Gas Pressurization
The process of verifying and adjusting pressure-measuring instruments using a clean, dry gas as the pressure medium defines a fundamental method in metrology. This procedure, known as pneumatic calibration, is typically employed for low to medium pressure ranges where hydraulic fluids would introduce contamination or hydrostatic head errors. It relies on high-purity nitrogen or compressed air to transmit pressure evenly through the system.
The compressible nature of the gas requires careful handling to ensure stable readings.
Temperature Effect
Gas compression generates heat, while expansion causes cooling, which affects pressure stability. During a pneumatic calibration, the system must reach thermal equilibrium before readings are taken. Insulated tubing helps minimize these thermal effects.
Cleanliness Requirement
Contamination of the calibration gas with oil or moisture can damage both the reference standard and the instrument under test. High-purity gas supplies and inline filters are standard components in these calibration systems. Moisture in the gas can condense during expansion, leading to localized pressure drops and flow restrictions.
Maintaining dryness is essential for achieving the highest levels of accuracy.
Calibration System
Deadweight testers and digital pressure controllers are the primary standards used in pneumatic calibration. These instruments are calibrated against primary national standards to maintain traceability. The system must be leak-tested before the calibration begins, as even a small leak can create dynamic gas flows that alter the static pressure at the test port.
This setup is verified using a reference gauge of known accuracy.
