pH / EC / TDS / PPM
What are the pros and cons of buying a combo meter?
The combination type meters are real handy for the convenience of being able to take both readings simultaneously, or with a single touch of a button to switch between modes. The problem IMHO with combination meters is pH sensors like to be stored in a fertilizer solution, but TDS probes like to be stored in distilled water. Storing the pH probe in plain or distilled water will damage the ph membrane, so the combination probe needs to be stored in a fertilizer solution so as not to damage the pH portion, so the TDS probe ends up being "dirty" from salt buildup. A friend has already lost one expensive probe on his Hanna from this same problem, and will only purchase "single function" pH or TDS meters in the future.
What is the difference between ppm and EC?
Total Dissolved Solids (TDS) is the best measurement of the nutrient concentration of a hydroponic solution. To estimate TDS, one can use a meter that measures the Electric Conductivity (EC) of a solution, and convert the number to TDS in parts per million (ppm). Many meters will do this conversion.
Total dissolved solids (TDS) is typically expressed in parts per million (ppm). It is a measurement of mass and determined by weighing, called a gravimetric analysis. A solution of nutrients dissolved in water at a strength of 700 ppm means that there are 700 milligrams if dissolved solids present for every liter of water. To accurately calculate total dissolved solids (TDS), one would evaporate a measured filtered sample to dryness, and weigh the residue. This type of measurement requires accurate liquid measurement, glassware, a drying oven, and a milligram balance. Example: 50 mL of the 700ppm solution would leave 35 mg of salt at the bottom of a crucible after drying.
Electrical Conductivity (EC) is expressed in siemens per centimeter (s/cm) or milliseimens per centimeter(ms/cm). It can be determined with an inexpensive hand held meter. Nutrient ions have an electrical charge, a whole number, usually a positive or negative 1, 2, or 3. EC is a measurement of all those charges in the solution that conduct electricity. The greater the quantity of nutrient ions in a solution, the more electricity that will be conducted by that solution. A material has a conductance of one siemens if one ampere of electric current can pass through it per volt of electric potential. It is the reciprocal of the ohm, the standard unit of electrical resistance. A siemens is also called a mho (ohm backwards).
For convenience, EC measurements often are converted to TDS units (ppm) by the meter.
The meter cannot directly measure TDS as described above, and instead uses a linear conversion factor to calculate it. Everyone’s nutrient mix is different, so no factor will be exact. The meter uses an approximate conversion factor, because the exact composition of the mix is not known. Conversion factors range from .50 to .72, *depending on the meter manufacturer, which do a good job of approximating a TDS calculation from the meter’s measurement of EC.
* All ppm pens actually measure the value based on EC and then convert the EC value to display the ppm value, having different conversion factors between differing manufacturers is why we have this problem communicating nutrient measurments between one another.
EC is measured in millisiemens per centimeter (ms/cm) or microsiemens per centimeter (us/cm).
One millisiemen = 1000 microsiemens.
EC and CF (Conductivity Factor) are easily converted between each other.
1 ms/cm = 10 CF
"The communication problem"...
So again, the problem is that different ppm pen manufacturers use different conversion factors to calculate the ppm they display. All ppm (TDS, Total Dissolved Solids) pens actually measure in EC or CF and run a conversion program to display the reading in ppm's.
There are three conversion factors which various manufacturers use for displaying ppm's...
USA 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
European 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Australian 1 ms/cm (EC 1.0 or CF 10) = 700 ppm
Hanna, Milwaukee 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
Eutech 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Truncheon 1 ms/cm (EC 1.0 or CF 10) = 700 ppm
Calculating the conversion factor
If your meter allows you to switch between EC and TDS units, your conversion factor can be easily determined by dividing one by the other.
Place the probe in the solution and read TDS in ppm. Change to EC on the meter and read EC in ms/cm.
Conversion factor = ppm / ec.
[Note: ms must be converted to us: One millisiemen = 1000 microsiemens (1.0 ms/cm = 1000.0 us/cm)
According to the chart below:
1.0 ms/cm = 500 ppm (USA Hanna)
1000 us/cm = 500 ppm
Conversion factor = ppm / (ms/cm * 1000)
.50 = 500ppm / (1000us/cm) ]
The answer is your meter's convertion factor and should be a number between 0.50 and 0.72 To improve accuracy, take ec and ppm readings from your res daily for about ten days. Average the conversion factors. The more data points that you use, the closer you will be to finding your true conversion factor.
