Know your Conductivity meter
Measuring Nutrient Strength
TPS Conductivity and TDS meters measure the ability of an aqueous solution
to carry an electric current. It does this by measuring the electric
current between two electrodes (the electricity flows by ion transport). A
nutrient-rich solution will have a higher conductivity than a
solution with less ionic salts (nutrients). The higher the nutrient level
the higher is the conductivity.
Microprocessor technology scales the Conductivity measurement
into either milliSiemens/cm (mS/cm) or microSiemens/cm (uS/cm). Using a
mathematical formula, the meters are also able to show the nutrient levels as
TDS in parts per million (ppM) or parts per Thousand (ppK). TDS is the concentration of a solution as
the total weight of dissolved solids. (1 ppM = 1 milligram/litre and 1 ppK
= 1 gram/litre).
Conductivity meters are favoured over TDS by commercial growers, simply because
they give the best estimate of the strength of a nutrient solution. TDS is
a "rough" estimate while Conductivity is exact. The total TDS is a mass
estimate and is dependent upon the mix of nutrients as well as the
concentration while Conductivity is only dependent upon the concentration of
nutrients.
The true ppM conversion factor is complicated by factors, including the
type of ionic salts present in a nutrient solution, their concentration,
and the temperature of the solution.
Our meters are capable of compensating for temperature. No meters have
ability to distinguish between different types of ionic salts. Conductivity measurements are also complicated by the fact that not
all salts conduct an electric current equally. Ammonium sulphate conducts
twice as much electricity as calcium nitrate, and more than three times
that of magnesium sulphate (Resh, 1989). Also, nitrate ions do not
produce as close a relationship with conductivity as do potassium
ions (Alt, D. 1980). Consequently, the higher the nitrogen to
potassium ratio in a nutrient solution, the lower the conductivity
values.
Millimho and micromho are commonly used by hydroponicists in North
America and in earlier scientific literature. The basis for this unit came from
the ohm, which is the unit used to measure electrical 'resistance'. A 1
ohm resistance with 1 Volt across it will conduct 1 Ampere of electrical
current. The electrical equation is V (volts) = I (ampere) * (times) R
(resistance) where R is measured in ohms. The reciprocal of resistance is
'conductance', with the mho (ohm spelt backwards) used to describe
conductance.
The scientific literature adopted Millimho per centimetre (mmho/cm) and
micromho per centimetre (mmho/cm), where 1mmho/cm = 1000 mmho/cm.
The metric equivalent for mho is Siemens, where 1mho/cm = 1mS/cm =
1000uS/cm. The metric system is used extensively throughout Europe,
South Africa, Australia and New Zealand.
For hydroponics, scientific literature generally uses deciSiemens per metre
(dS/m) to measure conductivity, with milliSiemens/cm (mS/cm) and
microSiemens/cm (S/cm) the established and accepted units of measurement
for soilless culture, where 1dS/m = 1mS/cm = 1000uS/cm as measured
by a Conductivity meter.
An old unit of measurement is cF (Conductivity Factor). These meters
use a scale of 0 to 100, where 0 represents pure water (zero ionic salts).
cF is not a recognised scientific measurement, and it has as its basis 1mS/cm
= 10cF. cF measurements were first introduced in the United Kingdom
during the early development of NFT (Nutrient Flow Technique).
Calibration Solutions
For hydroponics applications there are two requirements. The
calibration solution should represents a value close to the expected
conductivity of a nutrient solution. The solution should use the
same or similar types of ionic salts known to be in the nutrient solution.
The calibration solution most suitable for hydroponics applications is
the KCl (potassium chloride) Standard. Standard values used in hydroponics
applications are 2760 uS/cm or 1413 uS/cm at 25 OC. TDS
standards are also available from TPS. Check out our Standard
Solutions page.
When to Calibrate
Meters should be calibrated when first used, and then regularly
thereafter. Whilst conductivity sensors are generally very stable long
term, a regular calibration check will ensure your system is operating
correctly.
Checking the calibration should be a routine
that uses a fresh calibration solution. The recommended shelf-life for a factory sealed calibration solution is
1 year. Do not mix used calibration solution with new solution. Once the
meter has been tested and/or calibrated, the solution should be discarded;
not returned to the reagent bottle.
Remember - the accuracy of your measurements can determine the success
or failure of your crop. Regular calibration checks in accurate standards
may avert disaster !
The Importance of Temperature Compensation
Conductivity requires mobile ions in solution, when the mobility
rises because of increases in temperature the conductivity measured also
rises. For every 1OC temperature change, the conductivity
of a nutrient solution will increase by around 2% (Resh, H.M., 1991).
This temperature coefficient varies with the type of salts in the
solution, the concentration of those salts and the temperature itself.
When calibrating meters, the calibration solution temperature should be
as close as possible to the nutrient solution to be tested, to minimise
temperature effected errors.
TPS conductivity meters have an Automatic Temperature Compensation (ATC)
feature, which scale the readings to a standard temperature of 25OC. When the temperature deviates from 25OC, then the meter
automatically compensates for temperature changes experienced in the
nutrient solution.
TDS meters suitable for soilless culture should have a range from 0 to
10,000ppM, and Conductivity meters from 0 to 20mS/cm (20,000uS/cm).
