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Know your meter

Measuring Nutrient Strength

Instruments Australia's Conductivity meters - Electroconductivity (EC) and Total Dissolved Solids (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 electroconductivity than a solution with less ionic salts (nutrients). The higher the nutrient level the higher is the conductivity.

Microprocessor technology scales the measurement of electroconductivity into either milliSiemens/cm (mS/cm) or microSiemens/cm (uS/cm). Using inbuilt scaling the meters are also able to show the nutrient levels as TDS in parts per million (ppM). TDS is the concentration of a solution as the total weight of dissolved solids. (1 ppM = 1 milligram/liter)

EC 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 EC is exact. The total TDS is a mass estimate and is dependent upon the mix of nutrients as well as the concentration while EC is only dependent upon the concentration of nutrients.

Salinity meters are designed for sodium chloride (salt) solutions where the conversion factor is 1uS/cm=.5ppm. Because nutrient ions are on average much heavier than salt ions the true TDS is under estimated by about 30% in a typical hydroponics solution.

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 distingish between different types of ionic salts. Electroconductivity 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 electroconductivity as do potassium ions (Alt, D. 1980). Consequently, the higher the nitrogen to potassium in a nutrient solution, the lower the electroconductivity values.

TDS meters can only give a rough estimate of TDS. Two very different measuring standards have emerged, TDS and EC.

Millimho and micromho are commonly used by hydroponicists in North America and in 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 meter (dS/m) to measure electroconductivity, 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 an EC 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 not a recognized scientific measurement, it has at its basis 1mS/cm = 10cF. cF measurements were first introduced in the United Kingdom during the early development of NFT (Nutrient Flow Technique).

The conversion formula is only an approximation. For hydroponics applications it is recommended that the conversion formula TDS (in ppm) x 0.64 = EC (in uS/cm) be used.

Calibration Solutions

For hydroponics applications there are two requirements. The calibration solution should represents a value close to the expected electroconductivity 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. It is formulated to an electroconductivity of 2764mS at 25 oC. For salt solutions a ppm conversion factor of 0.5, hydroponics TDS users should use a conversion factor of 0.64 to determine ppM.

When to Calibrate

Meters should be calibrated soon after purchase or if it hasn't been used for some time. There is only a need to recalibrate the meter again if it goes out of calibration. Checking the Calibration should be a routine that uses a fresh calibration solution; not one that the laboratory supplied and guaranteed 2 years ago.

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.

Calibration solutions should be stored in a dark place. Components have been added to kill algae which develop in KCl when it is exposed to sunlight.

The Importance of Temperature Compensation

Electroconductivity requires mobile ions in solution, when the mobility rises because of increases in temperature the conductivity measured also rises. For every 10 degrees C temperature change, the electroconductivity of a nutrient solution will increase by 2% (Resh, H.M., 1991).

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.

Our conductivity meters have an Automatic Temperature Compensation (ATC) feature, which scale the readings to a standard temperature of 25 degrees C. When the temperature deviates from 25 degrees C, then the meter automatically compensates for temperature changes experienced in the nutrient solution.

Meter Ranges

TDS meters suitable for soilless culture should have a range from 0 to 10,000ppm, and EC meters from 0 to 19.9mS (19,999uS).

Brisbane-based Instruments Australia manufacture the multi-range, hand-held LC81 and LC84 for agricultural applications. They also manufacture the purpose-designed HP2-DS dual dosing controller for hydroponic applications. The latter monitors and maintains pre-set levels.

Meter Life-Span

The expected life-span of a meter is as long as you continue to get accurate results. An experienced grower will monitor crop development, and keep a log book. Re-testing the meter using fresh calibration solution should be a priority if any inconsistencies are noticed.

Leaving instruments to fry on the car dashboard, and wiping probes with a dry rag are certainly not recommended. This kind of treatment will alter a meter's accuracy, and reduce its longevity. Meters should be stored in a cool, dry place when not used, and the stainless steel electrodes cleaned in alcohol at least monthly. The probes should be dry prior to calibration.

Ideally, hydroponicists should adopt EC meters as the standard meter for measuring electroconductivity of nutrient solutions. These meters use the accepted units of measurement for electroconductivity, they are more accurate than TDS meters, and they are supported by scientific and hydroponics literature.

For those growers already using EC meters, they need only worry themselves with temperature compensation adjustments during the calibration procedure.

Check the age of calibration solutions. Only use fresh solution to calibrate and test meters.

Acknowledgements (Reference Practical Hydroponics & Greenhouses)

Prepared by Trevor Croll,

Australia, 42 Pearse Street, Keperra, Queensland Australia, 4054.
Phone 61 7 3855 1115