Rick wrote:

"Nikki Casali" wrote in message
...

It's a month since I started using a permanent pH probe to control my
tank's CO2 injections. The probe being used is made by AquaMedic. At the
the first calibration I used buffer solutions 7 and 10. The instructions
say to use 7 and 9. I used the 10 instead to adjust the calibration
curve. The calibrated probe's readings matched the colorimeter's readings.

It's a month later. Time to recalibrate. I first checked to see how far
the probe had drifted. It was less than .1 according to the buffer
solutions. In this calibration routine I used 7 and 9.18. Calibration
done I then placed the probe into the tank water. It read 6.85. The last
reading before calibration was 7.25. I now checked with the colorimeter
which said the tank was 7.3. The probe is now off by -0.4!

I recalibrated with 7 and the original 10. Still reads 6.85!

It seems to me that I have somehow trashed the probe by recalibrating
it. Is there anything I can do to recover the probe? A temporary
solution I have, before I can get a replacement, is to calibrate the
probe to the known tank pH. That is, placing the probe in the tank water
and adjusting it until it reads around about 7.3.

Anyone have a similar experience?

Nikki



without using the calibration mode simply check the PH level of the buffer
you are using. You are then using a known solution. If the meter reads
correctly then your water PH is 6.85.

Thanks for the reply!

OK, I checked the 7.01 pH buffer solution with a colorimeter - the only
pH gadget I can rely upon. It gives 7.0 pH. I noticed that the pH probe,
which initially read 6.85 pH in the tank, drifted over 30 minutes to a
stable 7.05 pH. The colorimeter also concurs with a reading of 7.1 pH.
Everything is now in agreement. But a pH of 7.05 translates to 37 ppm of
CO2 with my water. Woops!

I reckon the buffer solutions pulled the probe's readings down
temporarily. Is this known to happen??


Nikki