Hanna PO4: LR vs ULR

[PopBot16]

I've had the Hanna LR PO4 Checker for a while now, HI713, and I just purchased the ULR version, HI774 a few days ago. According to the accuracy specs, the HI713 is supposed to be ±0.04 ppm ±4% of reading, and the HI774 is supposed to be ±0.02 ppm ±5% of reading (see info at links above).

I've taken two sets of readings with both over the last two days, same time of day (7pm), same procedures, and all water used was from the same sample. These are the readings:

Day 1: LR - 0.09 ppm ; ULR- 0.23 ppm;

Day 2: LR - 0.15 ppm; ULR- 0.22 ppm;

I apply the accuracy specs in the way that I assume they are applied, it would seem like the ULR is telling me that PO4 ranged from 0.20 to 0.26 on Day 1 and 0.19 to 0.25 on Day 2. The LR is telling me that PO4 ranged from 0.05 to 0.13 on Day 1, and 0.10 to 0.20 on Day 2.

Does that seem right? While I like the reading of the LR better (haha), it reads lower, and I don't want a false sense of security regarding my PO4. I know I have a PO4 issue either way, and it needs to be lower (currently trying GFO after avoiding it for many years). I'm not chasing numbers, but I guess the question, which one do I use going forward? I'd rather not do both each time (and have to buy both sets of reagents). Thoughts?

 

[nereefpat]

Tough to say in that range. When you get under 0.1, I would definitely trust the ULR. I've seen many times where people report zero with LR and the ULR reads 0.03 or more. But in the 0.1-0.2 range, I would just call it somewhere in the middle. I know that's not terribly satisfying.

That being said, go slow with GFO, especially if you have a healthy tank without big algae issues.