Any serious grower needs a sure fire method of accurately testing the strength of their source water, nutrient solution and run-off so they are equipped to respond to their plants’ ever-changing needs. So, to help us in our quest for perfect plant nutrition, we have all sorts of nutrient products, additives, meters, monitors, feed charts, calibration fluids and acronyms buzzing around our local grow store. You’ve probably heard people talk about the “EC” of their nutrient solution, or perhaps they talk in ppm (parts per million) – or they recommend you use a “TDS meter”. What’s with all this jargon? And how does it all fit together?

Well, we thought enough was enough. So we’ve put together this quick, no-nonsense and impartial guide to understanding how to measure the strength of your nutrient solution so we can all be clear about what we’re talking about – once and for all!

Worldwide, there is one standard parameter for measuring pH, but there are many more for measuring the strength of a nutrient solution. The two major measurements in use today are:

• EC  – Electrical Conductivity
• TDS – Total Dissolved Solids

EC

First, some basic concepts: when we add nutrients to water we create a nutrient solution. The more nutrients we add, the more concentrated the solution, and the more readily it will conduct electricity. So, the electrical conductivity (EC) of your nutrient solution can be seen as a quick and easy measure of how much nutrient is dissolved in it overall. Put another way, measuring the conductivity of a solution means measuring the electrically charged ions. Pure water will not conduct anything, but tap water already contains other minerals, metals and salts so it does conduct a small amount. Remember, it’s always important to measure your source water to see what you’re dealing with.
To measure conductivity we can use an EC meter, also known as a conductivity meter. It has two electrodes that, when dipped in the solution, measure its electrical charge by passing a small charge between them.

What is EC measured in?

Siemens are to “electrical conductivity” what feet or meters are to “length” – it’s the unit of electrical conductance. It’s important to get this distinction really clear in your head right now. EC is the scale (also known as the ‘parameter’) and siemens are the units. When dealing with the very low amounts of conductivity associated with EC in nutrient solutions, the preferred units are mS (millisiemens; one thousandth of a siemen) and µS (microsiemens, one millionth of a siemen) per centimeter.
EC is the most widely accepted measurement for the strength of nutrient solutions, and is the standard in Europe and many other parts of the world. The one notable exception is North America which prefers to use TDS.

TDS

TDS (Total Dissolved Solids) is the preferred scale for measuring the strength of a nutrient solution here in North America. It quantifies the concentration of dissolved solids contained in a solution. TDS is arguably a better parameter for measuring nutrient concentration, since it measures by quantity or weight.  In other words, you can have two glasses of water with equal parts TDS but different EC levels, since one glass may have more or less conductive elements (say salt vs. calcium.)
The problem is that a true TDS measurement is difficult to achieve (and would also defeat the purpose since evaporation is required).  Therefore, if one wants to eliminate the estimating that the conversion factor does, an EC meter is better.  If we lived in a perfect world, and every nutrient company and TDS meter used the same non-linear scale, a TDS meter is preferable.  But since there are so many different variables, an EC meter lends itself to more consistency.

What is TDS measured in?

Once again – make sure you get your head around this – TDS is a scale, or a parameter, just like time, length, temperature and volume. The unit of TDS is ppm (parts per million.) A TDS reading of 50 ppm means there are 50 milligrams of dissolved solids in each liter of water, or 50 mg/l.

How do TDS Meters work?

If EC meters (conductivity meters) work by measuring conductivity in a nutrient solution and expressing this in siemens, how to TDS meters work out how many parts of nutrient there are per million of water? Sorry to break it to you, but the answer is, they don’t.
TDS meters work in actually the same was as EC meters! Both measure the electrical conductivity of the nutrient solution they are dipped in. The difference is in how the information is displayed.
A TDS meter will measure the electrical conductivity, and then use a conversion factor to display the strength of the nutrient solution in ppms. Now here’s the slightly tricky bit. The conversion factor from EC to TDS varies from meter to meter.

Conversion Factors

TDS NaCl

NaCl is a conversion factor based on Sodium Chloride (regular table salt.)
The conversion factor range is 0.47 to 0.5.
Non-linear meters based on NaCl typically use: 0.5 x the EC level (if converting from µS to ppm or mS to ppt) or 500 x the EC level, if converting from mS to ppm.
TDS 442™

442™ or Natural Water™ is a proprietary scale based on properties of naturally occurring fresh water.  The 442™ part is an abbreviation of 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride.
The conversion factor range is 0.65 to 0.85.
Non-linear meters based on 442™ typically use: 0.7 x the EC level (if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.

TDS KCl

KCl is a conversion factor based on Potassium Chloride.
The conversion factor range is 0.5 to 0.57.
Non-linear meters based on KCl typically use: 0.55 x the EC level if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.

TDS 640

A less popular conversion factor.
The conversion factor range is 0.64 to 0.67.
Non-linear meters based on 640 typically use: 0.64 x the EC level if converting from µS to ppm or mS to ppt) or 640 x the EC level, if converting from mS to ppm.

Yes, four different possible conversion factors means that four different meters that give measurements in ppm may all give different readings from the same solution! However, all EC meters should give the same reading in the same solution as there’s no conversion factor necessary.
I know, I know … TDS sounds like a confusing thing – but it’s really just a measure of the total ions in solution. For every gallon of water you have X mg’s of stuff in it. If one of your friends starts talking about their nutrient solution in terms of TDS, be sure to find out what scale they are using. Many growers, especially in Europe, in an effort to avoid confusion, use EC. If you are still confused, contact the manufacturer of your nutrients and find out what they recommend. Remember to ask them what TDS scale they use if they give you dosages in terms of ppm.
Likewise, if you are working with a TDS meter that only has a ppm display, remember you need to be sure of the conversion factor being used. TDS comes into its own when you need to measure individual elements in applications such as nutrient and water quality, tissue analysis results and soil analysis. Results from these laboratory tests will give individual elemental readings in ppm or mg/l. Remember, a TDS meter will only give you an approximation of the overall nutrient concentration, based on the conversation factor used.
Below is a table to show the relationship between the various methods of displaying the strength of a nutrient solution.

Jargon Buster

• EC = Electrical Conductivity
• TDS = Total Dissolved Solids
• PPM = Parts Per Million
PPT = Parts Per Thousand
• µS (or µS/cm) = micro-Siemens (one millionth of a siemen.)
• mS (or mS/cm) = milli-Siemens (one thousandth of a siemen.)
• NaCl = Sodium Chloride (EC-to-TDS conversion – EC x 0.5)
• KCl = Potassium Chloride (EC-to-TDS conversion EC x 0.55)
• 442 = 442 Natural Water™ (EC-to-TDS EC x 0.7)  (The “442” is an abbreviation for 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride.)

Making Sense of your Meter

Here are some popular TDS meters along with their conversion factors, where applicable.

Towards A Clearer World

There is a drive towards some standardization in the hydroponics industry to create less head work for all concerned. Nutrient manfacturers, if you specify dosage with in ppms, please also state what TDS scale you are using. This includes calibration fluid!

by Everest Fernandez