*Article* **The Short-Term Performances of Two Independent Gas Modulated Refractometers for Pressure Assessments**

**Clayton Forssén 1,2, Isak Silander 1, Johan Zakrisson 1, Ove Axner <sup>1</sup> and Martin Zelan 2,\***


**Abstract:** Refractometry is a powerful technique for pressure assessments that, due to the recent redefinition of the SI system, also offers a new route to realizing the SI unit of pressure, the Pascal. Gas modulation refractometry (GAMOR) is a methodology that has demonstrated an outstanding ability to mitigate the influences of drifts and fluctuations, leading to long-term precision in the 10−<sup>7</sup> region. However, its short-term performance, which is of importance for a variety of applications, has not yet been scrutinized. To assess this, we investigated the short-term performance (in terms of precision) of two similar, but independent, dual Fabry–Perot cavity refractometers utilizing the GAMOR methodology. Both systems assessed the same pressure produced by a dead weight piston gauge. That way, their short-term responses were assessed without being compromised by any pressure fluctuations produced by the piston gauge or the gas delivery system. We found that the two refractometer systems have a significantly higher degree of concordance (in the 10−<sup>8</sup> range at 1 s) than what either of them has with the piston gauge. This shows that the refractometry systems under scrutiny are capable of assessing rapidly varying pressures (with bandwidths up to 2 Hz) with precision in the 10−<sup>8</sup> range.

**Keywords:** refractometry; pressure; short-term performance; Fabry–Perot cavity; gas modulation; modulation techniques; metrology
