7 Conductivity

The conductivity of a solution is an expression of the capacity of that solution to conduct an electric current. Distilled or de-ionised water will conduct virtually no electricity at all and will therefore have a conductivity reading of zero. As salts are dissolved in the water so the conductivity of the solution increased. The conductivity of a nutrient solution is therefore a measurement of its strength as indicated by the actual amount of salts dissolved.

Conductivity is usually expressed in terms of CF. CF stands for Conductivity Factor and the units of measurement are usually milliSiemens, microSiemens or Siemens. The difference between these is purely in the placing of the decimal point. 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.

CF Meters

CF meters usually read in milliSiemens or CF units but 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 2.0 milliSiemens (expressed as 2.0mS/cm2). We can call this a standard working solution.

Therefore Standard working solution =2.0mS. =2,000 uS = 20 CF units.

Parts per Million

Conductivity is sometimes expressed in terms of parts per million (ppm). 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 in a CF meter. It is therefore not possible to make an 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 unit=~ 640 ppm.

Therefore Standard Working Solution= 2.0mS= 20 CF =~ 1280 ppm.

Many different meters are available from Growrooms UK. We have the cheaper digital meters like Eutech or Hanna and we also stock the Amazing Conductivity Truncheon, Saltesta Wand and pH Wand from New Zealand which use flashing LED's to show CF or pH reading. These are accurate and reliable and cannot be too highly recommended. They also carry a full two year warranty which is quite remarkable for such sensitive instruments.

pH and Conductivity in RockWool systems

Plants grown in RockWool slabs need slightly different nutrient regimes than in other systems. pH should be somewhat lower, at 5.8 rather than 6. Conductivity should also be slightly lower than in other applications. It is very important to remember that the readings in the tank may not reflect the reality within the slab. Nutrient should be extracted from the slab regularly for measurement. In a well maintained system there should not be too much variation between tank readings and those obtained from within the slabs. If Conductivity begins to rise in the slab it will be an indication that irrigation is either too infrequent or too short. It is neccessary to trickle nutrient into slabs for just long enough to get a 10 - 15% run off. The Timer should be adjusted to provide this amount of nutrient at least four times during the daylight period. More frequent irrigation may be beneficial. Remember that as plants grow they will take up much more nutrients and watering cycle will need to be extended regularly throughout life of the crop. The regular check of nutrient from within the slab itself is the most important routine for the rockwool grower. If the Conductivity begins to rise then the grower will increase irrigation to ensure run off at each watering cycle. If Conductivity remains high then it is a simple matter to flush the slabs.