Tommo
11-04-2012, 10:22 PM
There have been a number of questions on the forum regarding the use of TDS meters etc. I think that members may find the following information useful (including purchasing calibration fluids):
"What is TDS/PPM/EC
Firstly, TDS (or Total Dissolved Solids) is measured in units of PPM (or Parts Per Million). EC (or Electrical Conductivity) is measured in either µS (MicroSiemens) or mS (MilliSiemens). In the USA we use PPM and internationally µS and mS are the preferred measurements. Both TDS and EC meters measure nutrient strength in the same way with the same accuracy; however the readouts are displayed differently. For example: a nutrient solution that measures 1000ppm, on a TDS meter, will be the same strength as a solution that reads 1430 µS or 1.43 mS on an EC meter.
TDS and EC meters measure the strength of a nutrient solution by measuring the flow of electrical current between two metallic probes. The higher the salt concentration in the nutrient solution the better it conducts electricity; therefore the higher the reading on the meter. It is extremely important that the batteries are in good, strong condition and the probes must always be clean from salt deposits. Weak and dirty probes will affect the accuracy of the reading and are the biggest causes of inaccurate readings.
PPM Conversion Factors
Not all TDS meters will give the same PPM reading. The main reason for this is that different meters will use different conversion factors. These conversion factors are based off the EC reading. For example: A meter that uses a conversion factor of 0.5 will display a reading of 0.5 times that of the EC reading. If the EC reading is 1000 µS then the ppm reading will be 500 ppm. It is as simple as multiplying 1000 by the conversion factor of 0.5. If the meter uses a conversion factor of 0.7 (the most common conversion factor used) then 1000 µS would be 700 ppm (1000 x 0.7). When purchasing a TDS meter it is very important to find out what the correct conversion factor is for the meter. It could make a difference of more than 200 ppm! Not only is it important to know what the conversion factor for the meter is, it is also important to know the conversion factor of the calibration solution. Generally speaking, Genesis calibration solutions are for meters that use a 0.7 conversion factor and the Hanna solutions are for meters with a 0.5 conversion factor".
[see link for the conversion factors used on some brands of meters]
Source: http://www.bghydro.com/bgh/static/articles/0406_tdspH.asp
As some of you may know from my previous posts: a pet hate of mine is the over reliance on the use (and misunderstanding) of TDS when measuring the carbonate hardness of water. Don't take my word for this, boil some RO water in an aluminium pan and measure the TDS before and after. I think you'll be surprised at the significant increase in the TDS readings...
"What is TDS/PPM/EC
Firstly, TDS (or Total Dissolved Solids) is measured in units of PPM (or Parts Per Million). EC (or Electrical Conductivity) is measured in either µS (MicroSiemens) or mS (MilliSiemens). In the USA we use PPM and internationally µS and mS are the preferred measurements. Both TDS and EC meters measure nutrient strength in the same way with the same accuracy; however the readouts are displayed differently. For example: a nutrient solution that measures 1000ppm, on a TDS meter, will be the same strength as a solution that reads 1430 µS or 1.43 mS on an EC meter.
TDS and EC meters measure the strength of a nutrient solution by measuring the flow of electrical current between two metallic probes. The higher the salt concentration in the nutrient solution the better it conducts electricity; therefore the higher the reading on the meter. It is extremely important that the batteries are in good, strong condition and the probes must always be clean from salt deposits. Weak and dirty probes will affect the accuracy of the reading and are the biggest causes of inaccurate readings.
PPM Conversion Factors
Not all TDS meters will give the same PPM reading. The main reason for this is that different meters will use different conversion factors. These conversion factors are based off the EC reading. For example: A meter that uses a conversion factor of 0.5 will display a reading of 0.5 times that of the EC reading. If the EC reading is 1000 µS then the ppm reading will be 500 ppm. It is as simple as multiplying 1000 by the conversion factor of 0.5. If the meter uses a conversion factor of 0.7 (the most common conversion factor used) then 1000 µS would be 700 ppm (1000 x 0.7). When purchasing a TDS meter it is very important to find out what the correct conversion factor is for the meter. It could make a difference of more than 200 ppm! Not only is it important to know what the conversion factor for the meter is, it is also important to know the conversion factor of the calibration solution. Generally speaking, Genesis calibration solutions are for meters that use a 0.7 conversion factor and the Hanna solutions are for meters with a 0.5 conversion factor".
[see link for the conversion factors used on some brands of meters]
Source: http://www.bghydro.com/bgh/static/articles/0406_tdspH.asp
As some of you may know from my previous posts: a pet hate of mine is the over reliance on the use (and misunderstanding) of TDS when measuring the carbonate hardness of water. Don't take my word for this, boil some RO water in an aluminium pan and measure the TDS before and after. I think you'll be surprised at the significant increase in the TDS readings...