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There are a number of situation in which you are interested in a measure of total dissolved solids (=TDS) in water. For example:
Drinking water: The TDS number will tell you what quantity of other substances have dissolved into your drinking water. You would like to know what is the quality of your tap water. And if you have a filter (either a reverse osmosis filter, or a faucet filter), the TDS meter will tell you how well it's doing its job.
Hydroponics and Gardening: If you grow plants by hydroponics, the concentration of nutrients in the water is important.
Aquarium owners: Finally, if you have a saltwater aquarium, you will want to know what is optimal for your fish.
In all these situations, you are interested in the concentrations of ions in the water. A TDS meter will help with measuring that. There are a couple of methods by means of which the TDS meters work. Here is a guide to some of the best TDS meters on the market.
TDS can be measured by one of two methods:
1) Gravimetric analysis: This involves adding chemicals to the water to cause the dissolved solids to precipitate out of the water. Another method is by evaporating the water and analyzing what is left. This is very accurate, but also time-consuming.
2) Electrical Conductivity: This works on the principle that how well water conducts electricity depends on the concentration of ions (atoms or molecules with an excess or protons or electrons) in the solution. You place the two electrodes of the TDS meter into the water, and it runs a small current between the electrodes. The more ions in the water, the better it will conduct.
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Calibration: There is a calibration fluid which has a known TDS level. You use that to set your TDS meter. If you see that the TDS meter is giving different readings every time you dip it in the calibration fluid, it must mean that there is a problem with the meter. (People notice that the more expensive meters are worth the investment—the cheaper models will have to be calibrated more often.)
What's in the water: Since TDS meters measure conductivity, they will tell you that there are salts dissolved in the water. But they won't be able to tell you which salts are in the water. And the meter won't be able to tell you about the solids that don't affect the conductivity.
Other uses: You can also use the TDS meter for bottled water. Also, if you are camping and you filter water from a pond or stream with a camping filter, it can help you know whether it's safe to drink.
For Drinking Water: If you have a reverse-osmosis filter in your home for your drinking water, you may want a TDS meter to see if the filter is working well. The filter should be replaced every 6 months or so—but if you live in a place with hard water, your filter may work harder, and maybe you should replace the filter more often. Besides the filter, there is also the membrane of the system that might have to be replaced.
How to use the TDS meter for your drinking water: The way to use the meter is to read the TDS figure in parts per million (ppm) on your regular tap water, and compare it with the TDS figure for you filtered water. If the ratio of TDS filtered/TDS tap water = 0.10, that means that 90% of the solids in the water are being filtered out. That's good performance. If that ratio is 0.20 or more, it means that the filter already isn't doing a very good job. You should replace the filters, or even the membranes of the filter.
Conversion factors: The TDS meter essentially measures electrical conductance (=EC). Electrical conductance is measured in the unit siemens, or millisiemens (1/1000 siemen) or even microsiemens (1/1 million siemens). Different salts and ions will conduct electricity differently. The TDS meter will convert the EC figure based on some sort of approximation of the different salts found in the water. If the scale on your TDS meter gives you a reading of electrical conductance of 600 microsiemens, it will convert to 420 parts per million (=ppm) of dissolved solids. (You multiply by 0.7, which is assuming an average combination of different salts in the water: 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride. There are other conversion factors, assuming having salts of one type more than another.)
Combination meters: People who have a few aquariums measure the water's TDS level, as well as the temperature, and maybe some other figures as well. (They want to know what temperature and salt level is optimal for their fish.) Look for TDS meters that can give you those other parameters as well.
HM Digital-- was established in 1987, under the name Heemang Electronics, and are located in Culver City, California. They make water testing and monitoring equipment for water treatment and purification systems.
M&A Instruments-- are manufacturers of industrial and scientific meters. They are located in Arcadia, California.
Hanna Instruments—was founded in 1978 in Limena, Italy by Oscar and Anna Nardo. They are leaders in the development of electro-analytical instrumentation. They make pH meters, photometers, titrators, colorimeters, refractometers, and more.
Milwaukee—is a manufacturer of measuring instruments and meters. Their headquarters is in Rocky Mount, North Carolina.