The SC24V is a differential pH sensor. This means that the reference is not a (liquid) junction but a glass sensor which does not respond to pH changes (within the applicable range of the sensor). 

Therefore the sensor is truly maintenance free and the output voltage of the sensor depends only on the salt concentration of the process. 

The sensor responds to pH changes rather than analyzes the accurate pH value. In that sense it is best to describe the sensor as pH control sensor rather than pH measuring sensor. 

A pH sensor measures the voltage that the pH membrane measures as function of the pH value of the process sample. 

This voltage is then compared with the mV output of a reference cell that is independent on the pH value of the sensor In most pH control applications the salt concentration is rather constant, so the output of the SC24V differential sensor is only dependent on the pH of the process. 

A rule of thumb is that a change in salt concentration of ±25% has an effect of less than 0.1 pH on the pH reading.