When we feed a nutrient solution to our plants, one of the most
critical factors to get right is the nutrient strength. But there
seems to be a whole lot of confusion lingering around how to
measure the strength of your nutrient solutionespecially among
beginners.
The two major measurements in use today are:
EC  Electrical Conductivity
TDS  Total Dissolved Solids
EC
Let's start with the obvious: the more mineralbased nutrients
you add to some water, the more concentrated the solution becomes.
Pure water does not conduct electricity, but the more mineral ions
we add, the more readily it will conduct. Therefore, the electrical
conductivity (EC) of your nutrient solution is a fairly reliable
measure of how much nutrient is dissolved in it overall.
To measure conductivity we can use an EC meter, also known as a
conductivity meter. It has two electrodes that, when dipped in the
solution, measure its electrical charge by passing a small charge
between them.
It's important to measure the conductivity of your source water
(before you have added any nutrients or other additive products) 
this not only gives you the "baseline" measurement so you have an
idea of the purity of your source water, but it also gives you an
idea of what "room" there is left for additional nutrients.
If EC is Electrical Conductivity, what units is this
measured in?
You may have heard growers say things like "When my plants are
in full flower I feed them up to EC 2.2"  but 2.2 what?
The answer is Siemens, or more accurately, millisiemens. (One
millisiemen is one thousandth of a Siemen.)
There's no need to get your head twisted over this. Siemens are
to "electrical conductivity" what feet, yards, meters or inches are
to "length"  it's simply the unit of electrical conductance.
The important thing to get straight is that EC refers to the
scale (also known as the 'parameter') and siemens are the units on
this scale. EC is the most widely accepted measurement for the
strength of nutrient solutions, and is the standard in Europe and
many other parts of the world. The one notable exception is North
America which, for some reason, prefers to use the rather
cumbersome and vague alternative: TDS.
TDS
TDS stands for Total Dissolved Solidsit quantifies the
concentration of dissolved solids contained in a solution.
Proponents of TDS argue that it's a more suitable parameter than EC
for measuring nutrient concentration, since it measures by quantity
or weight rather through the implication of electrical
conductivity.
The problem with TDS measurements are they are great in theory,
but fairly absurd in practice. The only way of accurately measuring
the TDS of a nutrient solution is to evaporate all the liquid and
measure the residuethis would kind of defeat the point!
What is TDS measured in?
Remember, like "EC"  TDS is a scale, or a parameter, just like
length, temperature and volume. The unit of TDS is ppm (parts per
million.) A TDS reading of 60 ppm means there are 60 milligrams of
dissolved solids in each liter of water, or 60 mg/l.
So do TDS Meters work in a different way to EC
meters?
No.
TDS meters work in actually the same was as EC meters! Both
measure the electrical conductivity of the nutrient solution they
are dipped in. The difference lies in how the information is
displayed to you.
A TDS meter measures the electrical
conductivity in exactly the same was as an EC meter, but it simply
uses an inbuilt conversion factor to display the strength of the
nutrient solution in ppms.
It's these "conversion factors" that form the proverbial can of
worms. They can vary significantly from meter to meter.
Conversion Factors
TDS NaCl
NaCl is a conversion factor based on Sodium Chloride (regular
table salt.)
The conversion factor range is 0.47 to 0.5.
Nonlinear
meters based on NaCl typically use: 0.5 x the EC level (if
converting from µS to ppm or mS to ppt) or 500 x the EC level, if
converting from mS to ppm.
TDS 442™
442™ or Natural Water™ is a
proprietary scale based on properties of naturally occurring fresh
water. The 442™ part is an abbreviation of 40% sodium
sulfate, 40% sodium bicarbonate, and 20% sodium chloride.
The
conversion factor range is 0.65 to 0.85.
Nonlinear meters based on
442™ typically use: 0.7 x the EC level (if converting from µS to
ppm or mS to ppt) or 700 x the EC level, if converting from mS to
ppm.
TDS KCl
KCl is a conversion factor based on Potassium Chloride.
The
conversion factor range is 0.5 to 0.57.
Nonlinear meters based on
KCl typically use: 0.55 x the EC level if converting from µS to ppm
or mS to ppt) or 700 x the EC level, if converting from mS to
ppm.
TDS 640
A less popular conversion factor.
The conversion factor range is
0.64 to 0.67.
Nonlinear meters based on 640 typically use: 0.64 x
the EC level if converting from µS to ppm or mS to ppt) or 640 x
the EC level, if converting from mS to ppm.
Yes, four different
possible conversion factors means that four different meters that
give measurements in ppm may all give different readings from the
same solution! However, all EC meters should give the same reading
in the same solution as there's no conversion factor necessary.
I
know, I know … TDS sounds like a confusing thing  but it's really
just a measure of the total ions in solution. For every gallon of
water you have X mg's of stuff in it. If one of your friends starts
talking about their nutrient solution in terms of TDS, be sure to
find out what scale they are using. Many growers, especially in
Europe, in an effort to avoid confusion, use EC. If you are still
confused, contact the manufacturer of your nutrients and find out
what they recommend. Remember to ask them what TDS scale they use
if they give you dosages in terms of ppm.
Likewise, if you are
working with a TDS meter that only has a ppm display, remember you
need to be sure of the conversion factor being used. TDS comes into
its own when you need to measure individual elements in
applications such as nutrient and water quality, tissue analysis
results and soil analysis. Results from these laboratory tests will
give individual elemental readings in ppm or mg/l. Remember, a TDS
meter will only give you an approximation of the overall nutrient
concentration, based on the conversation factor used.
Below is a
table to show the relationship between the various methods of
displaying the strength of a nutrient solution.
EC (mS)

