Evaluate potometer repeats and measurement limits
Use means and variation while checking whether the apparatus provides trustworthy uptake data.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Complete potometer rate calculations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does replication help reveal?
What you will learn
- Calculate a mean and range from repeated uptake rates
- Explain leaks, normalisation and proxy limitations
Repeat rather than trust one reading
For supplied repeated rates of 0.08, 0.10 and 0.12 cm³/min, the mean is 0.10 cm³/min and the range is 0.04 cm³/min. Report variation as well as the mean. A repeat that differs needs investigation; it should not be deleted merely because it weakens the expected trend.
Check for leaks, unintended extra bubbles, damage and changing environmental conditions. The shoot is prepared under the teacher’s procedure, including underwater cutting where appropriate, to avoid introducing air that disrupts the water column.
Compare like with like
Different shoots can have different leaf areas. Either use the same suitable shoot across matched conditions or record comparable leaf area and, where appropriate, compare uptake per unit area. This improves comparison but does not control every physiological difference.
Leak-free uptake still is not identical to directly measured transpiration: water can be retained or used, and rapid environmental changes can alter the balance. Written evaluation of apparatus limitations does not certify practical technique.
Sources for this lesson
Pause and explain
How should a leaking apparatus be treated?
Worked example
Find mean and range for 0.08, 0.10 and 0.12 cm³/min.
Show the worked solution
- Sum 0.30 and divide by 3 for the mean.
- Subtract smallest from largest for the range.
Answer Mean = 0.10 cm³/min; range = 0.04 cm³/min.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate mean and range for the supplied three rates.
Give me a hint
Mean uses all readings; range uses extremes.
Compare my reasoning
- Mean = (0.08 + 0.10 + 0.12) / 3.
- Range = 0.12 − 0.08.
Mean 0.10 cm³/min; range 0.04 cm³/min.
Look for these in your work
- Calculate the mean
- Calculate the range
Two shoots take up 0.20 and 0.30 cm³/min with leaf areas 100 and 200 cm². Compare rates per cm².
Give me a hint
Divide each uptake rate by its own leaf area.
Compare my reasoning
- 0.20 / 100 = 0.002 cm³/min/cm².
- 0.30 / 200 = 0.0015 cm³/min/cm².
The smaller-area shoot has the higher rate per cm²: 0.002 versus 0.0015, despite lower total uptake.
Look for these in your work
- Calculate both normalised rates
- Separate total and per-area comparisons
Every repeat is high because a connection leaks. Why can a precise mean still be unreliable?
Give me a hint
Precision and validity are different.
Compare my reasoning
- Similar repeats can cluster around a biased value.
- A systematic leak affects interpretation even with small spread.
A consistent apparatus error can produce repeatable but unreliable uptake values; repair and remeasure rather than trusting the mean alone.
Look for these in your work
- Distinguish repeatability from validity
- Address the apparatus cause
Common mistakes
- Discarding a surprising reading without investigating it.
- Assuming matching leaf area makes all shoots physiologically identical.
What can you explain now?
How should a leaking apparatus be treated?
Compare with the explanation
Repair/check it and repeat affected measurements
A leak undermines the interpretation of displacement.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Give one reason for repeats, one apparatus check and one uptake-proxy limitation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.