WQ-pH sensors are built with a glass ion selective electrode (ISE). Therefore, even though deionized water has a pH level of 7, a pH reading of <7 occurs as a result of the low ionic strength of the water. The sensing technology used in the WQ-pH sensor has lowered accuracy in low ionic strength samples. To counter this, always add ISA to samples and calibration standards to ensure that they have a similar ionic strength. Use a 1:50 ratio for adding ISA to samples or calibration standards. This reduces the margin of error associated with ionic strength variations.