pH electrodes are commonly designed to produce and output signal equal to 0 mV when immersed in a solution with pH 7. But a constant output value at pH 7 occurs as a result of certain adverse events, such as: A broken bulb in the pH electrode, A short circuit in the cable or connector, or Magnetic force leakage in the cable or connector (for example, in case of excessive humidity).

In these situations, the instrument displays a false reading at pH 7 and the user may not be aware that corrective measures are necessary; particularly if the user is anticipating a pH neutral reading. To avoid this problem, HANNA® developed F.S.T. technology for HI 9813-6. With this special security system, a system failure will force a fixed value close to pH 4, rather than pH 7, and render the problem clearer to the user.

For added confidence in your measurements, the meter is equipped with two LEDs and a "CHECK" button. While pressing this button, upon immersing the electrode in the Cal-Check solution HI 50021P, one of the LEDs will light up to indicate the status of the meter. The green LED ("OK") indicates the probe is working correctly, while the red LED ("C&C" Clean & Cal) indicates the electrode needs attention.