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GrowFAQ :

What is the difference between ppm and EC?

  Added by: pegs  Last edited by: snoofer  Viewed: 134 times   Rated by 23 users: 9.75/10
Total Dissolved Solids (TDS) is the best measurement of the nutrient concentration of a hydroponic solution. To estimate TDS, one can use a meter that measures the Electric Conductivity (EC) of a solution, and convert the number to TDS in parts per million (ppm). Many meters will do this conversion.

Total dissolved solids (TDS) is typically expressed in parts per million (ppm). It is a measurement of mass and determined by weighing, called a gravimetric analysis. A solution of nutrients dissolved in water at a strength of 700 ppm means that there are 700 milligrams if dissolved solids present for every liter of water. To accurately calculate total dissolved solids (TDS), one would evaporate a measured filtered sample to dryness, and weigh the residue. This type of measurement requires accurate liquid measurement, glassware, a drying oven, and a milligram balance. Example: 50 mL of the 700ppm solution would leave 35 mg of salt at the bottom of a crucible after drying.

Electrical Conductivity (EC) is expressed in siemens per centimeter (s/cm) or milliseimens per centimeter(ms/cm). It can be determined with an inexpensive hand held meter. Nutrient ions have an electrical charge, a whole number, usually a positive or negative 1, 2, or 3. EC is a measurement of all those charges in the solution that conduct electricity. The greater the quantity of nutrient ions in a solution, the more electricity that will be conducted by that solution. A material has a conductance of one siemens if one ampere of electric current can pass through it per volt of electric potential. It is the reciprocal of the ohm, the standard unit of electrical resistance. A siemens is also called a mho (ohm backwards).

For convenience, EC measurements often are converted to TDS units (ppm) by the meter.

The meter cannot directly measure TDS as described above, and instead uses a linear conversion factor to calculate it. Everyone?s nutrient mix is different, so no factor will be exact. The meter uses an approximate conversion factor, because the exact composition of the mix is not known. Conversion factors range from .50 to .72, *depending on the meter manufacturer, which do a good job of approximating a TDS calculation from the meter?s measurement of EC.

* All ppm pens actually measure the value based on EC and then convert the EC value to display the ppm value, having different conversion factors between differing manufacturers is why we have this problem communicating nutrient measurments between one another.

EC is measured in millisiemens per centimeter (ms/cm) or microsiemens per centimeter (us/cm).

One millisiemen = 1000 microsiemens.

EC and CF (Conductivity Factor) are easily converted between each other.
1 ms/cm = 10 CF

“The communication problem”…
So again, the problem is that different ppm pen manufacturers use different conversion factors to calculate the ppm they display. All ppm (TDS, Total Dissolved Solids) pens actually measure in EC or CF and run a conversion program to display the reading in ppm’s.

There are three conversion factors which various manufacturers use for displaying ppm’s…

USA 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
European 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Australian 1 ms/cm (EC 1.0 or CF 10) = 700 ppm

For example,

Hanna, Milwaukee 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
Eutech 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Truncheon 1 ms/cm (EC 1.0 or CF 10) = 700 ppm

Calculating the conversion factor

If your meter allows you to switch between EC and TDS units, your conversion factor can be easily determined by dividing one by the other.

Place the probe in the solution and read TDS in ppm. Change to EC on the meter and read EC in ms/cm.

Conversion factor = ppm / ec.

[Note: ms must be converted to us: One millisiemen = 1000 microsiemens (1.0 ms/cm = 1000.0 us/cm)

According to the chart below:
1.0 ms/cm = 500 ppm (USA Hanna)
1000 us/cm = 500 ppm

Conversion factor = ppm / (ms/cm * 1000)
.50 = 500ppm / (1000us/cm) ]

The answer is your meter’s convertion factor and should be a number between 0.50 and 0.72 To improve accuracy, take ec and ppm readings from your res daily for about ten days. Average the conversion factors. The more data points that you use, the closer you will be to finding your true conversion factor.

When reporting your PPM in a thread, please give the conversion factor your meter uses. For example: 550 PPM @0.7 or give the reading in EC, which should be the same meter to meter.

It may also be advisable to give the starting value of your water; there is a huge difference between RO and distilled water with a PPM of approximately 0 and hard tap water of PPM 300 @.5 (notice the conversion factor so others can work out the EC) or well water with a conductance of 2.1 ms/cm.

A note to Organic Growers:
An EC meter has fewer applications for a soil grower because many organic nutrients are not electrically charged or are inert. Things like Superthrive or Fish Emulsion, blood meal, rock phosphate or green sand cannot be measured with a meter reliably when they are applied or in runoff. Meters can only measure electrically charged salts in solution.

“The solution”…
When reporting your PPM in a thread please give the conversion factor your meter uses for example 550 PPM @.7 or give the reading in EC (the EC shoul d be the same meter to meter).

0.5 ppm
0.64 ppm
0.70 ppm
50 ppm
64 ppm
70 ppm
100 ppm
128 ppm
140 ppm
150 ppm
192 ppm
210 ppm
200 ppm
256 ppm
280 ppm
250 ppm
320 ppm
350 ppm
300 ppm
384 ppm
420 ppm
350 ppm
448 ppm
490 ppm
400 ppm
512 ppm
560 ppm
450 ppm
576 ppm
630 ppm
500 ppm
640 ppm
700 ppm
550 ppm
704 ppm
770 ppm

600 ppm
768 ppm
840 ppm
650 ppm
832 ppm
910 ppm
700 ppm
896 ppm
980 ppm
750 ppm
960 ppm
1050 ppm
800 ppm
1024 ppm
1120 ppm
850 ppm
1088 ppm
1190 ppm
900 ppm
1152 ppm
1260 ppm
950 ppm
1216 ppm
1330 ppm
1000 ppm
1280 ppm
1400 ppm
1050 ppm
1334 ppm
1470 ppm
1100 ppm
1408 ppm
1540 ppm
1150 ppm
1472 ppm
1610 ppm
1200 ppm
1536 ppm
1680 ppm
1250 ppm
1600 ppm
1750 ppm
1300 ppm
1664 ppm
1820 ppm
1350 ppm
1728 ppm
1890 ppm
1400 ppm
1792 ppm
1960 ppm
1450 ppm
1856 ppm
2030 ppm
1500 ppm
1920 ppm
2100 ppm
1550 ppm
1984 ppm
2170 ppm
1600 ppm
2048 ppm
2240 ppm

  Last modified: 03:09 – Feb 18, 2001  
Quicklink:  http://overgrow.com/growfaq/741
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faq:741 “What is the difference between ppm and EC?”