The pressure which is measured by a sensor for the volume within the Specimen Chamber.

Custom modifications of a microscope can lead to a more accurate pressure measurement. If this is the case it will typically be detailed in the methods section of a paper.

Ideally the specimen chamber pressure should be directly measured inside the chamber; often this is not possible as the value is measured by a sensor located outside the chamber, due to manufacturer design decisions. This can result in a pressure difference between the measuring sensor and the specimen.

In the case of in-situ experiments, or where a specific sample environment is used, the pressure gradient within the specimen chamber can be exploited. c.f. sample environment pressure