You have a bad probe. If you calibrated it to a 7 pH solution first and then sloped it to
a 4 solution and it would not go to 4 something is wrong with the probe. I doubt it is
the 4 solution. I take it this meter has two screws, calibrate and slope IIRC ?


I looked for a manual for but no luck.

Something for you to read, so you know where you are at.

Measuring pH with a Meter
http://www.advancedaquarist.com/issues/feb2004/chem.htm

A Comparison of pH Calibration Buffers
http://reefkeeping.com/issues/2005-02/rhf/index.htm


--
Boomer

If You See Me Running You Better Catch-Up

Former US Army Bomb Technician (EOD)
Member; IABTI, NATEODA, WEODF, ISEE & IPS

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"Arjan Bakker" wrote in message
...
:
: ok you have a point there.
: Tomorrow I will get some fluid with PH 9 or 10.
: But it is still strange that I cannot calibrate a new probe,
: to ph4 !!
:
: I tried with an old probe, and I cannot get this one lower than 4.5,
: I then put some lemonjuice in water, tried the old probe and... 3.5.
: I guess there is something wrong with the calibration fluid provided with
: the probe!
: I'll try again tomorrow.
:
:
: "Pszemol" wrote in message
: ...
: "Arjan Bakker" wrote in message
: ...
: 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!!!)
:
: In an ideal world - maybe... :-)
:
: But real probe does not have linear characteristic
: so it is a curve. If you want acurate measurements
: you are interested in calibrating the meter as close
: to the measured value as possible...
:
: