pH Meters in Hydroponics
pH meters are just one piece of equipment that is worth investing a few extras dollars in, in order to purchase a high quality scientific meter. That is, the cheaper pen meters may seem appealing based on price, but where pH meters are concerned it is safe to say that you always get what you pay for. This is a lesson I myself learned many years ago. At one point I owned several pen meters so that I could cross check pH readings. In some cases there were massive variants between the readings even though all the meters had been calibrated prior to use (I,e. one meter may have told me pH was 5.5 while another was reading pH 6.5). In the end, a year on and a hard lesson learned, I purchased a very high quality scientific meter from a laboratory supplier that in those days cost me over a $1,500.00. Cost aside, it was the best decision I ever made and I never looked back thereafter (finally accurate readings every time and confidence in the equipment I was working with). The good news is that nowadays you can get about the same quality meter for several hundred dollars through hydroponic retail stores. We even sell them through our store on Manic Botanix.
A combination pH, EC meter (top) and a quality scientific pH meter (below)
Calibrating and using a pH Meter Correctly
There are many factors that influence pH measurement. Compensation for or elimination of these factors is the key to accurate and precise pH measurement.
I’ll avoid wading into the technicalities that surrounds these factors (e.g. zero potential and theoretical values etc) but taking a pH measurement without first calibrating your pH meter is not best practice. Put simply, if you are looking for very accurate pH readings every time that you use your pH meter to monitor your solution you should calibrate first. Other than this, there are several other factors that you need to be aware of to ensure you get an accurate a pH reading.
Temperature plays a key role in pH measurement
In simple terms, pH measurement should always be performed together with temperature measurement because only pH values measured at the same temperature can be compared.
The pH value of the solution changes with the temperature. I.e. an increase in any solutions’ temperature will cause a decrease in its viscosity and an increase in the mobility of its ions in solution. An increase in temperature may also lead to an increase in the number of ions in solution due to the dissociation of molecules. As pH is a measure of the hydrogen ion concentration, a change in the temperature of a solution will be reflected by a subsequent change in pH. See following table.
The variation in Nernst slope with temperature for an ideal electrode
Because of these temperature variants, as with EC meters, when calibrating your pH meter the calibration solution temperature should be as close as possible to the nutrient solution to be tested to minimise temperature induced errors.
Purchase a Quality Automatic Temperature Compensated (ATC) Meter
Reference pH electrodes function on the basis of a chemical equilibrium between a metal and a solution of its ions which generates a potential, and which is reflected in the pH reading. This is affected by temperature because the solubility of the metal salt in the reference electrolyte solution varies with temperature.
This brings us back to investing a few extra dollars in quality equipment. Most good modern scientific pH meters have an auto buffer recognition facility whereby values of pH buffers at various temperatures are stored in memory. Meter standardisation and temperature coefficient of variation correction is therefore automatically done if the correct buffer is used. Meter manufacturers generally outline specific pH buffer types to be used for calibration. Be sure to read the literature that comes with the pH meter that you purchase. More importantly, be sure to follow manufacturer recommendations.
Speak to your hydroponic supplier for more information on ATC pH meters.
Practical hints for successful calibration:
1) Always use fresh buffer solutions – never place a used buffer back into its bottle. This will lead to a contaminated buffer solution that gives an imprecise calibration.
2) A two-point calibration (e.g. 4.0 and 7.0) is recommended for optimal accuracy.
3) Calibrating with buffers that lie within your pH measuring range increases the accuracy of the measurement. Therefore, because in hydroponics optimal pH is between 5.5 – 5.8, calibrating with pH 4.0 and pH 7.0 buffers is ideal.
Stirring during the calibration process also plays a role in the accuracy of the pH reading. I.e. stirring affects the pH. I have watched several growers calibrate their pH meters by using a small medicine measuring cup and then leaving the meter to sit in it without stirring for a minute or so before calibrating. I have then watched them dip the electrode of the meter into an aerated nutrient tank/reservoir and stir it around and around – often quite vigorously. This is a bad practice but one I expect many growers use. It is important to note that the stirring speed during the calibration process should be identical with the stirring speed during the pH measurement. When stirring during the pH measurement is not required, calibration should also be carried out without stirring. With hydroponic solutions it is best to calibrate first without stirring and then measure the nutrient solution pH without stirring. Further, it is best to take a sample from the nutrient tank/reservoir in e.g. a glass or medicine cup/vial and take a pH reading from this rather than placing the probe into the tank/reservoir.
Best Practice for Calibration
- Thoroughly rinse the pH electrode sensing area with distilled water. Dab the electrode with a soft paper tissue to remove the distilled water (don’t rub the electrode surface – dab gently).
- Dip the electrode into buffer pH = 7, so that the diaphragm of the electrode is well immersed. If you meter has specific requirements, re temperature settings, be sure to follow the manufacturers instructions.
- Start the calibration on the meter following manufacturer recommendations.
- Rinse the electrode sensing area with distilled water. Dab the electrode with a soft paper tissue to remove the distilled water (don’t rub the electrode surface – dab gently). Dip in buffer pH = 4 and repeat the procedure.
- Take a nutrient sample from the tank/reservoir and measure the pH.
- After taking the pH reading from the nutrient solution rinse off the electrode with distilled water, put storage solution in the cap and put the cap on the end of the pH electrode.
If in doubt get your pH meter checked out
pH meters are precise scientific equipment that rely on an electrolyte gel for reading pH. Sometimes this gel can become contaminated or dry out. What this means is that your pH readings will not be accurate. In fact, it can be miles out. Fortunately, new probes can be purchased or in many/most cases probes can be regelled. If you have any doubts about your pH meters accuracy have it looked at by your hydroponic retailer.
As a tip, I tend to keep a liquid pH test kit around in my grow room to run an occasional side-by-side test with my meter if in doubt of its accuracy. These kits are cheap (about $10.00) to purchase through hydroponic stores and are accurate enough to tell you if your pH meter reading is questionable.
pH meter storage (when not in use)
pH glass electrodes need to be kept moist at all times. Therefore, it is important that the measuring tip of the pH probe remains hydrated when not in use. pH probe storage solutions are available for this purpose. If the pH electrode dries out during storage, a regeneration procedure is required to restore the hydrated glass layer and the reference junction in order to make the electrode operable.
Use the solution recommended by the manufacturer. However, as a general rule, store your pH electrode in the same solution as the reference electrolyte of the electrode. In most cases this is a 3 mol/l (325g/L @ 99% purity) potassium chloride (KCl) solution.
pH electrode storage solutions are sold through hydroponic stores. Ask your supplier for more information.
Cleaning the pH Electrode
For optimal pH measurement response time, it may be necessary to clean the pH electrode’s glass bulb and reference junction of contaminants and precipitations. Follow the steps outlined in the pH electrode manual. Typically, it is recommended to soak the electrode for a few minutes in warm deionized water, or in a cleaning solution to maintain the probe.
Read about pH in hydroponics science here……