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Aneroid barometer & pressure corrections

2nd Mate

times asked: 33; times asked
questions: 2; questions
last asked: 2026; last asked

The slides, written out

AI-drawn illustration. Simplified to explain the idea, not to scale.

Slide 1 · ANEROID BAROMETER

ANEROID BAROMETER Measuring Atmospheric Pressure Without Liquid CORE PRINCIPLE An aneroid barometer measures atmospheric pressure mechanically using a sealed, partially evacuated metal capsule. HOW IT WORKS 1 PRESSURE CHANGES Atmospheric pressure acts on the flexible capsule. 2 CAPSULE RESPONDS When pressure rises, the capsule is compressed. When pressure falls, it expands. 3 MECHANICAL MOVEMENT A spring or other restoring arrangement provides the opposing force. 4 MOVEMENT AMPLIFIED Levers and mechanical linkages magnify the tiny capsule movement. 5 POINTER INDICATES PRESSURE The amplified movement drives a pointer across a calibrated scale, normally in hPa or millibars. BRIDGE HABIT TAP → READ → RECORD → CHECK A gentle tap helps release mechanical friction so the pointer can settle before reading. | |

Slide 2 · SLIDE 2 OF 4

CORRECTING THE BAROMETER READING Before Recording Pressure 1 INSTRUMENT / INDEX CORRECTION Accounts for the individual instrument's error. Use the correction certificate, correction card or comparison with a standard instrument. 2 HEIGHT / SEA LEVEL CORRECTION A barometer located above sea level measures a lower pressure than the corresponding sea-level pressure. Therefore, when reducing station pressure to sea level, the appropriate height correction is applied according to the applicable table or method. TRADITIONAL APPROXIMATE RULE Height in ft × 1.10 ÷ 30 ≈ correction in hPa WORKED EXAMPLE Barometer height = 30 ft Increase by 10% 30 × 1.10 = 33 ft Correction 33 ÷ 30 = 1.10 hPa Correction = +1.1 hPa (to be added) IMPORTANT Do NOT apply a height correction twice. If the barometer is already adjusted to indicate sea-level pressure, follow the correction certificate or posted correction instructions. Where applicable, the correction table may use height and temperature as its arguments. Follow the instrument's correction table or certificate. NOTE Use the height correction table from the instrument manual. The arguments are the height above sea level and the outside air temperature.

Slide 3 · SLIDE 3 OF 4

WHY ANEROID BAROMETERS Suit Shipboard Use Aneroid barometers are more practical for shipboard use, but they are not immune to movement. 01 SHIP MOVEMENT Mercury barometers are difficult to operate correctly at sea. Rolling, pitching and vibration disturb the mercury column and make accurate readings more difficult. 02 ROBUST & LIQUID FREE No mercury column or fragile mercury column. No mercury spill risk. Built with durable components for routine shipboard use. 03 COMPACT INSTALLATION Can be conveniently mounted on the bridge. Install it where concussion, vibration and ship movement are minimized. 04 EASY TO READ Pointer and calibrated dial give a quick and clear reading. No need to read a moving mercury meniscus. SHIPBOARD BEST PRACTICE Mount Securely Minimize Vibration Keep Conditions Stable Tap Gently Read Accurately

Slide 4 · SLIDE 4 OF 4

ANEROID BAROMETER 6 Points You Should Remember 01 PRINCIPLE A partially evacuated flexible capsule responds to atmospheric pressure. 02 MECHANISM Capsule movement is amplified by mechanical linkages and levers. 03 READING Tap gently before reading to reduce the effect of mechanical friction. 04 CORRECTION Apply the instrument correction and the appropriate sea-level / height correction according to the instrument's correction certificate or table. 05 SHIPBOARD LOCATION Mount securely where concussion, vibration and ship movement are minimized. 06 AVOID DOUBLE CORRECTION If the instrument already indicates sea-level pressure, do not apply the height correction again. REFERENCE INSTRUMENT A mercury barometer may be used for comparison where available, but calibrated precision aneroid and digital barometers are also used for marine pressure observations. EXAM LINE "Aneroid barometers are preferred for practical shipboard use because they avoid the operational difficulties of a mercury column, while still requiring proper installation, calibration and correction." REFERENCE BASIS WMO Guide to Instruments and Methods of Observation (Marine Observations), NOAA / National Weather Service Marine Observation Guidance.

In exams

Asked 33 times across 2 different questions, most recently in 2026. Asked most in 2nd Mate (33).

Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.

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