# Take Home Lab – Measuring the mass flow rate from a compressed air can

The mass flow rate of a fluid appears many times in an introductory fluids class (pump power, conservation of mass, momentum, etc.). This take home lab requires students to use at least two different approaches to measure the mass flow rate out of a compressed air can. My experience with this is that some students struggle with finding a second way to do the measurement. This has led to some rather creative, if not physically appropriate, measurement methods.

It is a nice experiment because there are substantial difficulties in taking accurate measurements. For example, it is hard to accurately measure the diameter of the straw connected to the can as it is so small. It is not unusual to have a measurement uncertainty of 50-100% in the diameter leading to an error/uncertainty of up to 400% for the straw area. These errors  can make a substantial difference to the resulting calculated mass flux. There can, therefore, be substantial differences between the two sets of measurements that are still within the bounds of uncertainty. There is also a repeatability problem with this lab that is easily observed and explained if one is paying attention to the data.

As with other take home labs that I use later in the semester, the students are required to do some basic error analysis to explain the differences between their two measurements. This is a very useful complement to their fluids lab class that runs in parallel with the main lecture class. The students need to estimate the uncertainty in each measurement they take and then use that data to estimate the uncertainty in their measured mass flow rate.

As with all the take home labs I will not publish details on specific methods for conducting the tests as I still use them in class and want my students to figure it out on their own.

Instructions to students

Introduction

In this class we have looked at a range of different flow analysis techniques. In this lab you need to use 2 different approaches to estimate the mass flow rate coming out of a compressed air can such as are used for cleaning computer keyboards.

Task

1. Run a series of experiments to establish the mass flow rate out of a compressed air can. You can borrow an air can from me when you are ready to do your testing. You may use an electronic scale and a stopwatch but otherwise only non-lab equipment is to be used without permission. If you would like to use something else you need to check with me.
2. Write a brief report that
1. Is a maximum of 3 pages including photos of you running your experiments.
2. Describes the experiment(s) you used to establish your result including
1. How the test was run
2. What data you collected including estimates of your measurement uncertainty
3. How you performed your calculations including diagrams (with control volumes), equations, and relevant theory
4. A quantitative discussion of the uncertainties in your measurements and calculations including an analysis of the differences between your two sets of measurements.

Due 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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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