The present disclosure discloses a calibration vessel for an optical immersion sensor to be calibrated, which is designed for measuring, calibrating and/or adjusting a measured variable. The calibration vessel includes a housing having a repeatably tightly sealable opening for introducing the optical immersion sensor. The housing provides a value for a calibration solution. The opening comprises a guide for aligning and positioning the optical immersion sensor to be calibrated in all possible spatial degrees of freedom, wherein the housing is designed such that the influence of the interactions between the light, emitted and received by the sensor, and the housing wall, in particular as a result of scattering, absorption, reflection, phosphorescence and fluorescence, on the measured value that can be ascertained by the sensor is minimal, wherein the volume of the calibration solution is minimized at the same time.