Engineering & Technology · Sketches, Models & Technical Communication
Schematics, flow diagrams and system boundaries
A small teaching tank receives water through a valve and sends water to a collecting vessel through an outlet. A layout drawing could show where those items sit on a table. For a different question—where can water travel?—their connections matter more than the lengths or bends of the pipes.
Use units and simple arithmetic from Measurement, Units & Engineering Evidence. This chapter introduces its own coordinates, projections and model vocabulary; multiplication and division are enough for the worked cases. The geometry, system, data and results are original teaching constructions. They are not observed experiments, build specifications or operating instructions. No conformance to a drafting, piping or NASA standard is claimed.
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Choose a diagram that makes connections, flow or sequence explicit, and state what crosses a chosen system boundary.
Choose the relationship before choosing the picture
A schematic uses symbols and connections to describe selected relationships. It may move components on the page to make those relationships easier to follow. In our water schematic, a line means a connected water path and an arrow marks the direction considered in the example. The line is not a measured pipe length. Bending it on the page does not add a physical bend to the design.
An electrical schematic similarly describes electrical relationships rather than the physical placement of every wire. A wiring layout, by contrast, may need to show terminals and routing. Read the legend and conventions of the actual diagram: identical-looking crossings or arrows can mean different things in different systems. Our water symbols are explained beside the figure and make no claim to a specialised piping standard.
Ask what belongs inside the question
The dashed boundary encloses the tank alone. Water enters from the valve and leaves through the outlet. A dotted arrow carries level information to a reader outside the boundary; it is not another water path. These different line styles separate material movement from information, even without colour.
If the question becomes how the valve setting affects filling, we might draw a larger boundary around valve and tank together. The valve then becomes internal, while the upstream supply becomes an input. Moving a boundary changes the bookkeeping and responsibilities being considered; it does not physically move a component. Inputs, outputs and stored contents will matter when we model filling.
Use a process diagram for what happens next
The second panel is a process diagram for a paper simulation, not equipment operating instructions. Begin with a simulated volume, compare it with a target, and either stop or advance by one time interval, update the volume using the chosen filling rule, and compare again. The decision branches carry conditions. Without the volume update the loop would keep checking the old value; without its return arrow a reader could think the comparison occurs only once.
A diagram should also show the limits of the question. This simple loop omits overflow protection, instrument failure and real valve delays. It explains repeated calculation only. Another diagram would be required to analyse a physical controller’s possible states and faults. Completeness is always completeness for a stated purpose, not a promise that a few boxes describe everything.
Repair a misleading water diagram
A first sketch places the valve near the tank but does not connect them. A reader cannot tell whether it is part of the supply, part of the outlet or simply nearby. Add an explicit supply → valve → tank connection and a tank → collecting vessel outlet.
Next label the boundary “tank only”. Identify incoming water, outgoing water and the tank’s stored contents. Draw the level-reading connection with a dotted line and label it “information”. No pipe length can now be inferred from the spacing between the symbols.
For the paper calculation, label the two branches “volume at least target” and “volume below target”. The first leads to stop; the second advances time, recomputes the volume using the chosen filling rule, and returns to the comparison. This closes the logical gap without adding irrelevant physical detail.
Try the reasoning
If two components are moved farther apart on a schematic, must the connecting pipe become longer?
Show the answer and reasoning
No. The schematic records the connection. A layout drawing and dimensions are needed to establish physical distance and routing.
What crosses the tank-only boundary in this figure?
Show the answer and reasoning
Incoming water, outgoing water and the outward level information. The valve and reader are outside; the tank’s stored water is inside.
Why label both branches of the process decision?
Show the answer and reasoning
The labels tell the reader which condition selects each path. An unlabelled fork does not define what the process should do next.
What to carry forward
A useful diagram states what its lines mean, which relationship it represents and where its question begins and ends.
References
Teaching resources and research supporting this lesson’s explanations and examples.