*3.1. The Dual Refractometry System Used in This Work*

To demonstrate the short-term performance of GAMOR based refractometry, two fully independent Invar-based DFPC refractometers, the aforementioned SOP and TOP, were connected to a DWPG. Figure **??** shows a picture of the experimental setup. While the SOP was firmly placed on an optical table (placed in the rightmost box on the optical table, in the center of the figure), the TOP is designed to fit in a 19-inch transportable rack (the standalone unit to the right). They were both connected to a DWPG (placed in the leftmost box on the optical table).

**Figure 1.** The dual refractometry system scrutinized in this work. It consists of three main components: the SOP (the system in the rightmost box on the optical table); the TOP-refractometer (the standalone system to the right); and the DWPG (the system in the leftmost box on the optical table). In addition to this, it comprises a common gas supply (seen between the SOP and DWPG boxes), a common vacuum system (not in the figure), a computer (for control and data acquisition), and various electronics—for the SOP, partly seen on the shelves, and for the TOP, in the rack. In the bottom part of the figure, a schematic showing the subsystems of the two refractometers and their connection to the DWPG is presented. The blue gas line represents the gas pressure under assessment and the red circles within the DFPCs represent evacuated cavities.

The two refractometry systems are virtually identical in terms of optical and electronic components, including the FP-cavity ensemble. As shown in the schematics above, each system has its own gas handling system; optics (including lasers, electro-optics, passive optics, and locking electronics); and data acquisition and digital control in the form of digital to analogue converter (DAQ) modules and a computer. However, they differ when it comes to the means by which they assess the gas temperature: while the SOP system uses a thermocouple that refers to a miniature fixed point gallium cell to measure the spacer temperature [**?** ], the TOP system assesses it through the use of calibrated Pt-100 sensors. Normally, when working individually, the two refractometry systems use their own designated gas supplies and vacuum systems. In this work, however, they were connected to a common gas supply and vacuum system.
