# When a function of the distance from the centre

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 these
methods should be the same. The percentage difference from volumetric flow rate is proportional to the
mass flow rate since the density of air is a constant, hence the same percentage difference will be
present. It can also be seen from Table 1 that there is no trend in the percentage difference. This is not
justifiable as only a limited amount of data was collected. If more data was obtained and more advanced
Figure 2: This graph shows the varying velocities as a function of the
distance from the centre
Table 1: This table represents the volumetric flow rate
calculated using the pressure transducer at three blower
speeds, also showing the percentage difference between
the two devices used.
7
equipment were used, it will increase the chance of the theory being matched to the experiment. For
example, at low speed, 30mmH2O, we can see that there is an anomalous result present. However, since
there was not enough time the experiment was not able to be repeated. Also, it is evident that at all three
speeds the volumetric flow rate reading from the Venturi is much greater than the Pitot tube. The
Venturi measures the average flow rate whilst the Pitot tube measures flow rate at specific point, taking
into consideration turbulent flow. Since Venturi uses average, the values for Venturi is expected to be
greater than Pitot tube.
There is a lot of errors that can be found in our experiment. For example, when recording measurements
from the IM, there is a chance of parallax error. The inclined manometer is angled hence there is an
error of ?2mmH2O. To minimise this error, it will advisable to have one person take the readings from
the same position, preferably at eye level. Also ensure a reference point is chosen as this will reduce the
chances in random error occurring, for example taking results from the bottom of the meniscus of the
liquid. 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 be
variations 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 it
was 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 tube
is 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 changes
in flow rate could have been more accurately calculated had smaller intervals been used closer to the
end 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 the
readings. 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 the
graph, Figure 2. However, it is evident that the velocity increases slightly around 0.002mm when
looking at 30mmH2O. At low speeds the flow is not fully developed which therefore means that the
maximum velocity would not be reached at the expected position. Since the error is justified in the
observation 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 it
to be compared to. The aim of the experiment could not be supported with evidence. However, when
comparing the approaches used to calculate the value for Q, the Pitot tube gave an approximate unlike
the Venturi which gave a true value based on the results obtained in the experiment.
9.0 Conclusion
Overall, the aims and objectives of this experiment have been achieved with great accuracy, since all the
percentage differences present are below 5%. At all speeds of flow the Venturi is greater than the Pitot
tube, suggesting that there is a consistent pattern being followed. This suggests that the results obtained
are 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