It should be written there in the manual...It's important because meter manufacturer's have different base reference points. These numbers are reflected in Micro-Seimens or ms/cm. So, be sure to reference the instruction manual of your meter to find out what the base reference is for your meter. Where did you buy the meter? Maybe you can check on the store where you bought it or call the manufacturer...

However, if you want to get an EC value, you may use this simple formula...Multiply the ppm reading by 2 and divide it by 1000. So, if your ppm is 500, you should have an EC value of 1...