if pH electrodes are properly calibrated, do not assume they are always
accurate. The reality is that different pH meters occasionally give
different readings on a given sample even when each is calibrated on the
same set of buffers.
The existence of pH meter error can be
determined by measuring a nutrient sample using two different pH meters.
With this exercise it is not uncommon to get pH readings that differ by up
to 0.5 pH units. This occurs because some electrodes can respond to other
'species', such as sodium, which are electrochemically similar to hydrogen