In contrast to the ZIROX normal pressure probes the vacuum probe is not calibration-free.
The reason are the special sensor heating (inside the probe tube) and several conditions at the measuring electrode (energy removal depends on gas pressure and thermal conductivity). Therefore, isothermal electrodes, necessary for the correct using of the NERNST equation, cannot be realized. But for a process control the using of empirically determined values is enough in most cases.
For the measurement of oxygen partial pressures the probe must be calibrated by the user
(test measurements).