EC readings can be used to determine the approximate TDS (‘total dissolved
salts’) of various solution types. However, growers must be aware of
the limitations with this method.
TDS is calculated by multiplying
the EC by a ‘conductivity factor’ - this derives the approximate concentration
of salts in 'ppm' (or mg/l). However, because the EC
is dependant on the nature of the
salts present, we must
use a specific conductivity
factor which matches the type
of solution being tested. For example:
Soft
'natural' waters^: Multiply the conductivity by the factor listed in
the second column of Table 2.20c. The factor depends
on the conductivity units employed by your meter. Hence if a 'natural'
water sample has an EC of 2.0mS/cm, we can approximate that the TDS is
1,100ppm
(2.0mS/cm x 550).
^
e.g. uncontaminated scheme water, bore water, swamp water.
Hard/ Alkaline waters: These waters (e.g. as found in Western
Australia’s North West, South Australia etc.) require a conductivity factor of around 650 (when measured in mS/cm).
Inorganic nutrients:
The typical Grow and Bloom hydroponic nutrient formulations have much higher
concentrations of ions like potassium and phosphate. Consequently,
these solutions require a much higher factor than the 'waters' mentioned above
i.e. ~800 when measured in mS/cm.
To further illustrate how
important it is to consider the type of solution being tested, note that
many
high phosphate flowering additives will require a conductivity factor of ~1,100 (when measured in mS/cm).
The problem with TDS in hydroponics?
It is not uncommon to find some hydroponic operators assigning a TDS value
of about 1,400ppm to the common 2.76 mS/cm conductivity standard i.e. they
have used a conversion factor of 500 - a factor which is only valid for
relatively pure sodium chloride waters. As discussed
above, 800 is a more realistic factor for hydroponics - because many
nutrient mixtures of around 2,200 ppm yield a conductivity of 2.76 mS/cm.
Additionally, quoting TDS values derived from conductivity readings, and
without quoting the factor used, can imply different conductivity values
to different people i.e. depending on what conversion factor they favour!
Consider the following scenario
You calibrate your TDS meter in a 2.76mS/cm Conductivity Standard to
1,800ppm. Using the same Conductivity Standard, your
friend calibrates his meter to 1,400ppm. If you then give your
friend a nutrient strength recommendation he will add 30% more
nutrient than what you intended!
In summary: Conductivity factors which attempt to accurately
convert conductivity readings into TDS must be customized to suit both the
concentration range and ratios of the particular salts present in the
water being analyzed. This requires prior knowledge of the
sample's composition and, logically, such an effort would be
counterproductive. Thus the inevitable conclusion is that the general use
of conductivity to determine TDS is, at best, only an approximate method.
Remember, the fundamental property being measured is conductivity. TDS is
a derived value and employs guesswork re the composition of the solution.
For these reasons, in the water
industry it is fast becoming international practice to simply quote
electrical conductivity (EC) values and not TDS.
