AXEON TDS-EZ HM Digital - Manual (Page 2)

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TDS
Total Dissolved Solids correlates to the ability of wate r to conduct electricity. It is also an index used to
determine the concentration of dissolved minerals. The more minerals that are dissolved, the more
conductive the water will be. A TDS meter is calibrated to read in parts per million (PPM). TDS is the
concentration of a solution as the total weight of dissolved solids. (1 ppm = 1 milligram/litre. TDS is a
mass estimate and is dependent upon the mix of nutrients as well as the concentration.
Conductivity
Electrical conductivity is a measure of the abili ty of a solution to carry a current and depends on the total
concentration of ionized substances dissolved in the water. (the electricity flows by ion transport).
Although all ions contribute to conductivity, their valences differ, so their actual and relative
concentrations affect conductivity. When the concentration of ions is high, conductivity is high, and the
resistance to electrical passage is low. No meters have the ability to distinguish between different types
of ionic salts. Conductivity measurem ents are also complicated by the fact that not all salts conduct an
electric current equally
How the TDS meter works
TDS meters are, in reality, conductivity meters. They work by applying a voltage between two or more
electrodes. Positively charged ions w ill move toward the negatively charged electrode, and negatively
charged ions will move toward the positively charged electrode. Because these ions are charged and
moving, they constitute an electrical current. The meter then monitors how much current is passing
between the electrodes as a gauge of how many ions are in solution. This measure of conductivity,
µS/cm is then converted to ppm by a factor of approximately 0.5, on a curve ranging from 0.47 to 0.55,
depending on the level. The factor is related directly to the level of conductivity. This meter is built and
calibrated according to an NaCl standard. Other meters may be calibrated to either a KCl standard (0.51
conversion) or the 442 standard.(0.7 conversion)
What the TDS meter actually detects
Since TDS meters are often used to test water "purity," it is important to understand what they do not
detect. As conductivity meters in disguise, TDS meters will only detect mobile charged ions. They will
not detect any neutral (uncharged) compounds. Such compounds include sugar, alcohol, many
organics (including many pesticides and their residues), and unionized forms of silica, ammonia, and
carbon dioxide. These meters also do not detect macroscopic particulates, as those are too large to
move in the electric fields applied. So if you see "rusty" looking water from iron oxide particulates, that
won't be measured. Neither will anything else that makes the water look cloudy. Bacteria and viruses
also won't be detected.
Total charged ions" is likely a much better term for what the meter measures. Fortunately, a
measurement of total charged ions is good enough for most purposes.
Temperature Compensation:
The conductivity of ions in water depends upon temperature. The ions are naturally moving around
faster as they get warmer. When the same numbers of ions are moving faster, the apparent conductivity
is increased.
Our meters are capable of compensating for temperature by simultaneously measuring the conductivity
and the temperature. The internal electronics then take the temperature into account, and normally
provide a value that is "corrected" to what the conductivity would be at a standard temperature (25°C).
How external factors may affect readings
While pure water has a TDS well below 1 ppm, uncertainties fr om carbon dioxide in the air (which gets
into the water and ionizes to provide some conductivity) and the TDS meter itself may yield results of 1
or 2 ppm even from pure water
The entire electrode assembly must be submerged in the sample without a lot of bubbles or solids
present between the electrodes. So, for example, you cannot typically get a good reading by holding it in
a stream of tap water because air often gets between the electrodes that way (resulting in an artificially
low reading).