EC (µS)

TDS NaCl
(EC µS x 0.5)

TDS KCI
(EC µS x 0.55)

TDS 640
(EC µS x 0.64)

Natural Water™ 442
(EC µS x 0.7)

0.6

600

300

330

384

420

1.2

1,200

600

660

768

840

1.8

1,800

900

990

1,152

1,260

2.4

2,400

1,200

1,320

1,536

1,680

3.0

3,000

1,500

1,650

1,920

2,100

Jargon Buster
EC= Electrical Conductivity
TDS= Total Dissolved Solids
PPM= Parts Per Million
PPT = Parts Per Thousand
µS (or µS/cm)= microSiemens (one millionth of a siemen.)
mS (or mS/cm)= milliSiemens (one thousandth of a siemen.)
NaCl= Sodium Chloride (ECtoTDS conversion  EC x 0.5)
KCl= Potassium Chloride (ECtoTDS conversion EC x 0.55)
442= 442 Natural Water™ (ECtoTDS EC x 0.7) (The "442" is
an abbreviation for 40% sodium sulfate, 40% sodium bicarbonate and
20% sodium chloride.)
Making Sense of your Meter
Here are some popular TDS meters along with their conversion
factors, where applicable.
MAKE & MODEL

TDS FACTOR

BLUELAB


TRUNCHEON

Displays EC and both NaCL (0.5) and 442™ (0.7)

GUARDIAN

Displays EC and both NaCL (0.5) and 442™ (0.7)

HANNA


HI 98300

0.65

HI 98301 / 98302

NaCL (0.5)

GROCHECK COMBO
HI 981404N / HI 981405N

442™ (0.7)

HI 983301N

442™ (0.7)

HI 983301N/5

NaCL (0.5)

HM DIGITAL


TDS3

NaCL (0.5)

AP2

None  just measures EC

COM100

Displays EC and TDS (user can select NaCl, 442 or KCl
factors)

OAKTON


EcoTestr, TDSTestr11, PTTestr35

User Adjustable TDS Factor between 0.4 and 1.0
Default setting: 0.71

MILWAUKEE


MW401, MW402

NaCL (0.5)

T75, T76

NaCL (0.5)

C65, C66, MW301, MW302

None  just measures EC

MW801

NaCL (0.5)

MW802

0.65

NUTRIDIP


TriMeter 202

NaCL (0.5)

TriMeter 203

442™ (0.7)

Handheld

442™ (0.7)
