Meters that use total dissolved (ionic) solids (TDS) measure conductivity and multiply the reading by a fixed or adjustable "TDS factor" to determine TDS. TDS values are usually expressed as parts per million (ppm) or ppt (parts per thousand). There are many limitations when using TDS. First, the TDS factor used is salt specific so if there are multiple or unknown salts in solution, its nearly impossible to determine the correct factor to use. Second, since ionic concentrations are not linear, the TDS factor changes with concentration. TDS values are generally not considered with low conductivity values.
Conductivity is greatly influenced by temperature. Most fluids increase in conductivity as temperature increases. Most ionic solutions will increase about 2% for each 1°C increase. Unfortunately, this temperature coefficient (TC) is not linear. In the case of high resistance water it can be closer to 5% or so per °C.
Many instruments adjust the conductivity value based on a TC and display a value that is said to be corrected or normalized to 25°C. The meter will automatically make corrections to the reading and display a value as if the sample was 25°C, no matter what the actual temperature is. Some instruments use a fixed TC of 2.0% per °C. Let us consider a meter that uses 2.0% TC to measure a 1413 µS standard at 25°C (77°F). If the standard is warmed to 30°C (86°F), the meter applies a correction of 5 degrees x 0.02% x 1413 µS = 141.3. Without correction (0.0% TC) the actual value of a 1413 µS standard of KCl at 30°C (86°F) is 1548 µS. As the meter corrects for temperature, it displays a value of 1548 minµS 141.3 = 1407 µS. When the sample cools to 25°C, it will again read 1413 µS as no correction is applied. Although conductivity cell response is immediate, temperature corrected values will fluctuate as the temperature measurement stabilizes.
Advanced meters offer adjustable TCs, usually from 0.0% to as much as 10% per °C. This is a beneficial feature for two reasons. First, by adjusting the coefficient to zero, non-compensated measurements can be recorded. This eliminates the possibility of using an incorrect TC. Methods such as United States Pharmacopeia 23 specifically call for non-compensated measurements. Second, by using a TC of zero, the ideal TC for a sample can be determined by performing tests of the conductivity values at various temperatures. Once the TC of the sample fluid is established, it can be entered into the meter for automatic temperature correction.TC labels on conductivity calibration standards often provide a temperature table listing conductivity values at different temperatures. Conductivity meters with a fixed TC should be calibrated to the conductivity value at the meters normalization temperature, usually 25°C. Calibration to values other than the normalization temperature would only be appropriate if the meter did not utilize temperature correction, or if the TC was adjusted to 0.0%. As a general rule its always best to calibrate and measure as close to 25°C as possible when a TC is applied. Recording the temperature during calibration and measurement is good practice.
Another feature of advanced meters is a selectable normalization temperature. This allows temperature compensated readings to be adjusted either to 25°C (77°F) or another value, usually 20°C (68°F). The advantage here is that 20°C (68°F) is often closer to the actual sample temperatures than 25°C (77°F). When using a normalization temperature other than 25°C it is important to calibrate to the appropriate value of the conductivity standard at the specified normalization temperature. For example, a 1413 µS standard at 25°C should be calibrated to its value at 20°C which is 1278 µS.
While emphasis is given to the conductivity accuracy, it is important not to neglect the temperature accuracy. Although temperature is directly related to conductivity measurement it is often overlooked. Meter temperature accuracy should be verified and calibrated if necessary prior to conductivity calibration.
Similarly, the TDS meter electroconductivity measurement is scaled to give a readout termed ppm. Most meters are factory calibrated using a conversion factor of 1mS/cm=500ppm, where 2mS/cm=1ppm.
ppm conversion factor is not accurate. It varies considerably, depending upon the solution being tested. In fact, TDS meters using the factor of 1mS/cm=500ppm under-estimate the true TDS by about 30% of a typical hydroponic solution.
