When looking at the percentage difference found in Table 1, it is evident that they are all below 5%,Appendix 11.7. This increases the validity of the results as the percentage differences are small.However, according to the Continuity Principle, theoretically, the results obtained by both thesemethods should be the same. The percentage difference from volumetric flow rate is proportional to themass flow rate since the density of air is a constant, hence the same percentage difference will bepresent.
It can also be seen from Table 1 that there is no trend in the percentage difference. This is notjustifiable as only a limited amount of data was collected. If more data was obtained and more advancedFigure 2: This graph shows the varying velocities as a function of thedistance from the centreTable 1: This table represents the volumetric flow ratecalculated using the pressure transducer at three blowerspeeds, also showing the percentage difference betweenthe two devices used.7equipment were used, it will increase the chance of the theory being matched to the experiment. Forexample, at low speed, 30mmH2O, we can see that there is an anomalous result present. However, sincethere was not enough time the experiment was not able to be repeated. Also, it is evident that at all threespeeds the volumetric flow rate reading from the Venturi is much greater than the Pitot tube. TheVenturi measures the average flow rate whilst the Pitot tube measures flow rate at specific point, takinginto consideration turbulent flow.
Since Venturi uses average, the values for Venturi is expected to begreater than Pitot tube.There is a lot of errors that can be found in our experiment. For example, when recording measurementsfrom the IM, there is a chance of parallax error.
The inclined manometer is angled hence there is anerror of ?2mmH2O. To minimise this error, it will advisable to have one person take the readings fromthe same position, preferably at eye level. Also ensure a reference point is chosen as this will reduce thechances in random error occurring, for example taking results from the bottom of the meniscus of theliquid. The results used to calculate Q was obtained using PT, which had a smaller uncertainty of?1mmH2O suggesting that the results from this device is of higher accuracy compared to IM.The Pitot tube needs to be parallel to the air flow to obtain accurate results. If this is tilted there will bevariations in the pressure difference and it will not measure the pressure difference at the height chosen.Hence, it was necessary to make sure the Pitot tube is aligned, however this was difficult to adjust as itwas present within the tube. Therefore it was required to be adjusted using the valves.
Also when traversing the Pitot tube by 2mm, it is up to an individual’s judgement whether the Pitot tubeis in-line with the reading required from the calliper. The calliper itself has an uncertainty of ?0.02mm.Although there is systematic error present, the chances of human error is greater. Similarly, the changesin flow rate could have been more accurately calculated had smaller intervals been used closer to theend of the tube, for example measurements were taking at intervals of 0.5mm.
The results obtained from the manometer fluctuated vigorously, hence it was difficult to determine thereadings. It was decided to not use these results as this will result in random error.According to the theory, the maximum velocity is expected to be halfway, at point zero metres on thegraph, Figure 2. However, it is evident that the velocity increases slightly around 0.002mm whenlooking at 30mmH2O.
At low speeds the flow is not fully developed which therefore means that themaximum velocity would not be reached at the expected position. Since the error is justified in theobservation made. The theory and observation do match, but only to a certain extent.It is not possible to identify which method is of greater accuracy since there is no true value of Q for itto be compared to.
The aim of the experiment could not be supported with evidence. However, whencomparing the approaches used to calculate the value for Q, the Pitot tube gave an approximate unlikethe Venturi which gave a true value based on the results obtained in the experiment.9.0 ConclusionOverall, the aims and objectives of this experiment have been achieved with great accuracy, since all thepercentage differences present are below 5%. At all speeds of flow the Venturi is greater than the Pitottube, suggesting that there is a consistent pattern being followed.
This suggests that the results obtainedare valid. Although there is a certain percentage of uncertainty in our results due to the equipment used,such as the barometer which has an uncertainty of ?1mmHg, if more time had been provided to conduct