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Hydroponics PPM & EC Explained

It’s important to understand how hydroponics metering works It’s important to understand how hydroponics metering works

You use a hydroponics total dissolved solids (tds) or parts per million (ppm) meter to measure the amount of dissolved materials in your nutrients water, and it’s crucial for your crops that you get the amounts right. When you see meters discussed, you notice that the terms “ppm,” “EC” or  “cF” are used. These are different standards for representing the concentration of solids in a solution:


Parts Per Million (PPM)

Parts per Million (ppm) measures the concentration of solids in a solution. The higher the ppm, the more nutrient salts there are in the solution. However, ppm is an incomplete indicator of nutrient appropriateness; it does not tell you how much of each nutrient you have. PPM is sometimes called total dissolved solids (tds). This is because ppm is actually a unit for measuring tds. The two terms are considered interchangeable in hydroponics.

Electrical Conductivity

Electrical conductivity (EC) is a measure of the ability to conduct electricity. Pure water does not conduct electricity very well if at all Water only conducts when salts are dissolved in it. When you dissolve nutrients into water, the nutrient salts break down into electricity-conducting particles called ions.

These ions make water a good conductor. The more nutrients you add, the more the solution conducts electricity. Therefore you can estimate how much nutrient is in the solution by measuring its EC.

There are several units for measuring EC. The most common one is microSiemens/centimetre (µS/cm), or just µS for short. Sometimes you will see micromhos/centimetre (µmho/cm) in older literature. It is the same thing as microSiemens/centimetre. One µmho is one µS.

EC is measured with a “conductivity meter.” A conductivity meter measures how well a solution conducts electricity. In hydroponics it is used to estimate the amount of nutrients (PPM) in your nutrient water.

The EC of a solution changes when the temperature changes. If you take EC measurements of the same solution at different temperatures, you will get different EC/ppm readings. Fortunately, most conductivity meters are built to automatically adjust for temperature differences so they give you accurate readings regardless of temperature. Find out about that before you buy a meter.

Conductance Factor

Conductivity Factor (cF) is another way of saying EC. It is more common in older measuring equipment and rarely used in conductivity meters. CF uses a scale of 0 to 100, where 0 is the cF of pure water. CF is simply EC (mS/cm) multiplied by ten.  One cF is 0.1 mS/cm and one mS/cm is ten cF.

You can multiply the EC reading by a conversion factor (aka conversion scale) to get the ppm reading. There are several conversion factors commonly used. Generally the factor is within the range between 500 and 900. Most manufacturers use 700 for conversion.

For example, if your EC reading is 1.

1 µS/cm x 700 = 700 ppm.

And if you want to convert a ppm value into EC, simple do the reverse.

For example, if the ppm value is 700.

700 ppm/ 700 = 1 µS/cm.

PPM meters and EC meters are both conductivity meters. A ppm meter is actually an EC meter that measures the EC of a solution, and then converts the reading to ppm using a conversion factor.

The conversion from EC into ppm by a ppm meter is really only an approximation.  You can safely use a ppm meter and rely on its ppm readings only if it is factory-programmed to use the same conversion scale as the nutrient manufacturer used when they wrote their nutrient instructions.

Hydroponics Nutrients Strength

That’s why there’s a push for standardizing ppm and EC measurements into one universally accepted EC standard and some people tell you that an EC meter is a better investment than a ppm meter. EC meters are more versatile because they more easily adapt into different conversion scales. You simply multiply the EC reading by whatever conversion factor the reference material uses.

The bottom line is that the concentration of nutrients salts in your nutrient water is a big factor that determines how healthy your plants are. Help your plants by contacting your nutrients manufacturer and asking them what type of metering scale and nutrients strength ratings they recommend for your crops. In future articles, we’ll discuss more about metering and managing your nutrients water to maximize your crop growth/yield and preserve crop health.

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Last modified on Friday, 09 November 2012 09:48

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