Lab / experiment Grade 7 Engineering & Design
Saltcrest Ridge Fog Catcher: Testing Mesh Panels for Water Capture
A grade 7 lab where students build three mesh panels, spray them under identical conditions, and use the collected volumes to design a better fog-catching panel.
Objectives
- Students will build three mesh panels that differ in one variable and test them under matched conditions.
- Students will record collected volumes across three trials and compute an average for each panel.
- Students will use their own data to propose a combined panel design and justify it.
Vocabulary
- Condensation
- Condensation is when water vapor in the air cools down and turns back into tiny liquid water droplets. It is how clouds form and why a cold glass gets wet on the outside on a warm day.
Also: fog catcher, mesh, trial, average
Teacher overview
Saltcrest Ridge is a fictional dry ridge that receives very little rain, while thick fog drifts across it most mornings. Some communities in places like this hang large mesh fences on the hillside. Tiny water droplets in the fog stick to the mesh, run down the threads, and drip into a gutter below. The question for this lab is which mesh catches the most water. Students build three small panels, spray each one the same way, and measure what each collects. Mesh spacing matters in both directions, because too open a mesh lets droplets pass through, while too tight a mesh blocks the moving air instead of letting it flow past.
Procedure
Put on your safety goggles before you build. Some frames have sharp wire ends. Bend or tape any sharp end before you start.
Stretch one mesh across each of your three frames. Pull each mesh flat and tape it at the edges. Label the panels 1, 2, and 3 by mesh type.
Set each panel upright in the tray. Lean it back a little so water can run down. Tape a collection cup at the bottom edge of each panel.
Measure 25 centimeters back from the first panel. Mark that spot with tape on the table. You will spray from this mark every time.
Set the spray bottle to its finest mist. Spray the first panel 40 times from the tape mark. Aim at the middle of the panel. Keep the bottle level.
Wait one minute after the last spray. This lets the drops run down into the cup.
Pour what the cup collected into the graduated cylinder. Read the volume in milliliters. Write it in the table as Trial 1.
Repeat the same steps for panels 2 and 3. Use the same 40 sprays and the same 25-centimeter distance each time.
Dry all three panels and cups with a towel. Then run the whole test two more times, so each panel has three trials.
Do not drink any of the water you collect. Wipe up spills right away so the floor stays dry. Wash your hands when you finish.
Data & Analysis
Record your volumes in the table. Add the three trials for each panel and divide by 3 to get the average.
| Panel | Mesh type | Trial 1 (mL) | Trial 2 (mL) | Trial 3 (mL) | Average (mL) |
|---|---|---|---|---|---|
| 1 | |||||
| 2 | |||||
| 3 |
Look at your averages. Which panel collected the most water? Name its mesh type.
Suppose a panel collected 6 mL, 8 mL, and 7 mL in its three trials. What is that panel's average?
Suppose the best panel in a class averaged 12 mL and the worst averaged 4 mL. How many more milliliters did the best panel collect?
Use the same two numbers from question 3. How many times as much water did the best panel collect as the worst?
You kept several things the same in every trial. Name two of them. Explain why keeping them the same matters.
Suppose a real fog fence has 400 times the mesh area of your panel. If your panel collected 12 mL in one test, how much would that fence collect in the same conditions?
Use your own data to describe a new panel. Combine one feature from your best panel with one feature from another panel. Say which data supports each choice.
A real fog fence needs more than good mesh. Name one constraint a hillside site would place on the design, and explain why it matters.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: Accept the panel with the highest average in the group's own data. In most classes the medium-tight mesh, such as window screen, wins. Very open mesh lets droplets pass through, and cheesecloth often soaks up water instead of shedding it into the cup.
Answer: 7 mL.
6 + 8 + 7 = 21 mL. 21 divided by 3 trials = 7 mL average.Answer: 8 mL.
12 mL minus 4 mL = 8 mL more collected by the best panel.Answer: 3 times.
12 mL divided by 4 mL = 3, so the best panel collected three times as much.Answer: Accept any two controlled variables with a reason. Correct answers include the number of sprays, the spray distance, the bottle setting, the panel angle, the frame size, and the wait time. The reason should say that changing more than one thing at a time makes it impossible to tell which change caused the difference in collected water.
Answer: 4,800 mL.
12 mL x 400 = 4,800 mL. Students should note this is an estimate, because a full-size fence also faces wind and real fog rather than a spray bottle.Answer: Accept any combined design supported by the group's own numbers. A common strong answer combines the mesh with the highest average and the mesh with the second-highest average (for example, by layering them), citing both averages. The explanation must point to the average volume that supports each borrowed feature, not to which panel looked best.
Answer: Accept any genuine site constraint with a reason. Correct answers include steady wind that could tear a large panel, the need to anchor posts in thin rocky soil, the cost of the mesh and the gutter, the distance water must travel to a storage tank, keeping animals and dust off the mesh, and the fact that fog arrives only on some mornings, so storage must carry the site through dry days.
Differentiation
Support
Build one panel with the class as a model before groups start, and hand these groups their frames with the mesh already taped on. Give them a data table with only two panels to compare instead of three. Provide a measuring cup marked in large steps rather than a graduated cylinder, and pre-mark the 25-centimeter spray line on their table with tape.
Extension
Have groups run a fourth panel that changes the tilt angle rather than the mesh, keeping the winning mesh from their first three trials. Ask them to report whether angle or mesh mattered more in their data, and to explain how they can tell from numbers rather than from what they expected.
ELL supports
Provide picture cards for each mesh type with its name printed underneath, and post a photo sequence of the procedure steps at the front of the room. Give the sentence frame "Panel ___ collected ___ mL on average, which was the most" for question 1, and "We kept ___ and ___ the same so that ___" for question 5. Allow volumes to be recorded as numbers alone.
Extensions
Have groups graph their three averages as a bar chart with the mesh type on the horizontal axis, then post the charts and look for agreement across the class. Ask students to write one paragraph on what it would mean if two groups got opposite results with the same meshes, and what they would check first. This is the same question a design team asks when two field sites disagree.