Process and Insight of Pascal Traceability

National metrology institute (NMIs) has the prime responsibility to design, development and maintenance of the primary pressure standards to establish the traceability chain for the calibration of the sophisticated industrial instruments using various pressure standards like ultrasonic interferometer manometer (UIM), air piston gauge (APG), force-balanced piston gauge (FPG), and optical interferometer manometer (OIM). The UIM is one of the most accurate pressure standards in the liquid column manometer category, with the expanded relative uncertainty obtained as 7.2 ppm (k = 2) of the reading in the pressure ranges from 1 Pa to 130 kPa. However, due to the toxicity of mercury and complexity in handling, most of the NMIs have been decommissioned the UIM. For the realization of Pascal in a quantum manner, some of the NMIs have been developing new pressure standards based on the Fabry–Perot interferometer. Some of them have established highly accurate dimensionally traceable piston gauges as a primary pressure standard. Barometric pressure metrology, National Physical Laboratory, India (NPLI) is also trying to develop a Fabry Perot interferometer-based pressure standard. The APG has been working as a reference pressure standard at NPLI since 2002 using UIM as a traceability source, having the expanded relative uncertainty of 12 ppm (k = 2) of the reading. However, recently, NPLI has established the APG as the primary pressure standard in the ranges from 6.5 to 360 kPa with the expanded relative uncertainty as 76 ppm (k = 2) of the reading contained by limitation in the measurement of dimension metrology. To further strengthen the low-pressure range, the FPG has provided traceability in the pressure range of 1 Pa—15 kPa, having expanded relative uncertainty of 25 ppm (k = 2). These pressure standards, i.e., UIM, APG, and FPG, have gone through many critical comparisons, bilateral comparisons, and internal calibrations to establish calibration measurement capabilities (CMCs). This review article presents the journey of design, development, and maintenance of these standards worldwide with special attention at NPLI.