Hanna Checker HI-736 Phosphorus Checker HC
BeanAnimal's Reef > Article > Hanna Checker HI-736 Opinion

Hanna Checker HI-736 Opinion

I have recently purchased a few of the “Hanna Checker” products designed and sold by Hanna Instruments. While there are many “reviews” of the Hanna Checkers online, most of them are shameless plugs that offer little or no critical feedback regarding the useability of these testers.

NOTE: I have updated this review based on several months with the HI-736 checker. The updated notes are at the bottom of the page.

HI-736 ULR Phosphorus Checker HC

The Hanna HI-736 Ultra Low Range Phosporus Checker HC is designed to measure phosphorus in the reef aquarium and has a range from 0 to 200ppb (Parts Per Billion). The meter (like the other checkers) is fairly easy to use but Hanna has made some rather illogical choices with regard to the user interface.

Hanna Checker HI-736 Phosphorus Checker HC
Hanna Checker HI-736 Phosphorus Checker HC

The procedure (pdf user manual) to measure a sample is fairly straight forward and is as follows:

  1. fill cuvette with aquarium water
  2. press button on meter to turn on
  3. place filled cuvette in the checker and press button to calibrate
  4. remove cuvette and add reagent, mixng until fully disolved
  5. place cuvette back in meter and press AND HOLD button until 3 minute timer starts.
  6. when timer finishes, the measured phosphorus level will be displayed on the screen.


While the procedure sound easy, steps 4 and 5 must be completed in less than 2 minutes or the meter will shut off and the reagent is wasted. While the process can be completed in the alloted time, there is no scientific or otherwise valid reason that Hanna has chosen to enforce such an illogical timeout period. When pressed (no pun) for an answer, Hannas support representative have indicated (no pun) that the timeout is a battery saving feature. Worse yet, this has been a common gripe with ALL of the checker models. In some instances, Hanna appears to have partially understood the complaint and has increased the timeout from 2 minutes to a whopping 3 minutes!

Most of the Hanna Checker HC products require a single press of the button to initiate the measurement process. The HI-736 requires the end user to “
PRESS and HOLD the button for several seconds to start the countdown timer. If the button is simply PRESSED then the meter does an instant reading that is extremely error prone. That is, the result is a wasted reagent packet and the need to start over. While this may sound like an unfair complaint, it is a real issue when using several Hanna Checkers that all have the simple “press” workflow.

As with the other checkers, the timeout period also means that if you walk away or become distracted for more than than 2 (or 3) minutes while the meter is measuring a sample, the reading will be gone from the screen when you return. The net result is yet another wasted reagent and more time!

Also of concern is that the HI-736 does not come with any means to measure the 10ml sample into the cuvette. Dipping the cuvette into the water and then trying to drip it back out to get exactly 10ml is tedious at best. I just don’t understand why Hanna would not include a decent quality 10ml syringe with the HI-736. The 10ml syringe would make consistently filling the cuvette a simple task and would ensure repeatability across all samples without guessing or tedium. Hanna refuses to admit that a basic syringe should be included in the kits. At the extremely high cost per test, this is simply not an acceptable answer. Why the end user have to source their own syringe to full the cuvette?

A PMASI club member indicated that tablet type reagents for the HI-736 would also go a long way to decreasing the tedium and ensuring repeatable results across samples, as it is hard to consistently get ALL of the powdered reagent into the cuvette. I am not sure if a tablet would work, but a capsule would be the perfect solution. The advice has been passed on to Hanna, but they do not appear to be interested.

All in all I am somewhat pleased with the meter but somewhat baffled at the choices Hanna Instruments has chosen to make with regard to the user interface and workflow. With a few minor changes the overall usability of the HI-736 checker (and other models) would be greatly improved. It also bothers me that even though Hanna Instruments has been made very aware of these complaints by numerous reviews and forum posts, each new Checker HC is released with the same problems. Changing a timeout variable in firmware is trivial and it is almost laughable that those involved in the design of the unit would not recognize the issue to begin with. The target market for these meters is hobbyists and small water treatment facilities, etc. It would stand to reason that most of the target users are not trained lab technicians and do not want to put up with the tedium of lab style testing when simple product enhancements could eliminate it.

Lastly, the HI-736 meter measures Phosphorus in ppb (parts per billion) but in most cases aquarists wish to measure Phosphate in the reef aquarium and do so in ppm (parts per million). Conversion from Phosphorus in ppb to Phosphate in ppm can be accompolished by using the follow equation:

Formula to convert ppb phosphorus to ppm phosphate
ppb phosphorus to ppm phosphate conversion equation

Too lazy to do the math? I have posted a conversion table for the Hanna HI-736 ULR meter below using the conversion factor 3.066 as shown in the above equation.

ppb phosphorus to ppm phosphate conversion table
ppb phosphorus to ppm phosphate conversion table


The HI-736 played a very important part in resolving my Phosphate issues as they related to my recent near total system crash due to “Old Tank Syndrome”. I used the meter daily in conjunction with my Lanthanum Chloride dosing regimine.

Reagent packets are fairly expensive compared to batteries. I would much rather replace the batteries (a single AAA cell) on a regular basis than waste time and reagent due to the absurdly short timeout. It is my earnest hope that Hanna will respond to the feedback from users and improve the user interface so that the timeout period is does not make the product cumbersome to use.

I have also purchased the HI-755 Alkalinity Checker and will be posting a similar review shortly.

HI-736 Review UPDATE 12/20/2011

I have been through several (6) boxes of reagent for the HI-736 checker and have found that my meter (checker) sometimes crashes for no reason. Battery life is acceptable, though the timeout is still rather bothersome (both with regard to the workflow and holding the result after the test). However, my biggest issue to date is the fact that each batch of reagent appears to read differently, sometimes slightly and in the case of reagent batch H006, extremely LOW. It appears that other users have had Hanna replace their reagent with a newer lot. It appears that quality control with regard to reagent (and many other aspects) of these checkers is in question. I have seen reports of reagents for other checker models being bad as well.  Moreover, Hanna offers a high range standard solution, but not a low range standard solution compatible with the HI-736. Again, for the extremely high cost per test, I do not feel that I should have to double check my test results from reagent lot to reagent lot and compare those results with other users and tech support to confirm my checker is functioning properly.

The general sentiment on the reef forums is that Hanna has “excellent customer support”. I simply do not agree, as there is a world of difference between kind representatives and excellent customer support.  While I too find the Hanna support personnel kind, I do not find them all that helpful with anything more than asking you to perform extra tests (at my cost) and followng their already published instructions. They simply do not appear to acknowledge that the checker products have workflow, packaging and accuracy issues and instead point toward user error of a flawless product as the only issue.



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