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Nutrient Solutions
 
 

Conductivity and pH of nutrient solutions and practical considerations of measurement and control.

Nutrient solutions conduct electricity, and the capacity of any solution to conduct an electric current is its Conductivity. Conductivity is measured by a conductivity meter by placing a probe carrying two electrodes into the solution and passing an alternating current through the solution between the electrodes. The current is carried by charged ions (cations and anions) in the solution. Divalent ions (with two charges like ) carry twice as much current as monovalent ions (like ). The conductivity of the any nutrient solution will thus be directly proportional to the sum of the cation or anion charges in that solution.

Conductivity can be expressed in many different units, one of the most useful for growers is CF or the conductivity factor. The conductivity of a solution can be readily calculated from its composition, by calculating the sum of the equivalents in mmol/ litre, like this :-

Cations

 

 

 

 

 

CF

Concentration mmols/litre

0.5

7

3

1

0.25

milliequivalents

0.5

7

6

2

0.25

15.75

The conductivity of nutrient solution expressed in parts per million (ppm) can just as easily be calculated by converting the ion concentrations to mol. Convert to mols by dividing the concentration in ppm by the atomic weight of the nutrient. For example

Cations

 -N

 

 

 

 

CF

Concentration ppm

7

273

120

24

5.75

Atomic weight

14

39

40

24

23

mmols/litre

0.5

7

3

1

.25

milliequivalents/l

0.5

7

6

2

.25

15.75

Conductivity Units .

Electrical conductivity is the inverse of electrical resistance and so the original conductivity unit was the reciprocal Ohm or Mho. Mho is not used today, it has been replaced by Siemens as the unit of conductivity

1 CF = 10 milliSiemens /cm normally written as the abbreviation 10mS/cm

Another common unit is microSiemens/cm (S/cm) :

1 mS/cm = 1000 S/cm = 0.1CF

The official Systeme International uses the metre as the basic unit and so the proper SI units for conductivity use metres instead of centimetres eg DeciSiemens/m or dS/m

1CF = 10 mS/cm =10 dS/m= 10,000 S/cm = 10,000S/m

Conductivity is often quoted without any units as EC

EC (US references) often = mS/cm

EC (European references) often = S/cm

Conductivity meters and controllers

 

There is an extremely wide range of instuments for measuring and controlling

 conductivity of nutrient solutions, ranging from simple pen type meters through to sophisticated controllers. Pen type meters are very useful and inexpensive  back up instruments for routinely checking nutrient solutions delivered to crops growing in solid media or for checking on automatic controlled recirculating solutions. The electrodes and measuring and display components are all integrated in single unit not much larger than a pen. Hand held conductivity meters usually have the measuring circuit and display in a calculator size unit and use a separate probe. Conductivity controllers usually combine conductivity and pH control circuits in a single cabinet with separate conductivity probes and pH electrode on leads upto about 2 m long. Conductivity probes typically use stainless steel pin or ring electrodes, or sometimes carbon pins embedded in epoxy resin. The probes usually contain a temperature sensor. Conductivity meters have many other uses besides hydroponics and hence are available in a number of different measuring ranges. When buying a conductivity meter it is important to specify the range required, 0-100 CF and 0-19.9mS/cm are common suitable ranges. Conductivity meters which read out only in ppm of salts are not suitable. Any instrument bought should be capable of being calibrated by its user, there are some instruments on the market that can only be calibrated at the manufacturers factory and these should be avoided. Growers should check the calibration of their conductivity meters and controllers regularly. Standard solutions for calibration are readily available and the calibration procedure is simple. Poor sufficient standard into a clean dry container deep enough to immerse the electrodes of the meter, and then adjust the meter to read the standard CF or EC. After calibration discard the small amount of standard used for this calibration and use fresh standard each time the meter is calibrated. The standard will specify the conductivity and temperature, usually 25.Temperature has a large effect on conductivity which varies for different salts, and it is important to use a meter with built in temperature correction. Some of the better quality meters allow the user to adjust the temperature correction, for nutrient solution the correction factor ( )should be 1.5% /. When the built in temperature compensation is automatically adjusted it is often set at 2%/.

* - Acidity, Alkalinity

Acids separate into hydrogen ion ( ) and anions when dissolved in water, while alkalis dissociate into cations and hydroxyl ions ( ). Water molecules can dissociate both ways, into or into . When a solution contains more   than   we say it is acid, and when there are more   than   it is alkaline.   is a logarithmic scale for expressing the acidity or alkalinity of a solution. The   scale runs from 1 to 14,  1 is strongly acid,  7 is neutral and  14 is strongly alkaline.

Nutrient solutions are best kept between 5 and 6 which is quite different to the pH ranges recommended for crops growing in soil. The pH of nutrient solutions is changed by the plants as the roots can exude   or other cations or can exude anions or   in exchange processes involved in the uptake of nutrient ions. If the nutrient solution pH strays outside the pH 5 6 range because of plant root activity or the addition of acid or alkaline water, then addition of acid or alkali is necessary to bring the solution back into the required range. Root damage may occur if the pH gets too low (probably below pH 4.5) and the availability of nutrients from the solution can be reduced outside the recommended pH range. Growers have to monitor solution pH on at least a daily basis, and when necessary add acid or alkali to correct the pH. Indicator papers can be used to monitor pH, they are cheap but not reliable enough for commercial growers. A very wide range of pH meters and controllers is available, ranging from pen type pocket dipping instruments through to very sophisticated meters and controllers. Only instruments that have 2 point calibration and automatic temperature compensation should be purchased by growers, instruments without these features will be cheaper but will provide misleading results in the long term.

   meters need quite frequent calibration, more commonly known as buffering. Standard solutions are available for a wide range of pH values, but growers need only pH 4 and pH 7 buffer solutions. Two small containers of standard buffer solution are needed for buffering a pH meter, together with a beaker of preferably distilled or deionised water, otherwise clean rainwater. The pH electrode should be rinsed in the distilled water, shaken free of any water and then immersed in the pH 7 buffer. The pH meter can then be adjusted to read pH 7. The electrode is then rinsed and shaken dry before being immersed in the pH 4 buffer. The calibration point marked slope is then used to make the meter read pH 4. The process of adjusting at pH 7 and then adjusting the slope may need to be repeated several times before stable set points are found. Discard the samples of both buffers used after calibration and use fresh buffer each time. pH electrodes have a short life, often only 18 months of continuous use, if stable set points cannot befound when buffering the electrode needs replacing or in some cases servicing by refilling with new electrolyte.

 

Both conductivity standards and pH buffers are offered for sale in small 25 or 50 ml bottles, but buying these small bottles and using them buy dipping the electrodes directly into the bottles for calibration on a number of different occasions will not give reliable calibration, it is much better to 500 ml bottles of standards so that fresh standard solutions can be used for each calibration.
   

Links

Most quality conductivity and pH meter and controller manufacturers have web sites where you can see the range of instruments offered and can read the specifications and sometimes the operating manuals for the instruments.

http://www.eutechinst.com/

http://www.hannainst.com.au/

http://www.autogrow.com

 

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