March/April – 1996
Story Title: Salinity Meters
Author: Steven Carruthers
Not all salinity meters are equal. STEVEN CARRUTHERS reports on the types of salinity meters commonly available on the market, their units of measurement, and offers some advice on how to calibrate and care for them.
In today’s hydroponic environment there are several types of electronic meters commonly available for measuring the salinity of water or a nutrient solution. The two most popular meters are electroconductivity (EC) and Total Dissolved Solids (TDS) meters.
Essentially, an EC meter measures 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. Microprocessor technology scales the measurement of electroconductivity into either milliSiemens/cm (mS/cm) or microSiemens/cm (mS/cm). EC meters are favoured by commercial growers, simply because they give the best estimate of the strength of a nutrient solution.
TDS is the concentration of a solution as the total weight of dissolved solids. These meters are widely used by hobbyists, and actually measure the electroconductivity of a solution. They do this by measuring the electric current between two electrodes. A greater concentration of nutrients will cause the electric current to flow faster than a solution with a lower concentration. Microprocessor technology uses an in-built conversion factor to scale the readout in so-called parts per million (ppm).
While there is no direct relationship between microSiemens and ppm for mixtures, ‘conversion factor’ can be applied to give a rough estimate of TDS.
An analogy that best describes this ‘conversion factor’ would be to consider the days when the car speedometer provided a unit of measurement for speed in miles per hour (mph). When Australia changed to the metric system, new vehicles expressed speed in both mph and kilmetres per hour (kph) for the first few years, and older models used ‘stick-on’ kph dials that gave a conversion between the two units of measurement, based upon an accurate’conversion factor’ of 1 mph = 1.609 kph.
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.
The difference with the speeedometer analogy is that the 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)
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.
When to Calibrate
When and how often should I calibrate my meter? You should calibrate your meter soon after purchase, or if it hasn’t been used for some time. You shouldn’t need to recalibrate again until it goes out of calibration. Like checking the car oil, it becomes a routine to check your meter, using a fresh calibration solution; not one that the laboratory technician guaranteed 2 years ago.
The recommended shelf-life for factory sealed calibration solution is 1 year, and 6 months to one year once it has been unsealed. If you need to blow the dust of the label in order to read the use-by-date, then its definitely too old.
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 – a film canister makes an ideal receptacle for decanting sufficient solution for calibration.
Calibration solutions should be stored in a dark cupboard, especially if it comes in a clear bottle. Exposed to sunlight, algae will soon develop and shorten the use-by-date of the solution.
The Importance of Temperature Compensation
Electroconductivity has a substantial dependence on temperature. Increases and decreases in temperature affect the nature of the ions and their ability to conduct an electric current – for every 10°C temperature change, the electroconductivity of a nutrient solution will change by 2% (Resh, H.M., 1991). More simply, small changes in temperature make a large difference in electroconductivity.
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.
Today, most electroconductivity meters have an Automatic Temperature Compensation (ATC) feature, which scale the readings to a standard temperature of 25°C. If the temperature deviates from 25°C, then the meter will automatically compensate for temperature changes experienced in the nutrient solution.
Temperature Compensation Table
Generally, manufacturers and resellers supply a Temperature Compensation Table with every purchase of calibration solution. It is usually printed on the label or found within the packaging. These tables give recommendations for calibrating meters for a wide range of temperatures. Most tables use a ppm conversion factor of 0.5.
The recommendations made in Table 2 and Table 3 are calculated using a ppm conversion factor of 0.64. These tables should serve as a useful reference guide for calibrating both EC and TDS meters.
Not all EC and TDS meters are suitable for all hydroponic applications. TDS meters that only measure up to 1999ppm, and EC meters that only measure up to 1990mS, limit the hydroponicist sphere of operation. I can think of several crops where the optimum electroconductivity exceeds these limits.
TDS meters suitable for soilless culture should have a range from 0 to 10,000ppm, and EC meters from 0 to 19.9mS (19,999mS). Sitest Pty Ltd has recently introduced dual range pocket meters. The TDScan10 gives an extended range from 0 to 10,000ppm with a good resolution, and the TDScan20 measures electroconductivity from 0 to 19,999mS/cm. Both meters have automatic calibration and temperature compensation features.
The popular Hanna pocket meters suitable for all hydroponic applications are the DiST2 for TDS, and DiST4 for EC. Both meters have automatic temperature compensation features. These meters only give two- or three-digit readings, and users need to multiply the reading by 100 or 10, respectively, to obtain a complete reading. The Sitest equivalent meters are the TDScan2 and TDScan4, respectively.
E.T. Grow Homes have the ‘Milwaukee’ range of pocket meters, which are identical to the Hanna range in design and function. The CON 610 (TDS) and CON 611 (EC) measure from 100 to 10,000ppm, and from 100 to 19,990mS/cm, respectively.
Brisbane-based TPS Pty Ltd 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.
Accent Hydroponics manufacture the Combo Dual pH/cF Monitor which can be fixed in the tank room to provide 24 hour direct current (DC) monitoring, or it can be used as an AC mobile meter in the field. Accent Hydroponics recently released its Australian designed and manufactured ph/cF dosing controller which also provides continuous monitoring and maintenance of pre-set levels.
NZ Hydroponics and Accent Hydroponics both manufacture stick-shape meters (the Truncheon and Salt Stick, respectively) that give readings in either ppm, cF or mS.
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.
Replace old batteries if the display reading, usually LCD, becomes faint. Pocket-meter batteries are rated for 70 hours, after which they suffer a drop in voltage. The meter may still give a bright LCD reading, but the accuracy of those readings should be challenged by re-testing the calibration of the meter – as the voltage drops, so will the accuracy of readings. Some meters have short connecting wires in the battery section, so exercise caution when replacing batteries.
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.
Using a neglected or abused meter is like trying to blast out of a sand bunker with a number 2 wood – it may eventually do the job but only after much non-scientific terminology has been uttered. The result is usually other than what is hoped for.
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.
For those who do not want to move away from TDS meters, they should recalibrate their meters using a factor of 0.64, or refer to a table that gives a closer estimate of TDS and Temperature Compensation values.
Check the age of calibration solutions.
Only use fresh solution to calibrate and test meters.
Whichever meter you choose, at the end of the day you need to know that your data is not flawed.
I would like to thank the following people and organisations for their assistance in the preparation of this article: Rick Donnan, Hydroponic Consulting Services; Dick Finlayson, University of NSW; Amir Antebi, Sitest Pty Ltd; Karen Patterson, Hanna Instruments (Aust) Pty Ltd; Eutech Cybernetics, Singapore; and Michael Schimcat, TPS Pty Ltd.