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Lab / experiment Grade 12 Engineering & Design

Bench-Testing a Small Pump: Building a Head-Capacity Curve

A grade 12 lab where students measure a small pump's flow at increasing discharge heights, plot the head-capacity curve, and estimate shutoff head from the graph.

Objectives

  1. Students will measure a pump's flow rate at several discharge heights and compute flow in liters per minute.
  2. Students will plot a head-capacity curve and interpret its decreasing shape and intercepts.
  3. Students will estimate shutoff head from their curve and connect lift to pressure in a full-scale system.

Vocabulary

Key words: head, capacity, shutoff head, discharge, head-capacity curve

Teacher overview

A centrifugal pump delivers less water as the height it must lift that water increases, and the graph of that relationship, with head on one axis and flow on the other, is what an engineer consults when selecting a pump for a given service. Manufacturers publish these head-capacity curves for full-scale equipment, and a small aquarium pump produces its own version on a benchtop. In this lab, students time how long their pump takes to deliver one liter at a series of discharge heights, build the curve from their own data, and push the discharge up until flow stops entirely at the pump's shutoff head.

Bench-Testing a Small Pump: Building a Head-Capacity Curve — Grade 12 Engineering & Design
Name: ____________________________ Date: ______________

Procedure

Step 1 (safety, before starting): This lab mixes water and electricity, so plug the pump only into a GFCI-protected outlet, keep the plug and your hands dry when connecting it, and keep cords off the floor where water could pool. Do not drink any water used in this lab. Wipe spills immediately.

Step 2 (setup): Place the pump in the bucket of water with the tubing attached to its outlet. Stand the meter stick vertically beside the bucket and tape it in place. With the marker, note the water surface level in the bucket as your zero reference; every lift height is measured from this surface, not from the tabletop.

Step 3 (mark the test heights): Using the meter stick, mark the tubing positions that put the tube's outlet at 20, 40, 60, 80, and 100 centimeters above the water surface. A strip of painter's tape at each height keeps the test repeatable.

Step 4 (practice run): Run the pump once with the outlet at 20 centimeters, discharging back into the bucket, so the tubing fills and the team can rehearse roles: one student holds the outlet at the marked height over the pitcher, one times, one records.

Step 5 (collect flow data): For each marked height, hold the outlet at the mark, direct the stream into the empty 1-liter pitcher, and time how many seconds the pump takes to deliver exactly 1.0 liter. Pour the liter back into the bucket after each trial so the water surface stays near your zero mark. Run two trials per height and record both times in the data table.

Step 6 (find shutoff head): Slowly raise the tubing outlet above the highest mark until the flow first becomes a dribble and then stops entirely. Measure and record the height of the outlet above the water surface at the point where flow effectively stops. This is your measured shutoff head.

Step 7 (compute and plot): For each height, average the two trial times and compute flow in liters per minute (flow = 60 ÷ average seconds per liter). Plot head in centimeters on the vertical axis against flow in liters per minute on the horizontal axis, and sketch a smooth curve through the points.

Step 8 (cleanup): Unplug the pump before removing it from the water. Empty, rinse, and dry the equipment, and return the tubing unkinked.

Data & Analysis

Outlet height above water (cm) Trial 1 time for 1.0 L (s) Trial 2 time (s) Average time (s) Flow (L/min)
20
40
60
80
100
Shutoff head (cm): 0
  1. Using your own data, what flow in liters per minute did your pump deliver at the 20-centimeter height? Show the division from your average time.
  2. Describe the pattern in your plotted curve: as lift increases, what happens to flow, and does the change look steady or steeper at higher lifts?
  3. Suppose a pump on another bench fills the 1.0-liter pitcher in 12 seconds at a 30-centimeter lift. Compute its flow in liters per minute.
  4. That same pump takes 30 seconds to fill 1.0 liter at a 90-centimeter lift. Compute its flow at that height, and then the percent decrease from its 30-centimeter flow.
  5. Using your curve, estimate your pump's shutoff head and compare it with the value you measured directly in Step 6. Why is the curve's intersection with the zero-flow axis the meaningful reading here?
  6. Full-scale systems face the same trade-off in different units. A distribution pump fills a tank whose water surface sits 92.4 feet above the pump. Using the conversion that one psi equals 2.31 feet of head, how many psi must the pump develop just to lift water that high, before any friction is counted?

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: varies by group's own data. Flow = 60 ÷ average seconds per liter. A pump that averaged 15 seconds, for example, delivered 60 ÷ 15 = 4.0 liters per minute.
  2. Answer: flow decreases as lift increases. Accept any description consistent with the group's plot; most small pumps show flow falling as the outlet rises, dropping toward zero increasingly steeply near shutoff.
  3. Answer: 5.0 liters per minute. Flow = 60 ÷ 12 seconds = 5.0 liters per minute at the 30-centimeter lift.
  4. Answer: 60 percent decrease. Flow at 90 centimeters = 60 ÷ 30 = 2.0 liters per minute. The decrease is (5.0 − 2.0) ÷ 5.0 = 0.60, or 60 percent.
  5. Answer: varies by group's own data. The estimate is where the smooth curve crosses zero flow, and it should sit near the Step 6 measurement. The intercept matters because shutoff head is defined as the lift at which the pump can no longer move any water.
  6. Answer: 40 psi. 92.4 feet ÷ 2.31 feet per psi = 40 psi of pressure just to reach the tank's water surface, before friction losses are added.

Differentiation

Support

Reduce the run to three heights (20, 60, and 100 centimeters) with the data table pre-labeled, and provide the flow formula worked once with a sample time so groups only repeat the pattern. Assign the timing role to a student comfortable with the stopwatch and rotate after each height.

Extension

Have groups tape two identical pumps' outlets into a single tee fitting, if a second pump is available, and test whether two pumps in parallel move more water at the same 60-centimeter lift than one pump alone, connecting the result to why utilities run pump stations with multiple units.

ELL supports

Label a photo or sketch of the setup with "pump," "outlet," "lift height," and "water surface" before starting, and post the roles (holder, timer, recorder) as picture cards. Provide a sentence frame for question 2: "When the lift gets higher, the flow gets ___."