Changes:  Salt (NaCl) tablets are a new material that will replace the KCl 
for making calibration solutions.

One(1) tablet mixed with 8 fl oz of Distilled Water should read 2.31 mS on 
newer meters.  Older meters should read 23.1 (100µS).  Be sure that tablets 
have totally dissolved prior to calibration.  The international standard for 
conductivity readings is in mS or milliSiemens, but the older meters (and 
some new meters) measure in 100µS or 100 microSiemens.  

What in the world are milliSiemens per centimeter (mS/cm) 
or microSiemens per centimeter (µS/cm)?

These are the units for electrical conductivity (EC). Meters set to read 100µS/cm will give 
values 10 times higher than those set to read mS/cm.  A mS is equivalent to 1000 µS.
For example: 20-10-20 at 100 ppm will read .67 mS/cm higher than the value of the water 
source and 6.7 µS/cm higher.  If the water source reads .32 mS/cm, it will read 3.2 µS/cm.  
The sensor simply consists of two metal electrodes (covered by the black cap on the meter 
to the left)  that are exactly 1.0 cm apart and are placed into the water. A constant voltage 
is applied across the electrodes. An electrical current flows through the water due to this 
voltage and is proportional to the concentration of dissolved ions in the water - the more ions, 
the more conductive the water resulting in a higher electrical current which is measured 
electronically. Distilled or deionized water has very few dissolved ions and so there is almost 
no current flow across the gap (low EC).  Its not the water that is dangerous when handling 
electrical items around water, but the dissolved ions or solids in the water.

Fertilizers differ in the amount of salts and therefore how much they will affect the conductivity
reading.  For example:  20-10-20 has an EC of .66 mS per 100 ppm N where 21-8-18 has an
EC of .42 per 100 ppm N.  Add those numbers to your water source reading to determine the
right value.  Example2: