Alternative EC/TDS Standards for GH Flora Series Nutrients
(aka Cal-O-Rama)

Last Updated: 4/20/2004

This article discusses the following abbreviations and terms commonly used with hydroponic nutrient solutions.

What started out in 1998 as a project between three growers to use the above terms interchangeably turned out becoming an in depth look at GH nutrients. While this article is specific to only GH nutrients it should be pointed out that by the time it ends it becomes clear that a Nutrient Profile, not a meter reading, is the only way to communicate the finer points of nutritive content when growers use dissimilar fertilizer products. Providing a nutrient profile removes any doubt as to the targeted nutritive value, this allows anyone with dissimilar products to custom mix their own products in a way that would most closely resemble the intended target profile, if not reproducing it exactly.

Unlike a TDS meter's ppm reading, a nutrient profile contains only the ppm for elements of nutritional worth, it does not consider non-nutritive ingredients which are not listed in the guaranteed analysis. A TDS meter, by definition, measures total dissolved solutes, which in addition to the nutritive elements would include buffers, dyes, fillers and preservatives in the fertilizer to name a few. Furthermore, a TDS meter is actually an electrical conductivity (EC) meter but with its circuitry modified to display ppm-like values, thus it can only provide an electronic estimate of ppm by using any one of several conversion values based on EC as opposed to using the actual weights derived from the percentages in the guaranteed analysis. For those reasons, metered TDS ppm will always be higher than the ppm of active ingredients for a nutrient profile as illustrated in the profile section of this table.

This article can be used as a guide for those without meters as a way to mix GH nutrients and feel as confident in the resulting concentrations as those using a meter. It can also be used by those with meters who believe they might not be getting the same results from their meter as others get from theirs.

If you have a question about what's going into your reservoir when you're mixing in your nutrients, about your meters accuracy, or about TDS versus EC, this should interest you, especially if you use GH nutrients. Although this is primarily geared to nutrient concentrations (TDS/EC), at the bottom of this section are some pH figures that resulted from mixing 5 ml of each GH component into 1 US gallon of water from various water sources.

The original goal was to find definite consistencies that could be relied upon from different people, with different water sources, using different meters, but who all use General Hydro nutrients. In other words, a way to rule out the major variables in comparing meter readings, such as hardware (meter type), method (calibration) and source water, and put the focus on getting the same measurable results from the nutrients. Since this was first published in March 1998, subsequent tests have showed tap source water to complicate meter reading comparisons in ways not anticipated, so the original goal has been modified to exclude source water taken straight from the tap, and now uses either distilled water or water produced through reverse osmosis.

Use the 5ml figures to compare meter readings.
Use the 1ml figures to predict concentrations.

It should be noted that results may vary considerably with TDS meters using a conversion value other than 0.70. This doesn't mean there is anything wrong with your meter. The most common generic meter conversion value is 0.50 x EC/uS = TDS/ppm, however, it has been recognized that fertilizer solutions have different properties than other solutions for which meters are also used, hence the industry has come to standardize on a 0.70 value for fertilizer solutions. If you don't know which conversion your TDS meter uses, and find your results differ considerably (usually a much lower reading) try using the Conversion Calculator below to resolve the differences.

To compare your meter results or calibration with those used to perform these tests, first calibrate your meter as usual, then measure the distilled or R/O water and note the reading, then mix 5ml of each GH component into one US gallon of distilled water and test the solution. Your meter should read approximately 910ppm or 1.30mS or 1300uS.

Use the 1ml figures for predicting concentrations before preparing a mix. For example, the second table below illustrates how to find the TDS/ppm for a GH mix consisting of 6ml/gal Micro, 12ml/gal Bloom, and 2.5ml/gal Grow.

Thanks to HooDoo and NFT, who were kind enough to go through the trouble to take measurements and report their findings more than once. Also, thanks to Potroast whose 0.50 meter and Grandmas antique measuring spoon led to the discovery of the issue of using tap versus distilled water.

It's important to understand, for your own reference, the results of the tests were originally determined using the NET ppm method. Simply put, the NET method was believed to be a way of using tap source water with a high ppm count, and by deducting the tap water ppm from the final ppm (after the nutrients were added) arriving at a figure which would equate to having used distilled source water (having a ppm of zero 0). The idea being to measure only the NET ppm, which would theoretically be those added only by the nutrients. The theory sounds good but the practice didn't pan out. Although high TDS source water will naturally yield a much higher final TDS, surprisingly the NET TDS was lower than that from actual distilled water tests. About 40ppm lower for the most part, but in one case 160ppm lower. Keep this in mind when a mix is prepared from straight tap water. For this reason distilled or RO water were used for the tests.

When conveying EC/TDS readings to others, the source water must be a factor since it can vary so widely. Suggesting to someone to mix in nutrients until their meter reads 600ppm, where your source water is 500ppm and his is 40ppm would lead to him mixing in 560% more nutrients than you intended him to.

Three meters were used with the tests, with either distilled or R/O water. One meter was a Hanna EC meter where mS readings were converted to ppm (TDS) using the current 0.70 conversion standard for comparison with the TDS meter displays. Another was an Oakton TDS meter, the third a Hanna TDS442. Both TDS meters employ the 0.70 conversion standard for fertilizer solutions. All meters were calibrated prior to the tests, and all readings were allowed to stabilize before recording them.

Each GH component was tested individually, as was the finished mix. 5ml volumes were used for the tests so that small differences in volume measurements would not be an issue. The finished mix consisted of 5ml of each component. Where 1ml figures are shown, they are simply one fifth of the 5ml figures. You may see some very small inconsistencies in some totals versus the sum of its parts, this is the result of values being rounded.

The average of all three meter readings were used. The rounded average was used for the 5ml figures, but not for the 1ml figures.

Since the measurement of the finished mix is probably slightly more accurate than that of each of the individual GH components (due to rounding functions in meters resolving at 10ppm) it could be said that the accuracy of the rounded average for the 5ml figure is +/-10ppm, and +/-2ppm for the 1ml figures. All figures are shown below for your reference.

Both European & US produced GH nutrients were used at the time of this determination. However, both products were labeled identically and European versions offered today may differ.

An interesting blurb from Hydroponic Food Production by Gordon Resh on salts and conductivity:
"Ammonium sulfate conducts twice as much electricity as calcium nitrate and more than three time that of magnesium sulfate, whereas, urea does not conduct electricity at all. Nitrate ions do not produce as close a relationship with electrical conductivity as do potassium ions (Alt, D. 1980). The higher the nitrogen to potassium the lower will be the electrical conductivity values for the nutrient solution."

Guaranteed analyses for brand name premixes often show subtle changes over time as a result of improvements or changes in labeling requirements. What was portrayed in this article are GH Flora formulas as they were being labeled on June 2003.

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