Hello,

I bought a second hand "Dupla ph Control" (old type),
ofcourse without a manual.

I think I figured out how this works,
but when I try calibrating with PH4 I cannot
get the PH control below PH5.
I must be doing something wrong!

Can anyone help me get a manual for this control,
or does anyone know what i'm doing wrong?

Regards Tamara