Choose the pressure zero: absolute and gauge
You will be able to: Convert between gauge and absolute pressure using an explicit reference.
Can a pressure gauge read zero while a fluid still pushes on a surface?
An open container’s water surface is pushed on by the atmosphere. A gauge referenced to that atmosphere reads zero there, even though the absolute pressure is about 100 kPa in our rounded example.
A useful starting point: Pressure grows with depth in a resting fluid →
Words and symbols before equations
- Absolute pressure P_abs
- Pressure referenced to a vacuum.
- Reference pressure P_ref
- The comparison pressure, often the local atmosphere.
- Gauge pressure P_gauge
- Difference P_abs−P_ref; can be positive, zero or negative.
- kPa
- Kilopascal: 1000 Pa.
What this picture assumes
Reference pressure is fixed at 100 kPa. This is a pressure-reference comparison, not a tank trajectory. Negative gauge readings mean below reference, not negative absolute pressure.
Connect the picture to the physics
P_abs=P_ref+P_gauge. A zero gauge reading means equality with the chosen reference; it does not mean no molecular impacts or no pressure force. Negative gauge pressure means below the reference, while absolute pressure remains nonnegative in the ordinary models used here.
For an open resting liquid, use atmospheric pressure as P_ref. At depth h, P_gauge=ρgh and P_abs=P_atm+ρgh. If the liquid surface is pressurized above atmosphere, include that additional surface pressure in the absolute value.
Pressure differences are unchanged if the same reference is subtracted everywhere. Therefore gauge pressures can be used consistently in a pressure-force or Bernoulli difference calculation. Mixing an absolute value at one point with a gauge value at another inserts an artificial offset.
| Quantity | Zero reference | Relationship |
|---|---|---|
| Absolute pressure | Vacuum | P_abs=P_ref+P_gauge |
| Gauge pressure | Selected surrounding pressure | P_gauge=P_abs−P_ref |
A worked example, step by step
A sensor is in water 2 m below a surface exposed to 100 kPa atmosphere. Use ρ=1000 kg/m³ and g=10 m/s². Find gauge and absolute pressure.
- The hydrostatic increase is ρgh=20,000 Pa=20 kPa.
- Gauge pressure relative to the atmosphere is +20 kPa.
- Absolute pressure is 100+20=120 kPa.
- At the surface, gauge pressure is 0 kPa while absolute pressure is 100 kPa.
Always identify the reference. Zero gauge pressure is not a vacuum.
A gauge reads −20 kPa with a 100 kPa reference. Is absolute pressure negative?
Compare with an explanation
No. P_abs=100−20=80 kPa.
Predict. Change one thing. Explain.
Hold the atmospheric reference at 100 kPa and vary the absolute pressure. Identify a negative gauge value, zero gauge value and positive gauge value. Explain what each says about the reference.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
P_abs=120 kPa = 100 kPa reference + (20 kPa gauge). Pressure is above reference.
Reference pressure is fixed at 100 kPa. This is a pressure-reference comparison, not a tank trajectory. Negative gauge readings mean below reference, not negative absolute pressure.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant force, motion or energy relationship to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA gauge reads −15 kPa where the reference is 100 kPa. (a) Find absolute pressure. (b) Explain the negative gauge sign. (c) State the absolute pressure when that gauge reads zero. (d) Explain a problem with mixing absolute and gauge values in a pressure difference.
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Compare with the answer and four-point rubric
- 1 point: 85 kPa.
- 1 point: The measured pressure is 15 kPa below the reference.
- 1 point: 100 kPa.
- 1 point: They have different zeros; subtracting them directly introduces a spurious reference-pressure offset.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Absolute pressure zero is referenced to what?
Vacuum.
RECALL 2What does a zero gauge reading mean?
Pressure equals the selected reference.
RECALL 3Can gauge pressure be negative?
Yes, when absolute pressure is below the reference.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Choose the pressure zero: absolute and gauge
- P_abs=P_ref+P_gauge.
- For an open uniform liquid: P_gauge=ρgh relative to the atmosphere.
- Use the same pressure reference at all compared points.
Remember: Always identify the reference. Zero gauge pressure is not a vacuum.
Conditions: Reference pressure is fixed at 100 kPa. This is a pressure-reference comparison, not a tank trajectory. Negative gauge readings mean below reference, not negative absolute pressure.
Refresh Kid · Unit 8 · Objectives 8.2.B · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 8.2, objectives 8.2.B. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. Fall-2026 corrections also checked. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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