Introduction to TDS
Definitions
The electrical conductivity (EC) of your nutrient results from motion of
mineral ions when the meter applies an electrical voltage. The ppM or Total
Dissolved Solids (TDS) value of a potassium chloride solution happens to
be very close to half of its conductivity value(in microSiemens/cm).
ppM (parts per million) is a common unit for measuring the concentration of elements in the nutrient solution. One ppm is one part by weight of the mineral in one million parts of solution.
Practical
Considerations
When using a meter to measure either the ppm of total dissolved solids
or conductivity of a liquid, it is necessary to periodically calibrate
the meter using a calibration
standard solution. There are, however, special considerations to be given
to each type of calibration. Whereas conductivity is an absolute measurement
with
calibrations that are transferrable from one type of solution to another,
ppm total dissolved solids calibrations are specific to one type of dissolved
solids
solution and must not be transferred from one type of dissolved solids solution
to the next. Doing this will result in some serious errors in measurement.
Although the basis for testing ppm of total dissolved solids is the conductivity of the solution, it is not correct to assume that this measurement is absolute. It is always necessary to calibrate all total dissolved solids meters with a parts per million total dissolved solids standard calibration solution that contains the same type of salts or mixtures of salts as the solution to be tested. Failure to do this will result in serious errors in the measurement of total dissolved solids. This is because total dissolved solids meters are calibrated by correlating the conductivity of the solution to the ppm dissolved solids and this correlation varies considerably from one species of dissolved solids to the next.
One similar conclusion can be made for all types of dissolved solids. Most pre-formulated parts per million total dissolved solids standard calibrated solutions are formulated with calcium carbonate (CaCO3), sodium chloride (NaCl), potassium chloride (KCl), or the 442 (40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) natural water formulation. If your test solution's major dissolved solids components are the same as any of these, you may want to choose the pre-made formulation that best approximates your test solution. Generally, CaCO3 is used for boiler waters, NaCl is used for brines, and the 442 formulation is used for lakes, streams, wells, and boilers. Alternatively, if the contents of the ppm standard calibration solution used for calibration are known, it is possible to cross reference from existing calibration curves to curves for different types of dissolved solids solutions. Curves and tables are available in various reference books.
The previous discussion and references are based on standard conditions of temperature (25 deg C). When measuring conductivity or total dissolved solids in other than standard conditions, certain corrections for these variations must be accounted for before going on to determine the final values of conductivity and total dissolved solids. Without some sort of correction for standard temperature, conductivity or total dissolved solids measurements at various temperatures are meaningless because they cannot be compared. Many meters overcome this by incorporating temperature sensing elements and temperature sensing circuitry into the meter so that the value given is corrected for standard temperature. Using a meter that does not have temperature compensation will require the operator to use look-up tables or formulas to correct for the temperature effect. A good discussion of the effect of temperature on conductivity and total dissolved solids testing can be found on pages 6 and 7 in the article "Theory and Application of Electrolyte Conductivity Measurement", Copyright 1982 by the Foxboro Company.
One more thing to consider is the relative cleanliness of your TDS or EC electrode. A polarized or fouled electrode must be cleaned for renewal of the active surface of the cell. To prevent damage, abrasives or sharp objects should not be used. In most situations hot water with a mild detergent is effective, however acetone removes most organic matter and chlorus solutions are good for algae, molds, and bacteria.
Conversion of TDS Values
When working with TDS (Total Dissolved Solids) measurements, there is
sometimes a need to convert units. Below are a few common measures
and conversions to
other units.
1 Grain = 0.0648g (grams) = 64.8mg (milligrams) = 0.00228 oz (ounces)
Convert grains to grams: multiply grains by 0.0648 grains/gram = grams
Convert ounces to grams: multiply ounces by 28.35 grams/ounce = grams
Convert grams to mg (milligrams): multiply grams by 1000 mg/g = mg
Convert mg to ppm in a liquid: divide mg into liters = ppm
Convert ppm to ppt: divide ppm by 1000 ppm/ppt = ppt
Or vice-verse:-
Convert ppt to ppm: multiply ppt by 1000 ppm/ppt = ppm
Convert ppm to mg: multiply ppm by liters = mg
Convert mg to grams: divide by 1000 mg/g = grams
Convert grams to ounces: divide grams by 28.35 grams/ ounce = ounces
Convert grams to grains: divide grams by 0.0648 grams/grain = grains
From the top, an example of converting grains to ppm and ppt:
5 grains x 0.0648 grains/gram = 0.324 grams
0.324 grams x 1000 mg/g = 324 mg
324 mg ÷ 1 liter = 324 ppm
(if in 2 liters . . . 324 mg ÷ 2 liters = 162 ppm)
(if in 0.5 liters . . . 324 mg ÷ 0.5 liters = 648 ppm)
324 ppm ÷ 1000 ppm/ppt = 0.324 ppt
You may already see the short cut
0.324 grams in 1 liter = 0.324 ppt (because 1 gram in 1 liter = 1 ppt)
For ounces, here is how to convert to ppm and ppt:
5 ounces x 28.35 grams/ounce = 141.75 grams
141.75 grams x 1000 mg/g = 141,750 mg
141,750 mg ÷ 1 liters = 141,750 ppm
(if in 2 liters
. . . 141,750 ÷ 2 liters = 70,875 ppm)
(if in 0.5 liters . . . 141,750 ÷ 0.5 liters = 283,500 ppm)
141,750 ppm ÷ 1000 ppm/ppt = 141.75 ppt
Conductivity To TDS Conversion Factors
1. Factor - the conductivity to ppm TDS conversion factor. Multiply conductivity
by this factor to get ppm TDS for the type of TDS reading needed.
2. 442 - a formulation that most closely represents the conductivity to ppm relationship, on average, for naturally occurring fresh water.
Factor = Actual TDS / Actual Conductivity @ 25 oC.
| Conductivity at 25 oC |
TDS KCl | TDS NaCl | TDS 442 | |||
| ppm Value | Factor | ppm Value | Factor | ppm Value | Factor | |
| 84 µS | 40.38 | 0.5048 | 38.04 | 0.4755 | 50.50 | 0.6563 |
| 447 µS | 225.6 | 0.5047 | 215.5 | 0.4822 | 300.0 | 0.6712 |
| 1413 µS | 744.7 | 0.5270 | 702.1 | 0.4969 | 1000 | 0.7078 |
| 1500 µS | 757.1 | 0.5047 | 737.1 | 0.4914 | 1050 | 0.7000 |
| 8974 µS | 5101 | 0.5685 | 4487 | 0.5000 | 7608 | 0.8478 |
| 12,880 µS | 7447 | 0.5782 | 7230 | 0.5613 | 11,367 | 0.8825 |
| 15,000 µS | 8759 | 0.5839 | 8532 | 0.5688 | 13,455 | 0.8970 |
| 80 mS | 52,168 | 0.6521 | 48,384 | 0.6048 | 79,688 | 0.9961 |
