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pH probes, HUGE discrepency

kinetic

kinetic

Supporting Member
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I have two lab grade pH probes (Neptune brand), and I've calibrated them twice now with #7 and #10 pinpoint solutions. They're in the same part of the sump, getting a lot of flow. They're literally right next to each other.

So sometimes they're .10 away from each other, then the other day they were nearly .8 from each other. One pH probe reading 8.35, the other 7.56. What the heck? Can these be trusted?

Then I used my seachem pH test kit, and it gives me a result that says between 7 and 9 LOL! what a horrible kit.

This new tank just won't cruise along whatever I do with it. I always had awesome results with my nano, and thinking more water meant better stability, but that's totally not what's going on. The larger tank has more complicated stuff, maybe I'm creating more problems by correcting things rather than it just sitting stable on it's own. Who knows?

Skimmer doesn't work, LiterMeter III auto water exchange is off by a lot, pH probes on the fritz, Calcium at 500ppm always without dosing, Magnesium always at 1600-1800ppm, and ALK drops in weird ways, pH is... who knows what it is.

Very sad. Thinking of going back to a small tank =P
 
Did you calibrate with pH7 then pH10 in that order?

Were the water temps the same on both time? (I believe there is a temperature correction and not sure if it is probe design specific)


When you calibrate your probes
1) gently dab off excess water (the glass probe is actually a very thin porous membrane)
2) insert into pH 7 buffer and swirl a bit till stable (you want it to equilibrium and flush the non buffer away).
3) set the intercept adjustment to 7
4) rinse probe with DI water and carefully dab it mostly dry
5) repeat #2 with pH10
6) set the slope adjustment to 10
7) repeat 4 and re-check pH7 and it should still read 7.
 
Thanks Tony, I wasn't dabbing or rinsing with DI water =X that might screw it up.

The temp was just the temperature of the calibration fluid, and they were done at the same time. Maybe I'll let the unopened calibration packets sit in my sump to warm up first as well just to be extra sure.
 
But even if you calibrations were off due to above reasons, shouldn't the delta between the two be more or less the same if your ph is not swinging wildly? You said sometimes they differe by 0.1 and some other time differ by 0.8, I would be worried about the readings.
 
Not sure if it is necessary but I float my PH calibration packets in my tank water about 30 mins prior to calibrating. I also calibrate both probes at the same time in the same packet (RK2 and RKlite), rinsing in DI as Tony mentioned.
 
thanks guys for all the input so far.

This is my plan for attack:

- Float calibration fluids for 30 minutes in sump
- Calibrate one in #7 and #10, rinsing and dabbing between each
- Calibrate second the same way, immediately after
 
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