just prior to the 3 minute timer expiring. (fingers crossed)

 

[BigKahuna]

Here is how I test and I seem to get consistent results week in and week out:

Wash a cuvette and cap thoroughly with RO/DI water
Dunk the cuvette in tank water several times
Fill to 10ml mark and cap
Wipe cuvette down with a microfiber cloth
Hold cuvette up to light and turn slowly, making sure there is no dirt or fingerprints on it
Open a packet and empty it onto a business card folded in half
Set my stopwatch for 3 minutes
Place the cuvette into the tester lining up the "b" in ppb on the tester to the "m" on the cuvette
turn on and press C1
Start stopwatch
Remove cuvette, add reagent and shake for 2 1/2 minutes
Turn cuvette on its side and wipe it down again while turning to remove fingerprints on the outside and air bubbles stuck to the walls on the inside.
Hold cuvette up to light and turn slowly, making sure there is no dirt or fingerprints on it
I then place the cuvette back into the tester, lining up the letters again and hold down C2 and get my results 3 minutes later

Week in and week out I get readings between 4ppb-8ppb. I have retested in the past and my results were exactly the same as the previous test for that day.

I did have the phosphate tester at one point and got WILD readings with it. One test would be like .02 then .12 then .06, etc. I since then switched to the phosphorous test and couldn't be happier. Consistent results!

Pretty close to the way I do it. I have the reagents pack cut and open ready to pour into the cuvette before I even turn on the tester. Also I start the 3 minute time the second I do C1 and gently tilt the cuvette back and forth after adding the powder to prevent bubbles for almost exactly 2:50 every time for consistency every time. I almost always have the solids completely dissolved by this time. I've done so many tests at this point I can ballpark guestimate what the reading will be by just looking at the how much blue color change of the solids I see when they are first added to the cuvette

Basically consistency is king.

1. Have a 3 minute timer ready to go before you start.
2. Have the reagents packet cut open and ready to go, I actually cut off two sides to make it pour out easier.
3. Start the timer when you begin C1 which will take a few seconds so you have a buffer to get it done in the time allotted.
4. Keep a microfiber cloth handy and wipe the cuvette every time you handle it and always put it into the tester in the same orientation.
5. Mix by slowly tilting the cuvette back and forth for 2:50, I play ping pong with the air bubble so you get a nice mix and prevent micro bubbles from forming.

 

[chercm]

Here is how I test and I seem to get consistent results week in and week out:

Wash a cuvette and cap thoroughly with RO/DI water
Dunk the cuvette in tank water several times
Fill to 10ml mark and cap
Wipe cuvette down with a microfiber cloth
Hold cuvette up to light and turn slowly, making sure there is no dirt or fingerprints on it
Open a packet and empty it onto a business card folded in half
Set my stopwatch for 3 minutes
Place the cuvette into the tester lining up the "b" in ppb on the tester to the "m" on the cuvette
turn on and press C1
Start stopwatch
Remove cuvette, add reagent and shake for 2 1/2 minutes
Turn cuvette on its side and wipe it down again while turning to remove fingerprints on the outside and air bubbles stuck to the walls on the inside.
Hold cuvette up to light and turn slowly, making sure there is no dirt or fingerprints on it
I then place the cuvette back into the tester, lining up the letters again and hold down C2 and get my results 3 minutes later

Week in and week out I get readings between 4ppb-8ppb. I have retested in the past and my results were exactly the same as the previous test for that day.

I did have the phosphate tester at one point and got WILD readings with it. One test would be like .02 then .12 then .06, etc. I since then switched to the phosphorous test and couldn't be happier. Consistent results!

i do not quite understand this part :

Place the cuvette into the tester lining up the "b" in ppb on the tester to the "m" on the cuvette

 

[rehype]

Thanks very much for sharing your results rehype. Very interesting, so after waiting 7 minutes, all of your tests are well within the stated error rate. 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.

No problem tom. I wanted to point out that after i wait the 7 minutes i still use the checkers built in timer for the additional 3 minutes. My guess is after the two minute mixing the presence of microbubbles can contribute to inaccurate readings. So waiting for what amounts to 10 minutes gives the most consistent results at least in my experience.

 

[CalmSeasQuest]

Basically consistency is king.

Thanks BigKahuna - I couldn't agree more.

i do not quite understand this part :
Place the cuvette into the tester lining up the "b" in ppb on the tester to the "m" on the cuvette

It's not really critical where/how you align the cuvette in the checker - as long as you do it the same way every time (I use the same alignment marks as djkms.)

No problem tom. I wanted to point out that after i wait the 7 minutes i still use the checkers built in timer for the additional 3 minutes. My guess is after the two minute mixing the presence of microbubbles can contribute to inaccurate readings. So waiting for what amounts to 10 minutes gives the most consistent results at least in my experience.

Thanks again rehype - 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.

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 :rollface 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.