chemistry

Inch of mercury

Inch of mercury is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Inch of mercury rather than just read about it. In short: Inch of mercury (inHg, ″Hg, or in) is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.

Key takeaways

Inch of mercury belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Inch of mercury to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Inch of mercury from memory before moving on to harder problems.

Reference excerpt

Inch of mercury (inHg, ″Hg, or in) is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity. Conversion to metric units depends on the density of mercury, and hence its temperature; typical conversion factors are:

In older literature, an "inch of mercury" is based on the height of a column of mercury at 60 °F (15.6 °C).

1 inHg60 °F = 3,376.85 pascals (33.7685 hPa) In Imperial units: 1 inHg60 °F = 0.489 771 psi, or 2.041 771 inHg60 °F = 1 psi.

Applications

Aircraft and automobiles Aircraft altimeters measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground. In North America and Japan, these altimeter readings are provided in inches of mercury, but most other nations use hectopascals. Ground readings vary with weather and along the route of the aircraft as it travels, so current readings are relayed periodically by air traffic control. Aircraft operating at higher altitudes (at or above what is called the transition altitude, which varies by country) set their barometric altimeters to a standard pressure of 29.92 inHg (1 atm = 29.92 inHg) or 1013.25 hPa (1 hPa = 1 mbar) regardless of the actual sea level pressure. The resulting altimeter readings are known as flight levels. Piston engine aircraft with constant-speed propellers also use inches of mercury to measure manifold pressure, which is indicative of engine power produced in engines equipped with a supercharger or turbosupercharger (naturally aspirated engines measure manifold vacuum instead). In automobile racing, particularly United States Auto Club and Champ Car Indy car racing, inches of mercury was the unit used to measure turbocharger inlet pressure. However, the inch of mercury is still used today in car performance modification to measure the amount of vacuum or pressure within the engine's intake manifold. This can be seen on "boost gauges (forced induction) or vacuum gauges (natural induction), which give a rough indication of the relative power being produced at any given time.

Cooling systems In air conditioning and refrigeration, inHg is often used to describe "inches of mercury vacuum", or pressures below ambient atmospheric pressure, for recovery of refrigerants from air conditioning and refrigeration systems, as well as for leak testing of systems while under a vacuum, and for dehydration of refrigeration systems. The low-side gauge in a refrigeration gauge manifold indicates pressures below ambient in "inches of mercury vacuum" (inHg), down to a 30 inHg vacuum. Inches of mercury is also used in automotive cooling system vacuum test and fill tools. A technician will use this tool to remove air from modern automotive cooling systems, test the system's ability to hold vacuum, and subsequently refill using the vacuum as suction for the new coolant. Typical minimum vacuum values are between 22 and 27 inHg.

Vacuum brakes Inches of mercury was the usual unit of pressure measurement in railway vacuum brakes.

See also Torr (millimeters of mercury) Bar (unit) Mercury barometer Millimetre of mercury

References

Worked examples

Example 1 — a first encounter with Inch of mercury

Start with the simplest possible case. Write down what Inch of mercury claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Inch of mercury before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Inch of mercury ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Inch of mercury

In research: Inch of mercury appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Inch of mercury in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Inch of mercury is common in secondary-school and first-year university syllabi. It links to neighbouring topics Customary units of measurement in the United States, Mercury (element), Units of pressure, so understanding it makes those chapters shorter.
In everyday life: Look for Inch of mercury outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Inch of mercury in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Inch of mercury means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Inch of mercury out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Inch of mercury in simple terms?

Inch of mercury (inHg, ″Hg, or in) is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.

Why does Inch of mercury matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Inch of mercury?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Inch of mercury.