Be aware that pH meters require constant up-keep (cleaning, calibrating
and correct storage) but even then may not give reliable readings (see
section on “Accuracy Problems with pH Meters”).
Colour indicators work on the principle that the colour produced by the
particular dye used in the indicator formulation is dependant on the pH of
the solution (Fig 1.3).
The test is conducted by removing a small sample of nutrient from the
nutrient tank, adding a drop of the indicator, mixing and then comparing
the final solution colour with those on a coloured reference chart. Using
Flairform’s pH test kit as an example, within the optimum hydroponic pH
range (5.0 to 6.5), the colour chart ranges from pink (pH 5.0), to orange
(pH 5.5), to yellow (pH 6.0) and through to pale green (pH 6.5).
Therefore, experience shows if you are aiming to adjust pH to 5.5 (orange)
then an accuracy of +/- 0.2 is achievable. Because of their fundamental
accuracy, reliability and easy of use, wide range pH indicators are the
preferred method for measurement of pH in hydroponics.
Note that pool and aquarium pH indicators are usually not suitable for
hydroponics because unlike Flairform's indicator, they produce the same
colour at pH 4, 5, and 6 and therefore cannot warn of the need to add pH
UP when the true pH is, say, 4.
Taking pH readings:
Step 1. Before measuring the pH
ensure that the nutrient is well stirred, especially after pH UP and DOWN
are used. This is one of the most common mistakes made when testing pH
(or conductivity). Also, ensure that the sampling container is clean.
Step 2. Using the sampling vial,
remove a small sample of nutrient from the nutrient tank, add a drop of
the indicator, mix, then compare the final solution colour with those on
the coloured reference chart.
Next section...
