Compressed air is derived from moist air, and its pressure and temperature increase during compression, which can easily lead to the precipitation of liquid water, forming a gas-liquid two-phase flow. It may also contain oil contaminants, solid particles, and vibration interference from compressors. Its quality depends on dryness, dust content, and oil content, requiring drying (adsorption-type dryers are superior to refrigeration-type) and purification pretreatment to meet measurement requirements.  

For well-pretreated compressed air, which can be treated as an ideal gas, temperature- and pressure-compensated smart flow meters are recommended, such as swirl flow meters (DN ≤ 200 mm), which offer a turndown ratio of 15:1 and require shorter straight pipe sections (5D upstream and 3D downstream), making them suitable for low-vibration environments. For pipelines with significant vibration or large diameters, an integrated throttling device can be used to avoid errors caused by liquid accumulation in impulse lines. Installation must follow standard procedures to prevent welding damage, and outdoor installations should be equipped with protective enclosures. Regular drainage is necessary to prevent liquid accumulation. If airflow fluctuates significantly or intermittent usage is brief (e.g., 2–3 seconds), shortening the sampling interval can improve measurement accuracy.  

If compressed air is only cooled and filtered without thorough drying, it may still precipitate water due to temperature drops, forming a gas-liquid two-phase flow. For example, when saturated moist air at 0.4–0.8 MPa and 45°C cools to 10°C, the precipitated water is typically <15 g/m³, with a mass gas fraction >99%. In such cases, the homogeneous flow model (ρ_h ≈ ρ_G/x) can be used for approximate calculations, with minimal error when the mass gas fraction is high (x > 0.98). Flow meters such as swirl or vortex flow meters (whose measurements are density-independent) remain suitable, while throttling devices require correction based on homogeneous density, necessitating prior communication with manufacturers. Additionally, automatic or manual drainage devices should be installed at low points in the pipeline network to minimize measurement errors.  

The quality and pressure of compressed air directly affect costs. If an enterprise has multiple compressor stations or different workshops with varying pressure requirements, a differentiated pricing system should be implemented. Alternatively, smart flow controllers can be installed in critical pipelines to automatically regulate flow and pressure, stabilizing the network and improving energy efficiency. In summary, compressed air measurement requires selecting appropriate instruments based on operating conditions, optimizing installation methods, and focusing on quality management and cost accounting to enhance measurement accuracy and economic efficiency.