Lab / experiment Grade 11 Engineering & Design
Head Versus Flow: A Presque Storage-Tank Orifice Lab
A grade 11 lab where students measure how outflow from a tank changes with water height, then fit the head-versus-flow pattern and link it to distribution pressure.
Objectives
- Students will measure outflow rate from a tank at several water heights and calculate flow in mL per second.
- Students will graph flow rate against head and decide whether the relationship is proportional.
- Students will convert head in feet to pressure in psi and relate it to distribution service pressure.
Vocabulary
- Flow rate
- Flow rate measures how much water moves past a point in a certain amount of time, like gallons per minute through a garden hose. Bigger pipes and stronger pushes give bigger flow rates.
Also: head, orifice, pressure, proportional
Teacher overview
The pressure a water tower delivers comes from head, the height of water above the point you measure. This lab lets students see the same idea that sizes real tanks: as the water height above an outlet rises, water leaves faster, but not in a straight-line way. Groups drain a tall container through a small hole at several marked heights, time the outflow, and plot flow rate against head. They then connect their results to how a utility uses an elevated tank to hold steady pressure across a service area.
Procedure
Step 1 (safety, 2 min): Use only clean tap water. Do not drink or taste any water in this lab. Wipe spills right away so the floor stays dry, and keep the catch basin under the container at all times.
Step 2 (set up, 8 min): Mark head heights of 6, 12, 18, and 24 centimeters above the hole on the outside of your container with tape and a marker. Place the container at the edge of the basin so the hole drains into it. Keep the small piece of tape over the hole until each trial begins. Picture the container as a scale model of the storage tank serving the fictional town of Presque.
Step 3 (trial at 24 cm, 8 min): Fill the container to the 24-centimeter mark with the hole plugged. Hold the graduated cylinder under the hole, pull the tape, and start the timer. Stop timing when you have collected 100 mL, and record the time. Refill and repeat once more at 24 cm so you have two times to average.
Step 4 (trials at 18, 12, and 6 cm, 20 min): Repeat the same 100 mL timing at each lower head height, taking two timings at each height. Try to pull the tape and start the timer at the same instant every trial so your times stay comparable.
Step 5 (calculate, 12 min): For each head height, average your two times, then calculate the flow rate as 100 mL divided by the average time in seconds. Record every flow rate in the data table, and plot flow rate against head on graph paper.
Data & Analysis
| Head (cm) | Average time to collect 100 mL (s) | Flow rate (mL/s) |
|---|---|---|
| 24 | ||
| 18 | ||
| 12 | ||
| 6 |
- Which head height gave the fastest flow rate, and does flow rate rise or fall as head increases?
- Compare your flow rate at 24 cm with your flow rate at 12 cm. When the head doubled, did the flow rate also double? What does that tell you about the relationship?
- Describe the shape of your flow-rate-versus-head graph. Is it a straight line, or does it curve?
- A classmate claims flow rate is proportional to head. Use two of your data points and their ratios to argue for or against that claim.
- A real elevated tank holds water 100 feet above a home. Using the rule that every foot of head adds 0.433 psi, find the pressure that head delivers, in psi.
- Using your results, explain why a utility places its storage tank high and keeps its water level from dropping too low, and what happens to service as the tank level drops.
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: The 24 cm head. Flow rate rises as head increases; the greatest head produced the fastest outflow because more water height pushes harder on the opening.
- Answer: No, it less than doubled. Doubling the head from 12 to 24 cm raises the flow rate by less than a factor of two, because outflow rises with the square root of head rather than in direct proportion. Accept any data-based answer noting the increase was smaller than double.
- Answer: A curve that rises and flattens. The graph should climb steeply at first and bend so it flattens at higher head, a concave-down curve rather than a straight line.
- Answer: Against the claim. If the relationship were proportional, flow rate divided by head would be the same at every point; students should show that this ratio is not constant across their two chosen points, so the relationship is not proportional.
- Answer: 43.3 psi. 100 feet of head × 0.433 psi per foot = 43.3 psi, which sits inside the usual 40-to-80 psi service range and above the 20 psi minimum.
- Answer: A higher, fuller tank means more head, which means higher pressure and faster available flow at hydrants and taps. As the tank level drops, head falls, so both pressure and the flow available for firefighting or peak demand drop with it. Accept any explanation grounded in the group's head-versus-flow results.
Differentiation
Support
Pre-mark the head heights on each container before class and provide a data table with the flow-rate formula printed at the top, so groups can focus on timing and calculating rather than setup. Allow calculator use for the division steps.
Extension
Have groups add a fifth head height between two they already tested, predict its flow rate from their graph before measuring, then compare the prediction with the measured value and explain any difference using the shape of their curve.
ELL supports
Provide a labeled diagram of the container showing head as the distance from the water surface to the hole, and picture cards for "head," "orifice," and "flow rate." Allow analysis answers to combine a short sentence with a labeled sketch of the graph's shape.