For many growers, a basic pH-only meter or pH test-strip kit might be all that they need. Read my pH blog and find out why. Ten-to-fifty dollars beats one hundred-and-fifty dollars or more in my book, especially if you don't need the extra bells and whistles.
This blog isn't specifically about the MW 802 or any other brand or model. It's not a comparison of products or prices. This time, I'm talking about the basic operation of a multi-function meter and what the readings are called and how they are useful. If you feel that you absolutely have to have a multi-meter, then this is a primer on terminology and basic operation.
Please understand the following before you rush out to spend money on a meter. A meter is only an extension of your knowledge, skills, observation, and intuition. It is a tool for obtaining comparative numerical data over a period of time. Leaf deficiency charts are very useful tools for seeing how your plants are growing. They are free. Read my leaf deficiency chart blog here. Observing the effect of temperature and pH on your plants is an immediate-result tool for controlling plant growth and health, and is easy, fast and inexpensive. Read my pH blog here.
I still maintain that if you are using commercial two-or-three-part elemental salts-based nutrients solutions like General Hydroponics and other manufacturers, and using them according to instructions, you really only have to keep track of pH and temperature changes. These companies have worked out precise measurements for almost every type of plant. The charts are readily available.
It really is dead-simple. If the bottles or data sheets say “add 2 teaspoons of each part to a gallon of water to get this plant to grow this way,” or whatever amount specified, then that's all that you do. Change your solution every one to two weeks, flush the reservoir occasionally, and watch temperature and pH flunctuations. A thermometer, an inexpensive pH meter or kit, and “pH up-and-down “ fluids will suffice in most situations.
So you are still convinced that you need a full-featured, “do-it-all” multimeter? Okay. Then here's what they measure and why. A simpiflied explanation is that all meters measure the 'salinity,' or elemental salt-content of the hydroponic nutrient solution. Here is my You Tube video talking about meter use. Here are the basic functions:
pH. In my opinion, one of the most important tools for growing healthy plants. Here's my blog on pH. Please read it.
Parts-per-million or 'ppm' means how much of something is suspended or dissolved in the solution, measured as one part by weight of the element in one million parts by weight of solution. The meter is not designed to measure stuff that can't conduct an electrical current, such as solid organics, dirt, and other non-conductive particles. Some growers prefer ppm readings over TDS or EC readings.
Total Dissolved Solids, or 'TDS.' Some growers also use the term 'Total Dissolved Salts.' Salts are chemically changed elements, called 'ions,' that are dissolved in a 'solvent;' usually water. The salts are 'solutes,' and the water/ salt mixture is called an 'electrolytic solution.' The solution allows an electric current to flow through it.
Meters have a circuit that will measure the strength of that current and convert it to a usable signal that drives either an analog (sweep hand) or digital (readout) display. It cannot differentiate between table salt (sodium chloride; not desirable for nutrient solutions) and potassium nitrate, a common elemental salt in many fertilizers. Measuring an electrical current is all it can do. A “salt is a salt” to the meter.
Electrical Conductivity, or 'EC.' Electrical conductivity is basically the same as TDS, just measured in different units. In laboratory settings, the difference in TDS and EC may be useful, but in fieldwork, there isn't any practical difference. My MW 802 meter measures both EC and TDS, and it has a calculator circuit that converts from one to the other automatically.
The MW 802 measures the conductivity in what is called millisiemens, and it is millisiemens per centimeter, or mS/cm. Some meters show measurements in microsiemens per centimeter, or uS/cm. There is a formula to convert from one to the other. Temperature and pH affect the reading, and the MW 802 automatically compensates for changes.
All meters will have a calibration button or knob. If you buy a meter, be sure to purchase the correct calibration solution. I also highly recommend buying probe-cleaning and storage solutions.
Some meters may only individually measure ppm, TDS, or EC. You might need to use a table to convert from one to the other. Those tables are available through meter manufacturers, solution manufacturers, and the tables are also widely available on the internet. Really, the choice for which type to use is what you want to use. There are also formulas that can be used for conversions. If you need the formulas, they are easily found at hydroponic sales sites, and meter and solution manufacturers' sites. Remember, temperature and pH affect the calculations.
Remember that “a salt is a salt.” No meter is a 'fix-all” genius computer. If your solution has more of one type of elemental salt than another, and it's not what your plant needs at that time, the meter will NOT give you that information. Observation and experience, along with pH and temperature monitoring will make the difference between a thriving plant and an ailing plant. Numbers are great for comparative data over time creating an average that, when combined with visual data, might assist in correcting deficiencies.
You already own three of the greatest tools for growing plants: Your brain, your eyes, and your ears.
Grow well. Have fun. Learn. Eat well. Live well.