Open Channel Flow Experiment
This experiment investigated the area/velocity method and turbine flow meter method of calculating flow rate. Its aim was to compare accuracy and practical usage for each method. Initial hypotheses predicted that the turbine flow meter would have a higher level of accuracy than the area/velocity method. This was confirmed by the results. It was concluded that for high accuracy a turbine flow meter would be better suited however as it can only be installed within a pipe it would be unsuitable for many natural environments such as a river.
On the 29th October 2013 an open channel flow experiment took place within the university lab. During this experiment the flow rate was measured in a tank using two methods. These were the area/velocity method and the turbine flow meter method. The purpose of this was to gather results to compare the accuracy and analyse the potential benefits and drawbacks of each method.
Initial hypotheses were that the turbine flow meter would be more accurate as there are few opportunities for human error whilst conducting the experiment.
A risk assessment was completed before the experiment began. This was discussed by the group and lecturer present. Everyone was made aware of the health and safety protocols should Any incidents occur.
Materials and Method
See Appendix 1
Data and Results
Shown in Figure 1 are the results found by completing each experiment and completing the calculations to obtain the total flow rate. These are then plotted on the graph in Figure 2.
Turbine Flow Meter
Figure 1: Calculated Results of experiments Figure 2: Graphical Comparison of Results
For full table of data - see Appendix 2
"The modern axial turbine flow meterÃ¢ÂÂ¦ is a reliable device capable of providing the highest accuracies attainable by any currently available flow sensorÃ¢ÂÂ¦ Typical measurement repeatability is ÃÂ±0.1% of reading". (J.G. Webster, 1998)
Assuming the turbine flow meter is calibrated correctly it is likely to be highly accurate therefore it displays some inaccuracy in the area/velocity method. Although it does show a definite trend matching the turbine flow meter reading.
Several human errors were made during the area/velocity experiment. Firstly, it can be shown (Francis and Minton, 1984) that the average velocity in the vertical occurs at 0.63 depth below the surface. This was printed incorrectly in the lab paper as 0.4 below surface (0.6 from bottom).
Any human errors which may have occurred were compounded by the scale of the turbine velocity meter not being small enough. Consequently, the turbine used to measure the water velocity was not fully submerged during some parts of the experiment. This included all measurements after the weir and a few of the lower flow rates at 0.2 of the depth (minimum 0.0340m).
The turbine meter flow method would be more suited to installations in situations where there is a high demand for accuracy and a liquid flowing through a pipe where it can be installed. It can be used within a water supply or a in a factory however it could not be installed into a stream or river without severe environmental impacts.
The area/velocity method would however be more suited to use in a natural environment without any major environmental impacts; providing a good representation of the flow rate and highlighting any changes occurring.
Overall there are positive and negative points to each method of measuring open channel flow. The turbine flow meter is highly accurate but requires installation into a pipe and the area/velocity can be easily set up and removed in a river or stream however it is less accurate than the turbine flow meter.
J.G. Webster (ed.), The Measurement, Instrumentation and Sensors Handbook, Boca Raton, FL: CRC Press, Dec. 1998.