When reporting your PPM in a thread, please give the conversion factor your meter uses. For example: 550 PPM @0.7 or give the reading in EC, which should be the same meter to meter.
It may also be advisable to give the starting value of your water; there is a huge difference between RO and distilled water with a PPM of approximately 0 and hard tap water of PPM 300 @.5 (notice the conversion factor so others can work out the EC) or well water with a conductance of 2.1 ms/cm.
A note to Organic Growers:
An EC meter has fewer applications for a soil grower because many organic nutrients are not electrically charged or are inert. Things like Superthrive or Fish Emulsion, blood meal, rock phosphate or green sand cannot be measured with a meter reliably when they are applied or in runoff. Meters can only measure electrically charged salts in solution.
When reporting your PPM in a thread please give the conversion factor your meter uses for example 550 PPM @.7 or give the reading in EC (the EC shoul d be the same meter to meter).
EC Hanna Eutech Truncheon CF
ms/cm 0.5 ppm 0.64 ppm 0.70 ppm 0
0.1 50 ppm 64 ppm 70 ppm 1
0.2 100 ppm 128 ppm 140 ppm 2
0.3 150 ppm 192 ppm 210 ppm 3
0.4 200 ppm 256 ppm 280 ppm 4
0.5 250 ppm 320 ppm 350 ppm 5
0.6 300 ppm 384 ppm 420 ppm 6
0.7 350 ppm 448 ppm 490 ppm 7
0.8 400 ppm 512 ppm 560 ppm 8
0.9 450 ppm 576 ppm 630 ppm 9
1.0 500 ppm 640 ppm 700 ppm 10
1.1 550 ppm 704 ppm 770 ppm 11
1.2 600 ppm 768 ppm 840 ppm 12
1.3 650 ppm 832 ppm 910 ppm 13
1.4 700 ppm 896 ppm 980 ppm 14
1.5 750 ppm 960 ppm 1050 ppm 15
1.6 800 ppm 1024 ppm 1120 ppm 16
1.7 850 ppm 1088 ppm 1190 ppm 17
1.8 900 ppm 1152 ppm 1260 ppm 18
1.9 950 ppm 1216 ppm 1330 ppm 19
2.0 1000 ppm 1280 ppm 1400 ppm 20
2.1 1050 ppm 1334 ppm 1470 ppm 21
2.2 1100 ppm 1408 ppm 1540 ppm 22
2.3 1150 ppm 1472 ppm 1610 ppm 23
2.4 1200 ppm 1536 ppm 1680 ppm 24
2.5 1250 ppm 1600 ppm 1750 ppm 25
2.6 1300 ppm 1664 ppm 1820 ppm 26
2.7 1350 ppm 1728 ppm 1890 ppm 27
2.8 1400 ppm 1792 ppm 1960 ppm 28
2.9 1450 ppm 1856 ppm 2030 ppm 29
3.0 1500 ppm 1920 ppm 2100 ppm 30
3.1 1550 ppm 1984 ppm 2170 ppm 31
3.2 1600 ppm 2048 ppm 2240 ppm 32
How do I tell if my PPM/EC is too high or too low?
It's simple to find out if you are using too much food or not enough by watching the nutrient concentration levels in your tanks day to day. Don't be concerned with the exact reading, rather watch how it rises and falls from each day to the next. The differences between when you put the solution into the tank and the readings you get several hours later or the next day are what tell you if your plant is eating, drinking or happy.
Start with 1.00 EC (or a SAFE nutrient strength). Next day, if it reads 1.4, it means your plants have been using water and your nutrient solution is becoming more concentrated. This means the concentration of nutrients is too high, so you dilute.
If the meter reads lower than the previous day, 0.7 say, it tells you that the plants are eating nutrients faster than they are drinking water, so you should increase your nutrient strength. If it remains the same, your feeding schedule is on target for now. The nutrient/water intake fluctuates with the growth of the plant, so you must continually monitor it day to day.
Your plants will tell you the optimum nutrient levels. When they are receiving optimum food and water, the readings remain constant. The more you do it, the easier it gets. The reason no one can tell you what PPM/EC levels to use is because every garden is different and every plant has different requirements due to their particular environment. That's why you have a ball park starting figure, but after that your plants will tell you almost exactly what they require.
