I don't think it matters,
in my opinion you calibrate the slope mv/ph.
If the ph value is below 7 the mv/ph is negative,
above 7 is positive. But the slope stays the same.
At least this is what I remember from chemistry-lessons.
(a long long time ago!!!)


"Pszemol" wrote in message
...
"Arjan Bakker" wrote in message
...
but when I try calibrating with PH4 I cannot
get the PH control below PH5.

Why would you calibrate pH meter with 4 for a reef?
You calibrate with buffers the way measurement is
BETWEEN them. For reef tank pH 8.2 you use buffers
7 and 9 or 7 and 10. Buffer 4 is for freshwater tanks
where you keep the water at below ph 7 for some fish...