The solids used in nutrient solutions can be stated in terms of PPM - that is, a ratio of one part to one million of another. The PPM of a solution is routinely been determined by a PPM meter. This device initially takes an electrical conductivity (EC, measured in mS/cm, millisiemens per centimeter) reading. (In some parts of the world, this measurement is called conductivity factor, or CF; the EC to CF ratio is 1:10.)
The meter then uses a conversion factor (usually 1 mS/cm EC=700 PPM at 68F) to provide a PPM reading. This figure, whether stated in EC or PPM, is often referred to as a TDS reading (Total Dissolved Solids).
OK, now in laymans terms:
An electronic PPM/TDS meter actually measures EC not PPM.
1 millisiemen (mS) = 1000 microsiemen (S)
The PPM figure is a result of a conversion factor (multiplier).
If the meter measures an EC of 1 mS it could convert this to the following PPM figures:
1 * 700 = 700 PPM (This is a 0.7 conversion)
1 * 640 = 640 PPM (This is a 0.64 European conversion)
1 * 500 = 500 PPM (This is a 0.5 conversion)
If the meter measures an EC of 1000 S it could convert this to the following PPM figures:
1000 * .7 = 700 PPM (This is a 0.7 conversion)
1000 * .64 = 640 PPM (This is a 0.64 European conversion)
1000 * .5 = 500 PPM (This is a 0.5 conversion)
It is therefore important to know what conversion factor any PPM figure is relevant to.
Most nutrients give specifications for PPM using a 0.7 conversion, thus if your meter is a 0.5 conversion you have to convert the recommended PPM strengths to 0.5 or else you will try to run too strong and the plants will suffer.
Lets say you have a 0.5 conversion meter and you measure a PPM of 1100 in your reservoir. Your nutrients recommendations say you need to be at 1500 PPM @ 0.7 conversion. So in this example we divide our PPM reading of 1100 by the 0.5 conversion factor and we get 2200 S. Then we take 2200 S and multiply it by 0.7 and we get 1540 PPM @ 0.7 conversion. So as you can see we are only 40 PPM @ 0.7 over our intended target of 1500, not bad, we will leave that alone.
Now lets look at a possible bad scenario. Your nutrients tell you to mix them to 1500 PPM @ 0.7 conversion (2143 S). Your meter measures using a 0.5 conversion, and you mixed to 1500 PPM @ 0.5 conversion on your meters scale. You now have actually exceeded the manufacturers recommendation by 600 PPM @ 0.7 conversion by running at 3000 S * 0.7 = 2100 PPM @ 0.7 conversion.
To convert a PPM figure back to an EC figure is a simple matter of division. In the following example we will convert a 0.5 conversion PPM figure to S:
1500 PPM / 0.5 = 3000 S
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