What is a good ph/EC/TDS meter?
Added by: MisterIto
I use and recommend the Hanna HI9813 Grocheck portable ph, ppm and EC meter. It is available from most larger indoor garden supply retailers for around $200. I have had one for several years and it never drifts more than .2 on the ph scale or 30ppm from the original calibration made. I keep the electrode immersed in Hanna storage solution and use a hand held trigger sprayer to clean the film and residue that accumulates on it. I feel this contributes to my consistent readings without any need for re-calibration at all.
Added by ~shabang~:
I've had the lower model waterproof Oakton pens for over 5 years with no problems. I've replaced the pH probe twice since I let it dry out too long, but I did that for piece of mind and not because it really needed it.
What is a good ph/EC/TDS meter?
You can find some great meters here .
How can I raise or lower the pH of my soil mix?
How can I raise or lower the pH of my soil mix?
Growing in soil and adjusting pH levels
A lot of gardeners have trouble with the pH of their soil. A high pH can lock out needed nutrients and mimic other problems like Fe and Mg deficiencies. The biggest mistake new growers make is to try and correct pH problems too quickly. The first step in determining if high pH is the real problem, is to pick up a good pH tester. Don't be afraid to shell out the cash for a good one, it's well worth it!
Here are some recommendations: (All sell for under $100.00)
1. Milwaukee makes two styles of hand-held pH meters. A small "pen" called the Sharp and the larger Smart Meter. Both are easy to use. The Sharp pens are splash-proof (although not totally waterproof), and have a large easy to read display. They also have a detachable, replaceable probe.
2. Oakton - Same type of pH tester as Milwaukee makes, but it's made a little better imho. These are totally waterproof. (It floats.)
3. Shindengen ISFET pH Meters are state-of-the-art pH pens and work with a totally different method of measurement. This pen uses a solid state Ion Sensitive Field Effect Transistor (ISFET) instead of the fragile glass electrodes used by traditional pH pens. They have replaceable tips that change from opaque to clear when they need to be changed.
What is pH, and what do the terms acidic and alkaline mean?
The acidity or alkalinity of the soil is measured by pH (potential Hydrogen ions). Basically it's a measure of the amount of lime (calcium) contained in your soil, and the type of soil that you have. A soil with a pH lower than 7.0 is an acidic soil and one with a pH higher than 7.0 is considered to be alkaline. A pH of 7.0 is neutral.
Adjusting your soil pH :
Once you have determined the pH of your soil with a good tester, you can amend the soil if needed to accommodate the plants in your garden using inexpensive materials commonly available at your local garden center.
Adjust soil pH slowly over several days time, and check pH often as you go. Radical changes in pH may cause osmotic shock damage to the roots.
Raising soil pH : (to make it more alkaline)
It is generally easier to make soil mixes more alkaline than it is to make them more acidic. The addition of dolomite lime, hardwood ash, bone meal, crushed marble, or crushed oyster shells will help to raise the soil pH.
In soil: add dolomite limestone to the soil; use small amounts of hydrated lime.
Raising hydroponic pH : (to make it more alkaline)
In hydroponics: use potassium silicate, provides silicon at an effective doseage.
In bioponics/hydro-organics: add small amounts of sodium bicarbonate or lime.
Lowering soil pH : (to make it more acidic)
If your soil needs to be more acidic, sawdust, composted leaves, wood chips, cottonseed meal, leaf mold and especially peat moss, will lower the soil pH.
bloodmeal/cottonseed meal during vegetative; bonemeal during flowering.
Lowering hydroponic pH : (to make it more acidic)
In hydroponics: use nitric acid during vegetative; phosphoric acid during flowering.
Contributed by: Spiritual.Fa
Stabilizing pH with Dolomite lime
The best way to stable PH is by adding 1 ounce of Dolomite Lime per 1 gallon of planting soil.
Dolomite Lime is available in garden nurseries. Buy the fine Dolomite powder (There may be several kinds of Dolomite like Rough, Medium, Fine)
Dolomite Lime has been a useful PH stabilizer for years, since it has a neutral PH of 7 when added to your soil it stabilizes your soil at PH 7.
