GO IN DEPTH

Building a Water Clock

Materials

• 1-liter plastic soft drink bottles
• Pins
• Timer or watch that can be read to seconds
• Rulers
• Marking pens
• 100-mL graduated cylinders

Bernard Gitton's Time-Flow clock
Luna04 / CC-BY-SA-3.0 , via Wikimedia Commons

Purpose

To build a feedback-controlled system (a water clock) and research ways to improve the system design.

Context

This activity should follow student encounters with more simple systems, such as pencils, scissors, etc. In this activity, students will begin to examine more closely the interactions between the parts of a system. The main goal of having students learn about systems is not to have them talk about systems in abstract terms, but to enhance their ability to attend to various aspects of particular systems in attempting to understand or deal with the whole system.

Motivation

Begin by letting students view This Week's WOW: Water Clock, a video of the largest water clock in North America, on display at the Children's Museum of Indianapolis. Students should jot down and describe some of the parts that make up the water clock.

Then let students read more about water clocks in A Walk Through Time. Focus the students' attention on the simple water clock, or clepsydras, which is described on the page.

As they read, ask students to write down the answers to these questions:

• What are the parts of a water clock?
• (An oscillator [the pendulum], a frequency divider, a minute counter [the minute discs], and an hour counter [the hour balls]. Water from a pump in the basement, just below the clock, is pumped through a pipe running up the middle of the clock into a reservoir at the top. The water then drips down onto a scoop at the top which is connected to the green, swinging pendulum. The pendulum causes the scoop to dump the water into a series of siphons. The siphons fill and empty into the minutes' globes. A siphon is a tube in an inverted "U" shape which causes the solution to flow up, without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity.)
• What is it designed to do?
• What advantage does it have over other devices such as sundials?
• (It could be used at night as well as in daylight.)
• What is the largest problem associated with water clocks?
• (The rate of flow of water is very difficult to control accurately.)

Discuss the answers with the class.

Development

How A Water Clock Works
In the first part of the activity, the class will investigate how a water clock works and the effect of one of its variables on its ability to be an accurate timepiece.

Tell students: Early water clocks were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. Other water clocks were bowl-shaped containers that slowly filled with water at a constant rate. Markings on the inside surfaces measured the passage of time as the water level rose on the inside of the bowl, a result of its slowly sinking. We're going to use a soft drink bottle to make a similar device.

Ask students to select what they consider to be the most important parts of the device.

Procedure
Do the following as a teacher-led exploration:

1. Use the pin to make a very small hole in the bottom or close to the bottom of the bottle. A hole smaller than the diameter of the pin is desirable. Let the students examine the hole.
2. Holding a finger over the hole, fill the bottle with water to a level just below the shoulder where it begins to have a smaller diameter. Mark this level on the outside of the bottle. Measure the time required for 100 ml (+/- 0.5 ml) of water to run or drip out of the bottle. Repeat the experiment with the starting water level about halfway up the bottle and with the starting water level very low in the bottle. Plot the times as a function of the distance the starting water level was above the hole in the bottle.

Discuss the results with the class using questions such as the following:

• What do the results tell you? (The drip rate will change if the water level in the bottle changes very much.)
• What does this tell you about a water clock? (It might not be very accurate.)

Now have each student write a one-sentence description of how they might improve the simple water clock made in this demonstration. Student answers may vary, but generally they should respond that the clock could be improved by making the drip rate more constant.

Building a Better Water Clock
In this part of the activity, students will build a feedback-controlled robotic system that will function as a water clock that will keep time accurately for at least two hours without human intervention.

Procedure: Divide the class into groups. The goal for each group is to construct a water clock that will keep time accurately (within +/- 1%) for at least two hours without human intervention. To accomplish this, the drip rate from the bottle has to be constant. A drip rate of 10-15 ml/min will give appropriately accurate data (when the volume that drips out is measured). At this drip rate, 1-2 L of water will be collected in two hours and this is a small enough amount to be manageable. Since the drip rate will change if the water level in the bottle changes very much, the water level in the drip bottle will have to be kept pretty constant in order to keep the drip rate constant.

The task is to design a feedback-controlled robotic system to keep the water level in the bottle constant enough to maintain a steady drip rate. The student groups will each have to decide what "constant enough" is. The robot will need to sense the water level in the bottle and add water as necessary (but not too much or the level will get too high).

You can restrict the kind of sensors the students may use to mechanical devices (like floats) or allow them to use any materials from the classroom (or readily accessible in almost any household), including photocells for electro-optical sensing, if you have them.

The source of water could range from a large (2-L) reservoir of water to the tap, again depending on the restrictions you wish to place on the design. The robots can also range from ones powered only by the force of gravity to ones that incorporate electrical components like small motors. The critical part of the robot is the control of water flow from its source into the bottle. Again, you may restrict the options from controlling the flow through tubing by squeezing it to control by electro-mechanical devices like solenoid valves, if available.

Provide students some time in class and outside of class to develop the concepts for their robots, check their ideas to be sure they meet the design criteria and are safe, and then provide at least one or two periods of in-class time for part of the construction, so you can judge how the group members are working together and to provide encouragement and reinforcement of their ideas.

The finished robots must have a prominent sign giving the conversion factor from volume of water collected to minutes from beginning of collection.

Students should present their finished robots to the class. Each project should be accompanied by a written report which details their design, including drawings illustrate and name all of the parts of the robot system they have designed.

Assessment

Actual testing of the finished robots could be an event open to the whole school as each is tested at two or three random times during a two-hour run to see whether it is keeping time to within the specified +/- 1% over the entire period.

Full assessment credit should be given to any group whose robot meets the specifications. Deductions for missing the goal should be decided by you and the students together in advance of the testing.

To push student creativity, you might want to set up some special awards and incentives for robots run entirely by the force of gravity or by springs or by electric motors and/or for devices that show the elapsed time continuously. Your own creativity in this regard should be restricted only by what is reasonably available in your school. The wider the range of options allowed, the more everyone will learn about what’s possible with robots.

Extensions

For an additional lesson related to the benchmarks in this section, go to the Science NetLinks lesson entitled Measuring Shadows.

Visit The Franklin Institute's Robo-Spot for more on robotics and links to a variety of robot resources. Students who are interested in doing research reports or science fair projects on robotics can use these links as a starting point.