Nutrient measurement and control

 


EC, CF and TDS(ppm) - just what are they?

First of all lets dispel the myth that they are all the same sort of measure - they are not.  TDS(ppm) refers to the actual quantity of a salt or salts in solution (Total Dissolved Solids) whereas EC and CF refer to the electrical conductivity (ie how easily the solution passes an electric current).  Now, we can measure EC (CF) quite easily by passing a high frequency AC current through the solution but we cannot directly measure TDS (ppm).  EC is measured in mS/cm (milli Siemens per centimetre) and CF is measured in units 1/10 of a mS/cm so the conversion between CF and EC is 1 EC (mS/cm) = 10CF.

So what about measuring the EC and then multiplying the answer by a conversion constant to tell us what the ppm is.  Unfortunately, this cannot be done for the simple reason that the conversion constant for every salt is different.   Beware of meters that provide a read-out in ppm, they will only give a correct reading for one particular salt - usually sodium chloride.  This is unfortunate as sodium chloride has a very different conversion constant from most hydroponic salts and will therefore give a reading that has a very large error.  In fact TDS meters usually are calibrated to comply with one of three standards namely NaCl (1mS = 500ppm), 442 (1mS = 700ppm)and some have an imperical standard of 1mS = 650ppm).  If you are following instructions from a book or nutrient supplier that is in ppm you need to ensure that you meter is using the same conversion coefficient as the instructions.

Back to the measuring process.  Alternating current is used to try to eliminate electrolysis and the accompanying electroplating of the electrodes. Even so some plating will occur and for this reason it is important to regularly (every two weeks or so) clean the face of the sensor with an abrasive that will remove any such hard plating.  Autogrow recommends using a kitchen cleaner such as Jif on a soft cloth or finger tip.  Note that plain Jif should be used - not the lemon scented variety as this contains oils that will give an incorrect reading.  After cleaning, rinse the electrode in fresh water and shake off the excess.  If the probe has a shroud, replace this fully, ensuring that it is impossible to trap air bubbles at the tip when it is stood vertically in water.  Do not touch the sensor face with your fingers as these may have oil from perspiration on them.   After cleaning, the calibration of the instrument should be checked and recalibrated if necessary.  When doing this allow plenty of time for the temperature of the probe to equalise to that of the test solution.  All modern EC measurement probes will have a built-in temperature sensor to compensate for the inherent temperature sensitivity of liquid conductivity and this must be given sufficient time to kick-in.  Most hydroponic supply stores carry EC (CF) standard solution. This is usually standardised to an EC of 2.77 mS (27.7CF) at 25 Deg C.   Note that most modern instruments have built-in temperature compensation and so calibration can be done at any temperature.  For good accuracy, it is important to allow the probe to stand in the cal solution for at least 5 minutes so that the temperature of the probe equalises to the temperature of the cal solution. Even better if you can store the calibration solution at a similar temperature to that of the nutrient tank.  Without temperature compensation the error in the EC reading would be about 2% for every degree Celcius that the solution deviates from 25C.

 

pH measurement

pH electrodes work like very weak batteries.  When placed in a solution they produce a tiny voltage that is proportional to the pH of the liquid.  Unfortunately, they have a very high internal impedance and so the voltage cannot be measured by a normal multimeter.  pH meters use special amplifiers which present very little load to the sensor in order to measure the voltage accurately.

pH sensors last between 1 and 2 years (3 years if you are really lucky) in a hydroponic system.  You should probably change it after 2 years in any event before it really lets you down at the wrong time.  To help them last as long as possible you should store them in plain clean water (not distilled) preferably with a little pH4 buffer added.  Never let them dry out.

pH instruments are very susceptible to moisture and extreme temperatures so always keep them in a cool dry place and take care not to wet them (even the waterproof ones).

pH electrodes drift quite quickly and their calibration should be checked (and corrected if necessary) every week or two.  For hydroponics, this is usually done at pH7 and then at pH4. This provides two calibration points on either side of the usual working value of pH5.5 to pH6.0.  Note that modern research has shown the optimum pH value in hydroponics to be lower than the optimum for soil (usually pH5.5 to pH6.0 in hydroponics compared with pH6.0 to pH6.5 in soil)

On-demand dosers

These are used to automatically maintain the EC and pH at their respective set-points.

Most sensible recirculating hydroponic systems use a reservoir tank that has an automatic water make-up valve (ball cock).  As the plants transpire, the water level in the tank falls and fresh water enters the system.  This will tend to dilute the nutrient and may also have an effect on the pH.  With an on-demand doser, the situation is continually corrected and the ideal EC and pH maintained at all times.  This avoids the sudden shock associated with infrequent hand dosing.

Autogrow introduced the worlds first "timed dose" doser about 15 years ago (well, as far as we can tell).  This gives a fixed size dose (user settable) when it detects that the EC is low and then waits for a few minutes (user settable) for the dose to mix-in before checking to see if another dose is needed.  This method is extremely easy to set up and is not prone to overdosing like many other methods.  For this reason other manufacturers are starting to copy the idea.

Another innovation from Autogrow is the  "smart" or "proportional" dose.  When this is activated the controller gives a large dose when the error is large and automatically reduces the dose size as teh setpoint is approached.  This is a little harder to set up but when set correctly provides very rapid dosing which is sometimes required in batch fertigation systems.

Many growers like to use a lower EC value during the heat of the day and raise it late afternoon through to the following morning.  The new Autogrow NutriDose 1, as well as our greenhouse controllers, have two EC settings corresponding to two time zones to allow this to happen automatically.  The NutriDose 1 can even add extra water to reduce the EC

During bright weather, plants grow strong with short internode spacing whereas in dull weather they tend to stretch and grow spindly.  To compensate for this, growers will increase the EC during dull weather and lower it in bright weather.  Some Autogrow greenhouse controllers will even do this function automatically, within limits specified by the grower.

So, this raises the question;  can you exercise the same level of control manually as can be achieved with modern automatic doser?

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