science

Perm (unit)

Perm (unit) is a science 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 Perm (unit) rather than just read about it. In short: A perm is a unit of permeance or "water vapor transmission" given a certain differential in partial pressures on either side of a material or membrane. Definitions U.S. perm The U.S. perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.

Key takeaways

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

Reference excerpt

A perm is a unit of permeance or "water vapor transmission" given a certain differential in partial pressures on either side of a material or membrane.

Definitions U.S. perm The U.S. perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.

Metric perm The metric perm (not an SI unit) is defined as 1 gram of water vapor per day, per square meter, per millimeter of mercury.

Equivalent SI unit The equivalent SI measure is the nanogram per second per square meter per pascal.

The base normal SI unit for permeance is the kilogram per second per square meter per pascal.

German Institute for Standardization unit A variant of the metric perm is used in DIN Standard 53122, where permeance is also expressed in grams per square meter per day, but at a fixed, "standard" vapor-pressure difference of 17.918 mmHg. This unit is thus 17.918 times smaller than a metric perm, corresponding to about 0.084683 of a U.S. perm.

References Michon, Gérard P. (April 29, 2003) "Permeability and permeance". Final Answers: Physics of Gases and Fluids. - accessed August 13, 2007

Worked examples

Example 1 — a first encounter with Perm (unit)

Start with the simplest possible case. Write down what Perm (unit) claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Perm (unit) 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 Perm (unit) 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 Perm (unit)

In research: Perm (unit) appears in science 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 Perm (unit) 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: Perm (unit) is common in secondary-school and first-year university syllabi. It links to neighbouring topics Customary units of measurement in the United States, Units of measurement, so understanding it makes those chapters shorter.
In everyday life: Look for Perm (unit) 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 Perm (unit) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Perm (unit) 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 Perm (unit) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Perm (unit) in simple terms?

A perm is a unit of permeance or "water vapor transmission" given a certain differential in partial pressures on either side of a material or membrane. Definitions U.S. perm The U.S. perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.

Why does Perm (unit) matter?

Because it connects several science 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 Perm (unit)?

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 Perm (unit).