The true ppm conversion factor is complicated by many factors, including the type of ionic salts present in a nutrient solution, their concentration, and the temperature of the solution.
Most meters are capable of compensating for temperature, but they do not have the 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.
With all these factors at play, it's easy to understand why TDS meters can only give a rough estimate of TDS. According to industry sources, TDS meter sales currently represent about 70% of all salinity meters sold in the Australian hydroponics industry. As a result, two very different measuring standards have emerged, making the choice of a meter something of a Pandora's Box.
ppm Conversion Factor
Prior to the 1960s, there were no international agreements in place as to which was the best unit to use to measure electroconductivity. Consequently, the scientific literature adopted millimho per centimetre (mmho/cm) and micromho per centimetre (mmho/cm), where 1mmho/cm = 1000 mmho/cm. The basis for this unit came from the ohm, which is still used to measure electrical 'resistance'. The reciprocal of resistance is 'conductance', with the mho (ohm spelt backwards) used to describe conductance. Millimho and micromho are still commonly used today by hydroponicists in North America.
The metric equivalent for mho is Siemens, where 1mho/cm = 1mS/cm = 1000mS/cm. The metric system is used extensively throughout Europe, South Africa, Australia and New Zealand.
In 1960, the 'Systeme Internationale D'Unites' (SI) adopted the the metric system of measurements, incorporating the recommendations of the 11th General Conference on Weights and Measures, which sought to extend the metric system of electrical units - rapid advances in science and technology fostered the development of several overlapping systems of units of measurements as scientists worldwide improvised to meet the practical needs of their disciplines.
Today, the scientific literature uses deciSiemens per metre (dS/m) to measure electroconductivity, with milliSiemens/cm (mS/cm) and microSiemens/cm (mS/cm) the established and accepted units of measurement for soilless culture, where 1dS/m = 1mS/cm = 1000mS/cm as measured by an EC meter.
Another popular unit of measurement used by hydroponicists worldwide to describe electroconductivity is the cF (Conductivity Factor) unit, as measured by a cF meter. These meters use a scale of 0 to 100, where 0 represents pure water (zero ionic salts). This is not a recognised scientific measurement, but rather a measurement of electroconductivity that uses milliSiemens as its basis, where 1mS/cm = 10cF.
cF measurements were first introduced in the United Kingdom during the early development of NFT (Nutrient Flow Technique). Today, cF units are widely used by commercial growers in Australia and New Zealand.
Over recent years, TDS meters have become popular among hobbyists. These meters are factory calibrated using a ppm conversion factor of 0.5, where 2mS/cm = 1 ppm. However, a closer estimate for hydroponic applications uses a ppm conversion factor of 0.64. According to Handreck (Growing Media, 1987), the conversion formula is only an approximation, but it is usually good enough to remember that TDS (in ppm) is approximately 2/3 EC (in mS/cm). To convert TDS readings to electroconductivity measurements, the following formula should be used:
TDS (in ppm) x 0.64 = EC (in mS/cm)
For example:2000 ppm x 0.64 = 1280 mS/cm (or 1.28 mS/cm)
Calibration Solutions
Not all calibration solutions are equal, and choosing the most suitable calibration solution for hydroponic applications could translate into better quality produce.
Basically, there are four 'types' of calibration solution available on the market, each using a Standard that has specific applications in mind (see Table 1).
For hydroponic applications, the appropriate calibration solution should be based upon two conventions. Firstly, it should represents a value close to the expected electroconductivity of a nutrient solution; secondly, 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 hydroponic applications is the KCL Standard, which is generally formulated to an electroconductivity of 2764mS at 25°C. While commercial manufacturers recommend a ppm conversion factor of 0.5, TDS users should use a conversion factor of 0.64 to calibrate meters.
For example:2764mS x 0.64 = 1769ppm at 25°C.
The difference of some 380ppm (2764mS x 0.5 = 1382ppm) could translate into better quality produce.
