There are two test methods "24V DC Cooling Fan" air volume and pressure, one instrument with wind tunnel testing, the other is measured by double box. But for the average user, there is no such device. Only as a reference, ultimately depends on the cooling effect according to data provided by the manufacturer. Fan main parameters including flow, pressure, speed, power, etc ... belong to rolling friction ball bearing rolling of metal beads, small contact surface, the friction coefficient is small; and oil-bearing for sliding friction, a large contact surface, after long-term use, "24V DC Cooling Fan" oil will evaporate, bearing easy to wear, friction coefficient, the larger post-noise, short life. In addition to good quality fan ventilation volume, high pressure outside, reliability is very important, bearing fan used in this form is very important. Always use high speed ball bearing fan and low-speed "24V DC Cooling Fan" using low-cost oil bearing.