Graphic organizer Grade 8 Engineering & Design
Haverlea Rain Tank Diverters: Combining Three Tested Designs
A grade 8 organizer for comparing three tested rain-tank diverters one feature at a time, then combining the best chamber, screen, and reset into a single design.
Objectives
- Students will name the two designs that differ by one feature alone and use that pair to judge the feature.
- Students will assemble one combined diverter from the supported version of each feature and record its source design.
- Students will check the combined diverter against three stated criteria and mark which entry is predicted rather than measured.
Vocabulary
- Rainwater harvesting
- Collecting rain, usually from a roof, and storing it in barrels or tanks for later use, like watering gardens. It saves treated drinking water for the uses that really need it.
Also: first-flush diverter, catchment, mesh opening, criterion, trade-off, iteration
Teacher overview
This organizer is a feature-level comparison frame rather than a ranking sheet. The three rows of Part 1 are the three parts of the design that were changed between prototypes, not the three prototypes themselves, and Part 2 assembles one diverter from the supported version of each part. The data come from the fictional town of Haverlea, where a class tested three first-flush diverters on a school greenhouse rain tank across four measured storms. The error worth watching for in Part 1 is a completed frame whose three rows all name the same prototype, which shows that the designs were ranked against each other instead of compared one feature at a time; the criteria table in Part 2 exists because no tested prototype meets all three criteria on its own. A second error to correct at the table is a combined-design grit figure written as 55 mL, the lowest value in the results, because the chamber effect and the screen effect were measured in separate comparisons and have to be reasoned about together rather than copied from the best row.
Organizer
The site. The greenhouse at a school in the town of Haverlea is watered from a rain tank fed by the greenhouse roof, which covers 60 square meters. Grit and leaf pieces wash off the roof and settle in the tank, where they clog the hose filter. A first-flush diverter sends the dirty water from the start of a storm into a small chamber instead of the tank. An inlet screen holds back the larger pieces.
What was tested. A class built three diverters for the tank. The three differ in chamber volume, screen mesh opening, and the way the chamber empties between storms. Each test storm dropped 10 mm of rain. On a roof of 60 square meters, 10 mm of rain delivers 600 L, because 1 mm of rain on 1 square meter is 1 L, so the four storms sent 2,400 L into the system. A crew checked that the chamber was empty before every storm, so the emptying method changed the staff-time column only and not the water columns.
The criteria set by the Windlebrook Valley Water Authority.
- Store at least 2,000 L across the four storms.
- Leave no more than 30 mL of settled grit in the tank after the four storms.
- Use no more than 10 minutes of staff time across the four storms, counting 5 minutes for each trip to open the drain tap and 2 minutes for each screen clearing.
Test results.
| Design | Chamber volume | Screen mesh opening | How the chamber empties | Water stored over four storms | Settled grit after four storms | Screen clearings | Staff time |
|---|---|---|---|---|---|---|---|
| A | 60 L | 4 mm | Drain tap opened by staff | 2,160 L | 75 mL | 1 | 22 minutes |
| B | 20 L | 1 mm | Drain tap opened by staff | 2,320 L | 55 mL | 4 | 28 minutes |
| C | 20 L | 4 mm | Drip orifice, empties itself in about 3 hours | 2,320 L | 110 mL | 1 | 2 minutes |
Part 1. Compare one feature at a time.
| Feature | Versions tested | Version the data supports | The two designs that differ by this feature alone, and the figures | What choosing that version costs |
|---|---|---|---|---|
| Chamber volume | 20 L (B and C) or 60 L (A) | |||
| Screen mesh opening | 1 mm (B) or 4 mm (A and C) | |||
| How the chamber empties | Drain tap (A and B) or drip orifice (C) |
Part 2. Build the combined diverter.
| Part of the combined diverter | Version chosen | Design it came from |
|---|---|---|
| Chamber volume | ||
| Screen mesh opening | ||
| How the chamber empties |
Criteria check for the combined diverter.
| Criterion | Target | Value for the combined diverter | Met | Measured or predicted |
|---|---|---|---|---|
| Water stored over four storms | At least 2,000 L | |||
| Settled grit after four storms | 30 mL or less | |||
| Staff time over four storms | 10 minutes or less |
Part 3. Answer both questions in full sentences. (a) Which entry in the criteria check is a prediction rather than a measurement, and what test would settle it? (b) The combined diverter puts a part taken from Design C on a chamber three times larger than the one it was tested on. State the question this raises and the measurement that would answer it.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Completed Exemplar
Part 1. Compare one feature at a time.