Mix the dry soil medium and dolomite together really well, give the mix a good watering then after the water has had chance to settle and leech into the soil a bit give the mix a really good stir. Then water the soil/lime mix and give it another stir
Best plan is to mix fine dolomite lime into your mix before planting. Fine Dolomite will help stabilize your pH; however, if the ph becomes unstable or changes, you can then use Hydrated Dolomite Lime. Add some of the hydrated lime to luke warm water and give it a good stir then water your plants with it. Give the plants a good watering with this hydrated lime added and your PH should fall or rise back to 7
Other Benefits of Dolomite Lime
Dolomite lime is also high in two secondary nutes that can often be overlooked by fertilizers; dolomite is high in both (Mg) Magnesium and (Ca) Calcium.
Are the liquid type Ph test kits accurate?
Liquid Ph test kits offer the grower an inexpensive alternative to the expensive test meters and give results which are accurate enough to accomplish the task just fine. Usually reading within +/- two tenths of a point (.02) of electronic meters, depending on the kit.
For the grower looking for the most accurate Ph control, there are many various electronic Ph testers available that when calibrated will give extremely accurate readings. Unfortunately for quality units, you may be looking at a higher price than you are able to pay, even the best units will suffer down time as probes go bad. Ph probes are especially prone to failure as the membrane ages, eventually drying out and failing.
The most common solution to fill the void is liquid type Ph test kit.
They're quick, reasonably accurate and very inexpensive.
Great backup in case your probe fails.
When using liquid Ph testers, there are some important things to keep in mind:
Always use a white background when looking at the color in the vial.
A non white back ground can alter the color subtly enough that you may end up being off by anywhere from a little to a lot depending on the color background being used.
Make sure that all color comparisons are done under natural light or normal household lighting and NOT HID light, as the red/blue spectrums of the light (HPS and MH respectively) will cause serious variations to the appearance of the solution, and in some cases you could be running your solution as much as a whole point above or below what it should be.
What pH level or range should I aim for?
Marijuana thrives in neutral to slightly acidic mediums. Soil and soilless mixes should aim for pH of 6. Hydro growers should go slightly lower which helps prevent pythium and other root rots-- 5.5 to 5.7 is ideal.
What ranges should I maintain for my hydroponic nutrients pH, TDS/EC and temperature?
I follow and highly recommend the following parameters for hydroponic nutrient solutions for aeroponic, “bubblers”, drip, ebb and flow, NFT, passive, rockwool and wick systems.
PH 5.1-5.9 (5.2 optimal)
TDS 500-1000ppm, EC .75-1.5
Temperature 68-78f, 20-25c (75f, 24c optimal)
The pH of the nutrient solution is a major determinant of nutrient uptake by the plant. If the pH wanders outside the optimum range of between pH 5.1 and pH 5.9, then nutritional deficiency and/or toxicity problems can occur. For hydroponic nutrient solutions used with inert media, keep the pH at 5.2 for optimal elemental uptake. It is at this point that roots most readily assimilate nutrients. These pH and TDS/EC recommendations may seem low relative to the normally suggested range, but are based upon information garnered from "Hydroponic Nutrients" by M. Edward Muckle and Practical Hydroponics and Greenhouses. They both document the low pH resulting in increased nutrient uptake and my experience has shown discernible health and yield improvements at a ph of 5.2 over higher levels.
On page 100, Hydroponic Nutrients displays both the assimilation chart for organic soil applications and another for inert medium hydroponics, which depicts the vastly different scenarios. The widely accepted soil based chart is frequently misapplied to water culture applications. His research and that done by others, documented in Practical Hydroponics and Greenhouses, indicate that iron and phosphorous precipitate in nutrient solutions at pH levels above 6. Stay below a pH of 6 by all means to avoid this problem and benefit.
The nutrient assimilation rate is further enhanced by the reduction in solution TDS/EC, which reduces osmotic pressure and allows the roots to draw the nutrients "easier". Young, established seedlings or rooted cuttings are started at 500-600ppm. The TDS is increased to 800-900ppm during peak vegetative growth. During the transition from early to heavy flowering, TDS is further raised to 1000-1100ppm. It is then reduced to 400-500ppm during the final 2 weeks of flushing. The plants demonstrate their preference for a lower TDS/EC when running a lower pH by clearly sustaining higher growth rates.
