HM Digital, Inc.

TDS and EC meters for hydroponics explained

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Courtesy of HM Digital, Inc.

As technology advances, questions inevitably follow. Due to the rapid growth of the hydroponics and indoor gardening industry, it is important for growers to be familiar with the technology, in this case instrumentation.

There is a myriad of nutrients and fertilizers available to help you cultivate the best possible plant yields, but what is consistent among all of them is they need to be measured to ensure proper dosage. Equally important is the measurement of water to know what you’ll be mixing with, and there is no better way to measure nutrient solution or water than with a digital TDS (Total Dissolved Solids) or EC (Electrical Conductivity) meter.

There are times when the instructions for a nutrient don’t correspond with the instructions for a meter. This can certainly be frustrating, but refrain from pulling out your hair just yet. This article will help you navigate the challenges.
These meters can be found under the following names: nutrient meters, “ppm” meters, TDS meters or conductivity meters.

Let’s get to the facts. These meters are either TDS meters or conductivity meters, with the majority being TDS meters. There is no such thing as a “ppm meter,” so strike it from your lexicon immediately. PPM is an acronym for “parts per million,” a scale used for parameters, including TDS, chlorine, carbon dioxide, etc. We’ll address scales and parameters in a moment.

These meters measure your nutrients, but they are really measuring substances in water, whether a beneficial nutrient or a harmful metal. Other industries aside from hydroponics use these products as well for fish, pools, industrial processes and perhaps most importantly, your drinking water. So instead of referring to the meters as nutrient meters, most instrument companies will often refer to the meters by their true names: TDS or conductivity meters.

Parameters and Scales

Water testing is scientific. The majority of us would rather be harvesting our bountiful crops than remembering our high school science, but if you’re spending your hard-earned cash on an expensive instrument, take the time to learn the basics of the science. It’s easy and will help you get the most out of your meter.

A parameter is the characteristic of a substance that is being measured. For the purpose of this article, it’s either TDS (Total Dissolved Solids) or EC (Electrical Conductivity). Other parameters you could be testing for are pH, temperature, light and humidity.
A scale is a particular range applied to the measurement of that parameter. For example, temperature is a parameter. Fahrenheit or Celsius is a scale.

TDS is most commonly measured in ppm (parts per million), PPT (parts per thousand) or mg/l (milligrams per liter). EC is most commonly measured in µS (micro-Siemens) or mS (milli-Siemens).

If your nutrient mix is calling for 1.0 EC, this is an incorrect determination, and you may find this confusing when using a meter. What this is most likely referring to is an EC reading of 1.0 mS; 1.0 mS equals 1000 µS.

Why are there so many ways to measure nutrients?

Our industry lacks standardization, the fault resting on the instrument manufacturers and the nutrient companies. Rest assured there is a movement to standardize everything, but until a standard exists, the best way to look at it is as a cooking recipe.

Open any cookbook and you’ll find that some ingredients call for teaspoons, others call for ounces and others call for cups. Each requires a different measurement tool. Your nutrient mixture is a recipe, so it’s important to use the right measuring tool.

What’s the deal with EC and TDS? What are they and is there a difference?

TDS and EC, while related, are two different parameters.
TDS stands for Total Dissolved Solids, and in layman’s terms, is essentially any inorganic, dissolved substance in water, other than the pure H2O. TDS includes salts, minerals or metals dissolved in water. As mentioned above, TDS is most commonly measured in ppm on an ascending scale. The higher the ppm, the more TDS there are in the water, with pure water being 0.0 ppm. Pure water can be obtained through filtration, purification, distillation or collecting raindrops on your tongue. TDS does not include particles that are floating in water. (These are known as suspended solids).

Conductivity, or electrical conductivity (EC), is the ability of a substance to conduct electricity. While you may not typically equate water with electricity (unless you’re at the Hoover Dam), most elements other than hydrogen and oxygen will conduct electricity to a certain degree, no matter how minute that conductance might be. Therefore, since water is two parts hydrogen and one part oxygen, measuring conductivity is an easy and quick method of determining the purity level of water.

Now that we understand TDS and conductivity, that brings us to the next burning question: How is TDS measured?

TDS is measured as weight (mg/L) or quantity (ppm). As discussed above, ppm stands for parts per million, so if you add 2,000 ppm of nutrients to pure water, that means that out of 1,000,000 parts of water, 2,000 of those parts are the nutrients. Or for the mathematically inclined, that’s 0.2 per cent. The only true method of measuring TDS is to weigh a water sample, evaporate the water and then weigh the remains. Not only is this very difficult to do, evaporation would defeat the purpose of measuring nutrients in a solution. Therefore, companies have developed TDS meters that determine the nutrient content and display that amount on a digital screen for ease-of-use.

What are TDS meters and how do they work?

A TDS meter works by measuring the electrical conductivity of the water. As mentioned earlier, most elements, other than the hydrogen and oxygen, will conduct electricity to a certain degree. A TDS meter will measure the EC and convert that electrical charge to an estimated TDS level, thereby telling you the nutrient quantity.

EC is measured by determining the amount of electrical charge between two sensors. The greater the charge, the higher the EC (and the higher the TDS level).

What’s a conversion factor and why are there so many of them?

Since TDS meters work by first measuring the EC level, that EC level needs to be converted to a TDS level (hence a conversion factor). A conversion factor will allow the meter to make the best possible estimate of the true TDS level. Since there are different types of water in the world that are more or less conductive than others, it’s necessary to use different conversion factors for different water. For example, due to the abundance of salt, seawater will be far more conductive than fresh water.

So then should I use a TDS meter or an EC meter?

Technically, when measuring nutrients, it’s better to measure the TDS level, but don’t forget that the true method of measuring TDS is very difficult and TDS meters are actually measuring the EC and then converting that EC to TDS. Confusing? Perhaps. The answer to this question is: use the meter (and conversion factor) that is recommended by the nutrient company. If your preferred brand of nutrients recommends you measure the nutrients using EC, then buy an EC meter. You’ll have much more accurate results than if you purchase a TDS meter and attempt to do the math. If you don’t know which meter you should use, give your nutrient company a call and ask.

The lowdown

Nutrient solutions are best measured with TDS or EC meters. TDS meters will give you an accurate nutrient level in water, expressed in ppm (parts per million). Ideally, a grower should filter his or her water down to a pure or near-pure level, prior to adding the nutrients. Pure water will be 0.0 ppm. Therefore, you will know what you’re starting with when adding the nutrients and have the perfect solution. If you do not have access to a water filtration or purification system, you can use tap water, but you will need to add the appropriate TDS level of the nutrients to the TDS level of the tap water. For example, if your blooming stage nutrient calls for adding 1,500 ppm of nutrients and your tap water is 250 ppm, you should be looking for a nutrient solution level of 1,750 ppm.

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