I typically use this lab as the 3rd or 4th lab of the semester. The lab is simple enough, they have to use two different methods to measure the flow rate out of a hose. They can use kinematics, conservation of volume, or even momentum (though this is a little more tricky). This is one of the labs where I ask my students to use their estimates of measurement uncertainty, through some basic linear error analysis, to estimate their calculated flow rate uncertainty. If their 2 measurements are not the same (within the bounds of uncertainty they calculated) they have to discuss why not. It is a great experiment for discussing errors because, even though the measurements are simple to make, they often have significant percentage errors that propagate into very large percentage error in their calculated flow rates. For example, measuring the diameter of the hose outlet can be tricky and a 10% error in the measurement becomes a 20% error in the hose area. There are also challenges with repeatability of the experiments as it is hard to get the hose to have the same flow rate each time you turn it on. I do not explicitly ask them to discuss repeatability but rather I discuss it when I return the graded reports and ask them to think about repeatability as part of their next take home lab.
As with all the take home labs I will not publish methods for conducting the tests as I still use them in class and want my students to figure it out on their own.
In this class we have looked at a range of different flow analysis techniques (conservation of mass, kinematics, Bernoulli, momentum, etc.). In this 3rd lab you need to use 2 different approaches to calculate the flow rate from a garden hose.
- Run a series of experiments to establish the flow rate our of the flow from a regular garden hose. There is a hose in the fluids lab that you could use. You can use buckets, measuring tapes, and stopwatches. If you wish to use anything other than that you will need to check with me first. You are not to use laboratory flow rate measurement devices such as the venturi meter.
- Write a brief report (3 page max) that:
- Includes photos of you running your 2 experiments.
- Describes how the test was run
- Includes diagrams showing what you measured
- Presents the theory and equations you used in your calculations
- Lists what data was collected and estimates of your measurement error (in a table)
- Error analysis (see class notes) to estimate your uncertainty in your calculation of flow rate.
- A table listing the two calculated flow rates and your uncertainty estimation.
- If the two measurements to not agree (within the error range you calculated) then discuss why not.
Due date in 2 weeks
An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (firstname.lastname@example.org). I also welcome comments (through the comments section or via email) on improving the demonstrations.