March/April – 1993
Story Title: pH Factor
Author: Steven Carruthers
Using and Calibrating pH & Conductivity Test Pens
Electronic digital pH and conductivity testers, now widely used for hydroponics, are designed to provide accurate testing and control of pH (the measure and acidity and alkalinity), and of the concentration of the nutrient solution in your hydroponic system.
Pen-type testers have been available for several years and are used where pH and conductivity accuracy is not critical. More accurate instruments are usually considerably more expensive. The two major manufacturers of pen-type testers are Hanna Instruments and Eutech Cybernetics. The latest technology from Hanna has brought the phepl, pHep2 and pHep3 test pens onto the market ( the pHep3 model offers push-button calibration) and all being waterproof. Eutech produces the pHScan-1 and pHScan-2 models. Hanna also produces the DiST-1 and DiST-4 waterproof conductivity pens.
The DiST-1 reads in parts per million (ppm); the DiST-4 in millisiemens (mS). Eutech manufacture the push-button TDScan 1, TDScan2, TDScan 3 and the TD5can 4 conductivity meters.
Recirculating hydroponic systems need to be accurately tested and adjusted as growing plants feed on selected salts in the nutrient solution. The removal of these salts results in a depletion of the concentration of the nutrient solution, and changes in the pH.
Left without adjustment, the nutrients in the solution will eventually (in theory) be used up until the solution is comprised of water and unusable salts. Hence, to maintain maximum growth, the nutrient solution needs to be maintained at its optimal concentration.
If plants remove acid salts, the nutrient solution becomes more alkaline. In the same way, if plants grow in size, they require different salts as they pass through different stages of maturity, and they require more and more nutrients to maintain their mass and growth. The concentration of the nutrient solution is adjusted by adding water to decrease its concentration or nutrient salts to increase its concentration, and the pH of the solution is adjusted with acid and alkaline pH buffers.
USING YOUR PENS
The eictrodes of the pens need to be kept wet or moist in order to work well. Before using the pens for the first time, clean the electrodes in water to remove traces of test fluids left over from testing in the factory, and soak the pens in electrode storage solution for an hour or so. it is very easy to maximise the growth of your pants by following three steps every day:
1. Top up the level of the nutrient tank with fresh water, and circulate or mix for a short time to mix the water with the nutrient.
2. Test the nutrient concentration with your conductivity pen. For most young plants the nutrient concentration should be around 500-750 parts per million, and for adult plants around 1 000-1 500 parts per million. Increase the nutrient concentration by adding a little at a time of the concen trated nutrient until the desired reading is reached. If the solution is too concentrated, remove some of the nutrient solution and add fresh water until the correct reading is reached.
3.Test the pH of the solution with the pH pen. Most plants require a pH in the range of 5.5 to 6.5. If the pH is too low, add dilute pH Raise buffer solution, a little at a time until the desired pH is reached. If the pH is too high add pH lower buffer solution, a little at a time until the desired pH is obtained.
WARNING. Do not use concentrated buffer, as it is too easy to overshoot the correct pH level and have to adjust the pH the other way. This is likely to cause the plants to go into shock if the pH change is severe.
CALIBRATING YOUR PENS
Between uses, and to maintain quick and accurate readings when in use, keep the tips of the pens soaking in a specially pre pared storage solution.
The purpose of this solution is to maximise electrode life. It should have an ideal level of ion activity for the following reason low ion activity encourages an exchange of ions between the electrode and the storage solution. It is essential therefore that the solution be specially made for the purpose, to minimise strain on the electrodes.
Due to a number of factors, your pens require periodic calibration. Each type of pen has specific calibration solutions-laboratory grade fluids guaranteed to con form to a set pH or conductivity.
For pH pens, the typical hydroponic calibration solutions are pH7.0 and pH4.0. The usual rule is to use the calibration solution closest to the pH of the nutrient solution. For this reason, most people use a calibration solution of around pH7.0. For greater accuracy, some people calibrate their pens at both pH7 and pH4 (this adjusts the “slope” of a theoretical line from pH7 to pHO).
For conductivity pens used for hydroponics, a typical solution is cF2.76 mS/cm (equivalent to a reading of 1382 parts per million).
The process of calibration is simple. Clean your pen with fresh water and dry. A dirty or wet pen will contaminate your calibration solution, rendering it inaccurate. Immerse the probe in the calibration solution, and check the reading. If your pen is correctly cali-brated the reading should be the same as the calibration solution
For example, if you are using pH7.0 calibration solution, the reading should read 7.0. If this is not the case, the pen’s reading needs to be adjusted. Eutech pHScan pens and Hanna’s pHep3 are self-calibrating. If the pen does not read 7.0, the solution is probably contaminated and should be replaced. For Hanna pens, use a jeweller’s screwdriver to turn the small calibration screw on the back of the pen until a reading of 7.0 is achieved.
For your conductivity pen, using cF2.76 calibration solution, adjust the pen until the pen reads 138 (DiST-I/TDScan 1) or 27 (Dist-4/TDScan 4).
Your pens are not waterproof. They are designed to be immersed only to around a third of their depth. However, people do drop pens into the nutrient solution, especially in the case of Hanna pens which need to be immersed in the solution in order to read them. (Eutech pens have a “hold” button that hold the reading so that it can be read after removing the pen from the solution.)
If the worst happens to you, DON’T PANIC! If you act prompt ly, there need be little or no dam age to your pen.
1.Pry open the top of the pen and take out the batteries.
2.Remove and dry the batteries. Spray a tiny amount of WD-40 or equivalent on a soft cloth and wipe the batteries.
3.Turn the pen upside down to drain any water out of it.
4.As soon as possible, take the pen to a service station and blow out any remaining water with compressed air. Leave up side-down on absorbent paper to blot any remaining moisture.
5.Once the pen is dry, replace the batteries, making sure they are inserted the right way.
If you have acted promptly, your pen should have suffered lit tle or no damage, and will now be back in working order.
TESTING SOIL pH USING ELECTRONIC INSTRUMENTS
Testing the pH of soil is easily achieved by mixing one part of air dried soil with four parts water. Place soil and water in a small flask and let sit for one hour, shaking vigourously for one hour. Once the soil has finally settled, test the water sample to obtain an accurate pH level.