What is conductivity?

The conductivity of a solution is an expression of the capacity of that solution to conduct electric current. Distilled or de-iornised water will conduct virtually no electricity at all and will therefore have a conductivity reading of zero. All salts are dissolved in the water – in making up a nutrient solution for example – so the conductivity of the solution increases. The conductivity of a nutrient solution can therefore be used as a guide to its ‘strength as it indicates the amount of salts dissolved.

Conductivity is usually expressed as ‘EC’. EC stands for Electrical Conductivity and the unit measurement is milliSiemens per centimetre squared (Ms/cm2). This is frequently shortened to ‘millSiemens’ and although strictly incorrect this is widely used in the industry. Sometimes conductivity is expressed as CF (conductivity Factor). There are a thousand microSiemens in a milliSiemen A CF unit is equal to a hundred microSiemens so there are 10 CF units to a milliSiemen.

EC or Conductivity Meters

Ec meters usually read in milliSiemens but occasionally in CF units. For the grower it is simply a matter of getting the decimal point in the right place. As a rule of thumb it can be said that a good conductivity level for most purposes would be 2 milliSiemens (i.e. 2.0 mS/cm2). We can call this a standard working solution:

Standard working solution = 2mS/cm2 = 2000uS/cm2 = 20 CF units

Parts per Million

Some meters display the ‘conductivity’ of a solution in terms of partsper millon (ppm). This is very misleading: ‘parts per million’ is a measure not of conductivity but of concentration, and the correlation between the two is highly variable for different salts.

You cannot measure ppm with the meter so how come there are ppm meters out there? The answer is that these meters are just ordinary EC meters with a superimposed scale of ppm measurement applied by the manufactures in response to consumer demand. The conversion is based upon an approximate and unverifiable relationship and each manufacturer uses a different conversion scale.

For this reason the grower would be best advised to think and work in milliSiemens.

However, if you already have a meter that reads in ppm you will wish to use it, and the following will be helpful.

One gram of any salt, dissolved in a Litre of water will make a solution containing 1,000 PPM of that salt. However different salts have a different capacity to conduct electricity so two solutions made with different salts could both be 1,000 PPM. but have quite different readings on a CF meter. It is therefore not possible to make a exact conversion from parts per million to CF units, however as most nutrient solutions have approximately the same ratio of constituent salts we can make an approximate conversion;

1 mS = 10 CF units @ 850 PPM.

Please be aware that this figure is not universally accepted and you will hear different, much lower conversion values being suggested. We believe this figure to be valid and we stand by it.

Therefore Standard Working Solution = 2.0 mS/cm2 = 20 CF = approx