EC standard 1.413 mS/cm
The nutrient solution needs to be monitored to ensure your plants are receiving the correct level of everything they require.
For this you will need a NUTRIENT METER, other names used are CF METER (conductivity factor) and EC METER (electrical conductivity).
To maintain your meter, whatever its name, you will need this calibration fluid.
Use this product in accordance with the instructions provided with your nutrient meter.
Check your meter regularly with the fluid to ensure continued correct readings.