1–15,000 Pa Absolute mode comparisons between the NIST ultrasonic interferometer manometers and non-rotating force-balanced piston gauges

The National Institute of Standards and Technology (NIST) Low Pressure Manometry Project maintains and operates primary standard ultrasonic interferometer manometers (UIMs) over the pressure range of 1 mPa to 360 kPa. Over the past decade a new type of customer gauge, the non-rotating force-balanced piston gauge or FPG (model 8601, DH Instruments, a Fluke Company¹) has been introduced to the standards community that covers the range of ≈1–15,000 Pa and is capable of both absolute and differential measurement modes. Since 2002, NIST customers² have requested that four different FPG units be compared to the NIST primary pressure ultrasonic interferometer manometer standards (UIMs). The results of the comparisons were that all four FPG units were within manufacturers stated uncertainty (0.008 Pa + 30 × 10⁻⁶ × P for absolute mode) when compared against the NIST UIMs at pressures between 10 Pa and 15,000 Pa (absolute mode). At pressures between 5 Pa and 10 Pa, the results were generally within manufacturer’s specifications. Below 5 Pa some of the FPG units were outside of manufacturer’s uncertainty specifications. The use of an isolating capacitance diaphragm gauge (CDG) was necessary during the comparisons to prevent humidified gas from the FPG from entering the NIST 160 kPa mercury UIM primary pressure standard. The results of these four different comparison tests will be discussed in detail, along with test conditions, equipment set-up, and test uncertainty analysis.