26th May 2021
Types Air Flow Meters – Part 1
Air flow is probably one of the most frequently measured media but is one of the more difficult media to measure in an accurate manner. Don’t be fooled by companies saying airflow is easy to measure and dismissing alternative technologies just because they want to sell you their products. Take time, assess, evaluate and research your air flow measurement application. Just look at the abundance of flow meter types and measurement methodologies utilised in measuring air flow; each one having it merits and quirks for the different applications and ranges. Some of the basic measurement processes still require the user to have an understanding of external effects on accurate air flow metering, which come down to some real basics, such as include pressure, temperature, humidity, specific heat capacity and viscosity. Then moving onto the application area, do you have a turbulent or laminar profile, or a mixture in transition, does this change with your flow range? Do you require upstream and downstream conditioning for the flowmeters? Do you know what type of measurement you require, volume or mass flow, actual or standard conditions? Does the air flow meter you specify need corrections for non standard conditions or from calibrated conditions? Even to the educated user, many metering installations throughout the world are not the installed in ideal setup, this maybe because of ignorance, lack of knowledge, cost, or in some cases just impossible to meet the ideal installation requirements due to physical restraints. Once you have considered the above then you can start to hone-in on the suitable air flow measurement techniques and products available in today's marketplace.
Types of Air Flow Meters for Volumetric Air Flow Metering
Variable area flowmeters, often known as VA or Rotameters, are one of the least expensive flow meter options. They give a quick visual indication and are suitable where accuracy is not a primary requirement. They need to be correctly sized for the air flow conditions and require scale corrections for flow conditions away from the stated tube operating conditions. The meters need to be installed vertically so the float can freely move upward and downward in the tapered tube without disturbance. They generally give a 10:1 turndown ratio and from 1 – 5% of FSD error.
Vortex Flowmeters can offer advantages where there is a requirement to measure air flow in elevated temperatures and pressures. The air flow stream is obstructed with a shedding bar, which, when an air flow stream is passed through the bore and over the bar, vortices are shed proportionally to air flow rate, usually measured by a transit and receive piezo crystals. They have no moving parts and can measure dirty streams, vapor flow and mist flow. The meters can be installed in varying orientations but require a conditioned flow profile and can offer a 30:1 turndown with a 1% accuracy. Higher grade meters are often fitted with integral temperature and pressure sensors to give a mass based measurement.
Laminar flow meters or sometimes known as viscous flow meters are high accuracy flow meters when used with clean air and are protected from debris and fluctuating temperatures. They provide a linear relationship for volume flow against pressure drop at a given air viscosity. The linearity is only applicable for Reynolds numbers in the laminar region. Measurement accuracy of 0.5% of reading is possible over a 20:1 turndown with a high grade differential pressure sensor. Optional installation of span tubes can often improve the linearity and extend the turndown range.