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Total Dissolved Solids (TDS) is a measure of the total amount of dissolved matter in a pool or spa. It comprises all of the ions (charged species) present in the water, but does not include uncharged entities (eg. insoluble organic and inorganic waste, suspended particles, etc.). A list of ions potentially present in pool or spa water is tabulated below. Note that this list is far from exhaustive.


The TDS level in parts per million (ppm) can be determined by measuring the electrical conductivity of the water and multiplying by a conversion factor. This factor varies between 0.50 and 0.90 depending upon the nature of the sample, but is nominally chosen as 0.67 for swimming pools.(1) The salt level can also be calculated by multiplying the TDS level by a second factor, although conjecture exists as to what this factor should be. Myron L, manufacturer of the popular analogue unit used by many pool shops, proposes 0.74,(2) whilst Palintest, makers of many of the test tabs and photometers used in the Pool Industry, suggests 0.82.(3) Either way, salt levels determined in this manner are only rough approximations. The "true" salt level can only be found using an analytical method that determines chloride (eg. using a chloride-selective electrode or performing a titration using a standardised silver nitrate solution). For our purposes, however, the TDS approximation method is perfectly acceptable.

TDS is usually measured using either a meter or a pen, both of which rely on a pair of platinum electrodes. Traditionally, many pool shops have used the analogue Myron L meter. Whilst robust and simple to use, it does suffer from a lack of serviceability (most internal components are "potted" in plastic). In 2001, Bio-Lab introduced TDS pens sourced from Hanna Instruments. Having a digital readout, these are often simpler to read than the analogue output from the Myron unit. Late in 2002, Hanna updated the pens' design, introducing the Model HI98312 which featured an automated calibration procedure much like that used in many pH pens. Importantly, this meant that only a specific calibration standard can now be used, as the microprocessor in the pen senses whether the standard falls within preset values. In other words, the option of calibrating the pen using any solution of known TDS or conductivity no longer exists.

Calibration is recommended on a weekly basis. With the advent of the HI98312 TDS pen, Bio-Lab has added Hanna's 12.88 mS/cm calibration standard to our range of test solutions. To order, request Part No. 6151. The standard is supplied in a foil sachet containing 20 mL of solution. When ready to use, it is recommended that the standard be transferred from the sachet to a resealable airtight container, where it should be kept when not in use. Cleaning the pen (Hanna units) or the test reservoir (Myron L units) with fresh water and drying it prior to immersion in the calibration standard will extend the life of the solution by effectively eliminating contamination or dilution. If this protocol is followed meticulously, the shelf life of the calibration standard is approximately 6 - 12 months.

Calibration and Measurement Using Hanna HI 98312 EC/TDS Pen (Rounded profile)

1. To calibrate, turn the unit on by pressing the MODE button. On brand new units, the word CAL will appear in the lower left hand corner of the LCD screen. On units calibrated previously, press the MODE button until the word CAL appears.
2. Immerse the pen in the 12.88 mS/cm calibration solution. This will automatically perform the calibration. If satisfactory, the LCD screen will display the word OK for 1 second before returning to normal measurement mode.

As supplied, the conversion factor (described above) on the Hanna HI98312 unit is usually preset to 0.50. This only needs to be changed the first time the unit is used. Once set, the new conversion factor remains in memory, even when the unit is switched off.

1. Turn the unit on by pressing the MODE button.
2. Press the MODE button until the word CONV appears in the lower left hand corner of the LCD screen.
3. Press the SET/HOLD button until the value next to the word CONV reads 0.67.
4. Repeat pressing the MODE button until normal measurement mode appears.

Note: The change to the conversion factor cannot be performed until the unit is calibrated using the 12.88 mS/cm standard. For further information, refer to the pamphlet that companies the pen.  To determine TDS values, select the parts per thousand (ppt displayed in upper part of the LCD screen) mode using the SET/HOLD button. To convert to parts per million (ppm), simply multiply the on-screen reading by 1000.

Calibration and Measurement Using Hanna DiST WP TDS Pen (Squared profile)

1. To calibrate, turn the unit on by pressing the button on top of the pen.
2. Immerse the pen in the 12.88 mS/cm calibration solution.
3. With a small screwdriver, turn the CALIBRATION grub-screw on the back of the unit until the reading on the LCD screen is 8.6 ppt. This value comes from multiplying 12.88 mS/cm by the 0.67 pool water conversion factor.

To determine TDS values in parts per million (ppm), simply multiply on-screen readings by 1000.

Calibration and Measurement Using Myron L TDS Meters

1. To calibrate, turn the unit upside down and remove the baseplate from the meter. In the centre of the unit is a variable potentiometer with a white or off-white "wheel".
2. Turn the unit the right way up and fill the test reservoir with the 12.88 mS/cm calibration solution.
3. Turn the "wheel" on the variable pot until the reading on the LCD screen is 8,600 ppm. This value comes from multiplying 12.88 mS/cm by the 0.67 pool water conversion factor and scaling from parts per thousand to parts per million. Note that some Myron L units only go to 5,000 ppm. In this case, turn the "wheel" until the reading is 4,300 ppm. Don't forget that all subsequent measurements must be multiplied by two in this case.

Determination of Salt Levels

Given the differences of opinion that exist on conversion from TDS to salt levels, and that the value is only a rough approximation anyway, Bio-Lab suggests taking 3/4 of the TDS value as salt. Consequently, if a saltchlorinated pool records a TDS of 5200 ppm, then the apparent salt level will be 0.75 x 5200 = 3900 ppm. Note that some older Myron L units have been "pre-calibrated" as salt meters. To confirm whether your unit is set up for TDS or salt, turn the unit upside down and check whether there is a cross to the immediate left of the number "442" (TDS calibration) or the term "NaCl" (salt calibration). Converting a Myron L salt meter to a TDS meter is simply a case of recalibrating using the 12.88 mS/cm calibration solution and the method described above.

 

References

1. "Using Conductivity Meters in Agriculture" (www.dpi.qld.gov.au/horticulture/3096.html), Queensland Government Department of Primary Industries.
2. "DS/pDS Meters, Conductivity or TDS & pH Instruments" (www.myronl.com/products/ds_pds.htm), Myron L Company, Carlsbad, CA.
3. "Pocket Sensors" (www.palintest.com.au/Australian%20images/Australian%20PDF%20files/product%20flyerpocket% 20sensors.pdf), Palintest Australia Ltd, Peakhurst NSW.

The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein