EC readings can be used to
determine the approximate TDS (‘total dissolved salts’) of various aqueous
solutions. However, growers must be aware of the dangers of dosing on the
basis of TDS rather than EC.
TDS is calculated by multiplying the EC by a
‘conductivity factor’ - this gives the approximate concentration of salts
in 'ppm' (or mg/L). However, because the EC is dependant on the
nature of the salts present,
for accurate results it is important to 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 18.40. The
factor depends on the conductivity units specified on the meter. Hence if
a 'natural' water sample has an EC of 2.0mS/cm, it can be estimated 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
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 heavier ions like potassium and phosphate. Consequently,
these solutions require a 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, many high phosphate “PK”
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. This
factor 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 (true) yield a conductivity of
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 use!
following scenario: You calibrate your TDS meter with 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 in TDS, he will add 30%
more nutrient than what you intended!