| Feature | Version the data supports | The two designs that differ by this feature alone, and the figures | What choosing that version costs |
|---|---|---|---|
| Chamber volume | 60 L | A and C share a 4 mm screen, and the emptying method does not affect the water columns, so for grit and storage they differ by chamber volume alone; grit fell from 110 mL to 75 mL, a drop of 35 mL | 160 L less water stored over four storms, 2,160 L instead of 2,320 L, which still clears the 2,000 L criterion |
| Screen mesh opening | 1 mm | B and C share a 20 L chamber, and the emptying method does not affect the water columns, so for grit they differ by mesh alone; grit fell from 110 mL to 55 mL, a drop of 55 mL | Three more screen clearings across the four storms, which adds 6 minutes of staff time |
| How the chamber empties | Drip orifice | A and B both used the drain tap and spent 20 minutes on tap trips, while C spent none | The chamber stays full for about 3 hours after the rain stops, so a second storm inside that window would find it partly full and divert less |
Part 2. Build the combined diverter.
| Part of the combined diverter | Version chosen | Design it came from |
|---|---|---|
| Chamber volume | 60 L | Design A |
| Screen mesh opening | 1 mm | Design B |
| How the chamber empties | Drip orifice | Design C |
Criteria check for the combined diverter.
| Criterion | Target | Value for the combined diverter | Met | Measured or predicted |
|---|---|---|---|---|
| Water stored over four storms | At least 2,000 L | 2,160 L, from 2,400 L of runoff less four diversions of 60 L | Yes | Measured on Design A, which used the same chamber |
| Settled grit after four storms | 30 mL or less | About 20 mL, from 110 mL less the 35 mL chamber effect and the 55 mL mesh effect | Yes | Predicted, and only if the two effects add |
| Staff time over four storms | 10 minutes or less | 8 minutes, from no tap trips plus four screen clearings at 2 minutes each | Yes | Predicted from the two measured rates |
No tested design meets all three criteria. All three designs meet the storage criterion, only Design C also meets the staff-time criterion, and none meets the grit criterion, so the combination is what the data point toward.
Part 3. (a) The grit figure of about 20 mL is the prediction, because no prototype combined a 60 L chamber with a 1 mm screen. Running four more storms on the combined diverter and measuring the settled grit would settle it, and the result may be higher than 20 mL if the screen catches material that the larger chamber would have diverted anyway. (b) The drip orifice emptied a 20 L chamber in about 3 hours, so on a 60 L chamber it would take roughly three times as long, near 9 hours. The question is whether the chamber is empty before the next storm arrives. The crew would time the 60 L chamber as it drains, compare that time with the shortest gap between storms in the school's rain records, and widen the orifice and time it again if the drain is too slow.
Differentiation
Support
Give these students the completed chamber-volume row from the exemplar as a model, and mark on their copy which pair of designs differs by one feature alone for each remaining row, since finding the controlled pair is the step that stalls the work. Ask for Part 1 and Part 2 only, and pair them with another group for the criteria check. The scenario in the town of Haverlea can be shortened to the results table and the three criteria.
Extension
Tell these students that the Windlebrook Valley Water Authority has tightened the staff-time criterion for the fictional site to 5 minutes, and ask what they would now recommend. Working through it shows that the 1 mm screen costs 8 minutes and fails, while every version with the 4 mm screen leaves at least 75 mL of grit and fails the grit criterion, so no combination of the tested versions passes. Strong responses name what they would redesign, such as a self-clearing screen, or which criterion they would ask the authority to reconsider, and say what evidence they would bring.
ELL supports
Provide a labeled cross-section of the diverter for the fictional greenhouse tank showing roof, screen, chamber, drain, and tank, with the three changed features marked in a second color. Pair each column heading with a one-line definition, and offer sentence frames such as "Designs ___ and ___ differ only in ___, so the change from ___ to ___ is caused by that feature" and "Choosing ___ costs ___."
Extensions
Have students plan the next round of testing for the fictional Haverlea site and write it as a short test sheet naming which storms to use, what to measure, and which prototype serves as the control. A second greenhouse at the site has a roof of 90 square meters, so a 10 mm storm delivers 900 L there, and students can work out whether the 60 L chamber should scale with the roof area and what test would show it. A further round can check whether the chamber effect is proportional by building a 40 L chamber and comparing its grit figure against the 60 L and 20 L results.