The optimum temperature for hydroponic solutions to be is 24c/75f. At this point, most elements are assimilated highest and atmospheric oxygen is most readily dissolved. Although increases in temperature increase the rate of photosynthesis, avoid exceeding the maximum listed of 25c/78f. Elevated temperatures make some elements more available, but reduce the solution's dissolved oxygen capacity, increasing root disease likelihood.
What is pH?
The pH scale measures how acidic, or how alkali a given solution is.
The term pH can be broken down into two parts; the first is the [p], this represents the mathematical symbol for -log (negative logarithm) of the number in question. The “H” stands for Hydrogen, and is represented by the chemical symbol [H] So the correct way to write this down is a small p, and a capital H (pH).
The pH scale is basically a rough guide as to how many Hydrogen ions are present in any given substance; the more hydrogen ions are present, the more acidic the substance becomes. The pH of distilled water is 7.0, this is neutral. Any solution with a pH below 7.0 (i.e. pH 1.0 to pH 6.9) is an acid and any solution with a pH above 7 (i.e. pH 7.1 to pH 14) is an alkali. The pH scale is logrithmic, that is, a pH 6.0 solution is 10 times more acidic than a pH 7.0 solution (pH 5.0 is 100x more than 7.0!).
Acidic solutions have a pH between 1 and 6.9 (your stomach contains HCl it is pH2).
Alkaline solutions have a pH between 7.1 and 14. (your small intestine is pH 9).
Neutral solutions are neither acidic nor alkaline so their pH is 7.
Acids all produce Hydrogen ions (H+). Acids like Hydrochloric acid produce lots of Hydrogen ions; this is because when Hydrogen Chloride gas dissolves in water the molecules of Hydrogen Chloride dissociate into Hydrogen ions and Chloride ions.
HCl = H+ + Cl-
Water also dissociates to produce ions, this time it is Hydrogen ions and Hydroxyl ions.
H2O = H+ + OH-
Sodium Hydroxide also dissociates to produce ions when it is dissolved in water, this time it is Sodium ions and Hydroxyl ions.
NaOH = Na+ + OH-
In each case, we can measure or calculate the concentration of Hydrogen ions present. We use the symbol [H+], we use square brackets to mean that it is the concentration of Hydrogen ions.
In HCl Hydrogen Chloride solution or Hydrochloric acid [H+] = 0.01
In H2O water [H+] = 0.0000001
In NaOH Sodium Hydroxide solution [H+] = 0.00000000000001
We count the decimal places from the first number, and that is where the pH scale is derived from.
HCl = pH2
H2O = pH7
NaOH = pH14
So to recap if the pH is low, it means that there is a high concentration of Hydrogen ions and if the pH is high it means that there is very low concentration of Hydrogen ions or none at all. Water and other neutral solutions are in the middle at pH7.
What is a pH buffer?
To more deeply understand pH, we must first explore the concept of chemical equilibrium. The pH of pure water is considered neutral, because this is the point at which the autoprotolysis of water is just as favorable of a process as the reverse reaction. The chemical equation for the autoprotolysis of water is as follows:
H2O <---> H+ + OH-
The equilibrium constant for this reaction is called Kw, which is equal to the product of the concentrations of hydrogen ion and hydroxide ion in solution in moles per liter (See Appendix 1 below to calculate the #moles/L)
The dissociation constant
Kw = [H+][OH-]
At this point, it is valuable to understand the p function itself. The p function of some value is equal to the negative logarithm of that value. So,
pH = -log[H+]
pKa = -log(Ka)
The value for Kw is 0.00000000000001 at 24°C, which is easier to write as pKw = 14. So, for pure water, we know that all H+ and OH- came from water molecules, and thus they are equal in number throughout the solution. Since they have the same value, we can use the above equilibrium expression as follows:
Kw = [H+][OH-] = [H+]2 = 10-14
Thus, [H+] = 10-7, and pH = 7.
Acids and Bases
Any chemical that increases the concentration of hydrogen ion in solution, or lowers the pH is an acid. Likewise, any chemical that increases the hydroxide concentration in solution is a base.
When an acid and its conjugate base are present in solution together, that solution is said to be a buffer, since it may react with acid or base without significant changes in pH. A hydroponic nutrient solution contains several conjugate acid-base pairs, since there are so many species present.
