How do i convert between EC, CF, & PPMs?

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 68°F) 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 (µS)

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 µS 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 µS. Then we take 2200 µS 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 µS). 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 µS * 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 µS:

1500 PPM / 0.5 = 3000 µS