Fan Speed test
An orifice plate and pressure sensor was to be used in order to provide an indication of flow velocity. When it came to the calibration of this equipment it was recognised that the pressure sensor was not sensitive enough to detect the negligible pressure drop that existed over the orifice plate. In order to combat this issue and establish an indication of flow velocity, a fan speed test was carried out. This test provided a relationship between the setting on the fan controller (voltage frequency regulator) and the actual air speed that was produced. This was carried out by mounting the hot-wire anemometer directly in front of the open duct, with the sensor in the centre of the pipe, and recording the readings for set positions on the dial of the controller. The results are shown below in the figure below.
The first thing to note here is the unexpected behaviour at around 80% on the dial. This was retested several times and the results were consistent - there was a noticeable increase in air speed when adjusting from 80% to 60%. If the data point at 83% is excluded, the results give a reasonable linear relationship. It is suspected that the behaviour of the frequency controller itself, rather than the fan, is behind this anomaly, and that with further investigation it would be possible to determine a cause. However, this wasn't a critical component of our experimental analyses - so no further investigation was done. The line shown in the above graph was simply used to select fan settings which would give a reasonable range of air speeds for the subsequent tests. The points chosen were 16.7%, 50% and 100% - giving a corresponding low air speed of 0.8ms-1, medium speed of 1.2ms-1, and high speed of 1.75ms-1. These settings will be referred to through the evaluation phase linked here.