For a solution containing an acid HA and its conjugate base A-, the following equilibrium exists:
HA + H20 <---> H3O+ + A-
For this protolysis equilibrium, the acid dissociation constant is given by:
Ka = [H3O+]*[A-]/[HA]
The pH is given by the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
The term pKa refers to the p-function of the dissociation constant for that acid in water, similar to pKw for water. Notice from the equation above that as long as the acid and conjugate base are within one order of magnitude in concentration, additions of acid or base will not greatly affect the pH.
The buffering capacity, or ability to resist change in pH, is greatest within one pH unit of the pKa for the acid. A complex equilibrium exists between the concentrations of all of the species present in the nutrient solution and the concentration of available hydrogen ions, making the nutrient solution a buffer over a very large range. This is why adding acid to pure water decreases the pH much faster than adding acid to the mixed nutrient solution.
Any species added to solution that can be either a proton donor (acid), or a proton acceptor (base), sets up a buffer.
You may have found that pure water you leave out in the air becomes slightly acidic over time. This is due to the absorption of CO2 from the atmosphere. The chemical process is as follows:
CO2 + H2O ---> H2CO3 <---> H+ + HCO3-
Carbon dioxide reacts with water to form carbonic acid, which dissociates in water to hydronium and bicarbonate anion. This increases the concentration of H+ in solution, reducing the pH. The pKa of carbonic acid is 6.4, which is about the pH of pure water that has been exposed to the air.
ex) Potassium bicarbonate
Potassium is K+, bicarb is HCO3-. usually with diprotic acids like carbonic and sulfuric, the first H comes off pretty easily but since the ion has a -2 charge it holds onto the second proton fairly strongly.
Potassium bicarbonate is KHCO3, which dissociates to K+ and HCO3-. The bicarbonate anion can act as either an acid or a base. This makes it amphoteric.
In chemical fertilization, EDTA salts are used as “chelators”. The purpose is to form a more stable species in solution by using bidentate bonds. This means that the metal ion (such as Mg2+) will have two bonds for each EDTA molecule attached. This entropy of formation is higher for the EDTA complex, preventing the metal ions that you want to stay in solution from reacting to form insoluble compounds. Chelation makes the nutrient species more soluble, and thus more readily available for uptake.
What effect does pH have on elements in solution?
The element of interest to the plant must be present in an ionic form that can be transported by the roots. Changes in pH mean changes in concentration of H+ and OH-, which drive changes in equilibrium between various salt forms. For example... if the pH is too high, any available OH- will react with manganese or magnesium, or any of the various components of the nutrient solution.
Mg2+ + 2 OH- ---> Mg(OH)2
Magnesium hydroxide is not available for passive transport into the root system, but Mg2+ is. On a similar note, contamination by chlorine is bad for your solution, because MgCl2 is insoluble as well, and has a high rate constant of formation.
1. Calculating Molar concentration
The molar concentration of a substance in solution is calculated by converting the mass of the substance into moles, and dividing that number by the liters of solution.
To make the conversion, you add up the atomic masses (from the periodic table of elements) for each atom in a single molecule of that substance. This is the molar mass. Divide the mass of the substance added to solution by the molar mass. This result is the number of moles. Divide this by the volume to get the molar concentration.
Let's do an example:
We add 2.5g Epsom salts to 2 liters of water. The chemical formula for Epsom salt is MgSO4·7H2O.
The atomic masses are as follows:
Mg = 24.3 g/mol
S = 32.1 g/mol
O = 16.0
H = 1.01
Now remember to multiply each mass by the number of that species present in the molecule.
Total mass = 24.3 + 32.1 + (11*16.0) + (14*1.01) = 246.5 g/mol.
Now we convert grams to moles: 2.5g / 246.5 g/mol = 0.0101 mol.
Since we used two liters, we divide number of moles by 2, and the result, [MgSO4·7H2O] = [Mg+] = [SO4-] = 0.00507 mol/L = 0.00507 M.
Note: since Epsom salt is an ionic species, it dissociates in solution.
How long does the seal last inside the probe?
Something to note for all pH meters is the age of the meter; the gel seal inside the meter is only usually guaranteed to keep its seal for 2 years after being manufactured, although the gel seal may last much longer this.
Digital meters such as the pHscan 1 have the month and year of manufacture on the inside of the battery lid. I suggest checking it when purchasing as some hydro stores may have slow stock turnover.
What does pH mean?
