µS/cm (microsiemens per centimetre) is a unit of electrical conductivity. 1,000 µS/cm = 1 mS/cm. Water-quality reports and lab analyses often use it because source waters have low conductivity and whole numbers are easier to read. Many meters switch automatically between µS/cm and mS/cm, which causes misreadings if you miss the unit on the display.
In practice
Typical values: distilled water about 0.5–3 µS/cm (rises as it absorbs CO2 from air), good RO water below about 20–50 µS/cm, rainwater roughly 5–50 µS/cm, many tap waters 200–800 µS/cm, and a flowering nutrient solution 1,400–2,200 µS/cm (1.4–2.2 mS/cm).
A rising RO output reading is the first sign of worn membranes: if your RO water creeps from 15 to 60 µS/cm, it is time to service the system.
The standard calibration solution for EC meters is 1,413 µS/cm (1.413 mS/cm), a potassium chloride solution at 25 °C.
µS/cm ÷ 1,000 = mS/cm. On a 500 scale, ppm ≈ µS/cm × 0.5; on a 700 scale, ppm ≈ µS/cm × 0.7.
Don't confuse with
- mS/cm: a thousand times larger; "1.4" and "1,400" can be the same solution.
EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.A calibration solution is a standard liquid of known pH or EC used to calibrate meters, such as pH 4.01, 7.00 and 10.01 buffers or 1.413 mS/cm EC standard.As a plant nutrient, chlorine is used as the chloride ion (Cl-) in tiny amounts for photosynthesis and osmotic balance; excess from water or salts is the usual problem.ppm500 is the TDS scale that converts EC to ppm with a factor of 0.5 (based on sodium chloride), so 1.0 mS/cm reads 500 ppm.ppm700 is the TDS scale that converts EC to ppm with a factor of 0.7 (the 442 natural-water standard), so 1.0 mS/cm reads 700 ppm.TDS (total dissolved solids) is the mass of dissolved material in water; grow meters estimate it from EC with a conversion factor and display it as ppm.
