What is the difference between PPM, EC, TDS, DS and CF?

EC, TDS, DS, CF, PPM, Excerpted from Chapter Five, Indoor Marijuana Horticulture, The Indoor Bible

EC, TDS, DS, CF, PPM

Pure distilled water has no resistance and conducts no electrical current. However, when impurities are added to pure distilled water, it conducts electricity. Your water analysis will indicate the impurities or dissolved solids found in household tap water. These impurities conduct electricity. For instance, when you add fertilizer to pure water, the EC climbs.

TDS stands for Total Dissolved Solids. This is just one of the ways to arrive at EC, Electrical Conductivity. EC is the most consistent measure of gross nutrient solution strength.

Nutrient (salt) concentrations are measured by their ability to conduct electricity through a solution. Dissolved ionic salts create electrical current in solution, the main constituent of hydroponic solutions is ionic salts. Several scales are currently used to measure how much electricity is conducted by nutrients including: EC = Electrical Conductivity, CF = Conductivity Factor, PPM = Parts Per Million, TDS = Total Dissolved Solids and DS = Dissolved Solids. Most American growers use PPM, to measure overall fertilizer concentration. PPM is not as accurate, nor consistent as EC, to measure nutrient solution strength.

The difference between EC, CF, PPM, TDS and DS is more complex than originally meets the eye. Different measurement systems all use the same base, but interpret the information differently. First let’s start with EC, the most accurate and consistent scale.

START BOX
EC = Electrical Conductivity
CF = Conductivity Factor,
PPM = Parts Per Million,
TDS = Total Dissolved Solids,
DS = Dissolved Solids,
END BOX

EC is measured in: (a) Milli-siemens per Centimeter (MS/CM) or (b) Micro-siemens per Centimeter (US/CM). One Micro-siemen/CM = 1000 Milli-siemens/CM.

PPM testers actually measure in EC and convert to PPM. Unfortunately the two scales (EC and PPM) are not directly related. Each nutrient or salt gives a different electronic discharge reading. To overcome this obstacle, an arbitrary standard was implemented which assumes "a specific EC means equates to a specific amount of nutrient solution". Consequently, the PPM reading is not precise at all, it is only an approximation, a ball park figure!

It gets worse! Nutrient tester manufacturers use different standards to convert from CF to the PPM reading.

1. Hanna	1 MS/CM = 500 PPM
2. Eutech	1 MS/CM = 640 PPM
3. New Zealand Hydro.	1 MS/CM = 700 PPM

PPM recommendations are inaccurate and confusing! To help you through this confusion, please see the following chart.

1 MS/CM = 10 CF e.g. 0.7 EC = 7 CF.

Conversion scale from PPM to CF and EC.

EC	Hanna	Eutech	Truncheon	CF
MS/CM	0.5	0.64	0.70	0
0.1	50 PPM	64 PPM	70 PPM	1
0.2	100 PPM	128 PPM	140 PPM	2
0.3	150 PPM	192 PPM	210 PPM	3
0.4	200 PPM	256 PPM	280 PPM	4
0.5	250 PPM	320 PPM	350 PPM	5
0.6	300 PPM	384 PPM	420 PPM	6
0.7	350 PPM	448 PPM	490 PPM	7
0.8	400 PPM	512 PPM	560 PPM	8
0.9	450 PPM	576 PPM	630 PPM	9
1.0	500 PPM	640 PPM	700 PPM	10
1.1	550 PPM	704 PPM	770 PPM	11
1.2	600 PPM	768 PPM	840 PPM	12
1.3	650 PPM	832 PPM	910 PPM	13
1.4	700 PPM	896 PPM	980 PPM	14
1.5	750 PPM	960 PPM	1050 PPM	15
1.6	800 PPM	1024 PPM	1120 PPM	16
1.7	850 PPM	1088 PPM	1190 PPM	17
1.8	900 PPM	1152 PPM	1260 PPM	18
1.9	950 PPM	1260 PPM	1330 PPM	19
2.0	1000 PPM	1280 PPM	1400 PPM	20
2.1	1050 PPM	1344 PPM	1470 PPM	21
2.2	1100 PPM	1408 PPM	1540 PPM	22
2.3	1150 PPM	1472 PPM	1610 PPM	23
2.4	1200 PPM	1536 PPM	1680 PPM	24
2.5	1250 PPM	1600 PPM	1750 PPM	25
2.6	1300 PPM	1664 PPM	1820 PPM	26
2.7	1350 PPM	1728 PPM	1890 PPM	27
2.8	1400 PPM	1792 PPM	1960 PPM	28
2.9	1450 PPM	1856 PPM	2030 PPM	29
3.0	1500 PPM	1920 PPM	2100 PPM	30
3.1	1550 PPM	1984 PPM	2170 PPM	.1
3.2	1600 PPM	2048 PPM	2240 PPM	32

Every element in a multi-element solution has a different conductivity factor in these approximate measurements. Pure water will not conduct electrical current, but as elemental salts/metals are added, electrical conductivity increases proportionately. Simple electronic meters measure this value and interpret it as total dissolved solids (TDS). Nutrient solutions used to grow marijuana generally range between 500 and 2000 parts per million (PPM). If the solution concentration is to high, the internal osmotic systems can reverse and actually dehydrate the plant. For general purpose, try to maintain a moderate value of approximately 800 to 1200 PPM. Nutrient solution concentration levels are affected by nutrient absorption by roots and by water evaporation. The solution weakens as plants use nutrients, but water also evaporates from the solution, which increases the nutrient concentration. Adjust the concentration of the solution by adding fertilizer or diluting with more water.

Super Size Secret: Use an EC meter to check the overall strength of your nutrient solution. EC is the most consistent measure of overall solution strength.