The degree of acidity/alkalinity of a solution is identified on the ph scale of 0 to 14, with a pH of 7 representing the neutral point. The pH scale is logarithmic, meaning small changes in pH represent large changes in the degree of acidity or alkalinity. For example, a solution with a pH of 5 is ten times as acidic as a solution with a pH of 6, but a solution with a pH of 5 is 100 times as acidic as a solution with a pH of 7. The pH of the nutrient solution is a major determinant of nutrient uptake by the plant.
What is a TDS meter and what does it measure?
Total Dissolved Salts meters are essentially little voltmeters that look at the voltage produced by a sensor, usually a couple of metal pins. The nute solution acts like a battery electrolyte and the pins function as do plates (electrodes) in a battery. The idea is that a nutrient solution is more electrically conductive when there are more nutrient salts in solution, so more salts means more voltage. A little math is done in the machine to convert the voltage to ppm (parts per million of dissolved solids).
There is a calibration adjustment so this math can be touched up to compensate for various factors. You will need a test solution to verify your meter once a week. Usually you will find a single measurement at about 1500-1700ppm is enough to verify it's reading what it's supposed to.
You need one that will read at least 0-2000ppm (or 0-1999ppm). You could use a 0-999ppm meter in a pinch if you added an equal volume of plain water to a sample from your tank-- you'd just double the meter reading.
It's best to simply get the correct meter.
There are other scales of measurement of nutrient concentration. In Europe, the "EC" (electrical conductivity) meters are preferred. They measure in units of millisiemens or mS instead of parts per million (ppm). The numbers are convertible one scale to the other, but most references and discussion here cite the ppm scale.
Waterproof meters are both more expensive and worth it.
How do I figure out the ppm of my fertilizer mix?
To figure out the ppm of your fertilizer (or fertilizer mix), you need to be able to measure grams and liters. Look at the 3 numbers on the side of a fert bag. These are the percent content of the nutrients. For every one gram of said fertilizer in one liter of water, it contributes 10 ppm of the given nutrient per percentage point. A 20-20-20 gives 200 ppm (10 ppm X 20) of each nutrient for each gram in a liter of water.
The formula is this:
grams of fert per liter = A/B
A=your desired ppm
B=10 ppm X the % of nutrient in mix
your ppm = C X B
B=10 ppm X the % of nutrient in mix
C= grams of fert per liter
So to make a 200 ppm-100 ppm-200 ppm NPK mix using a 13-0-44 (potassium nitrate), a 12-62-0 (monoamonium phosphate), and a 33-0-0 (ammonium nitrate) you would work backwards from your sole P and K sources (it makes it easiest in this case), and make up the N at the end. I have rounded numbers to the nearest 0.1 g for the following. You would use 0.5 g of potassium nitrate (200 ppm/(10 ppm X 44 K)) and 0.2 g of monoammonium phosphate (100 ppm/(10 ppm X 62 P)) in one liter. This would give you 89 ppm N (10 ppm X 13 N X 0.5 g + 10 ppm X 12 N X 0.2 g), 124 ppm P (10 ppm X 62 P X 0.2), and 220 ppm K (10 ppm X 44 K X 0.5 g). 111 ppm are needed to raise the N to the 200 ppm level, so we can use 0.3 g of the ammonium nitrate (111 ppm/(10 ppm X 33 N)) to bring us up to finish.
The actual mix would yield a 188 ppm N, 124 ppm P, 220 ppm K mixture in one liter of water. To get more precision, you need to mix larger batches or get a better scale (you would need to make a 10 liter batch of the above with a scale that is only accurate to the gram).
If you mix your own fertilizer, you can adjust your N source to meet your pH needs, rather than being dependent on adding acid or base, which is nice.
This works for formulating hydro mixes, as well as for us dirt farmers
thanks for all the info on meters and krap ive soil grown b4 but hydro is relitivly new to me
and i was having trouble figuring out what meters to buy but this thread has help immensly this is a truly wonderfl site
thanx again >chronic
Ebay have some good cheap TDS/PPM meters,i got mn for £15 and its pretty acurate to.
Those 'HM Digital' brand ones that are often on there are good.. might be the ones you mean as they're usually £15 ish. Had mine a couple of years and the accuracy still seems within a few ppm.
very informative, cheers for taking the time and effort to do this IKDJ2003
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