Conversion from 3 Micropascals to Pascals

Information about the Micropascal (µPa)

The micropascal is a unit of pressure equal to one millionth of a pascal. Its symbol is µPa. It is one of the smallest pressure units in practical use, and it owes its place to a single number: 20 micropascals, the reference pressure against which every sound level in air is measured.

That figure is the threshold of hearing — roughly the quietest sound a healthy young ear can detect at a frequency of about 1,000 hertz. When acousticians defined the decibel scale for airborne sound they needed a fixed pressure to compare against, and they chose that threshold. Zero decibels means a sound pressure of 20 micropascals; every decibel figure quoted for a road, a machine or a concert is a ratio to that number.

The scale that follows is steep. Twenty decibels is ten times the reference pressure, 200 micropascals. Sixty decibels, ordinary conversation, is a thousand times it, or 20 millipascals. A hundred and twenty decibels, the threshold of pain, is a million times it: 20 pascals. The ear covers a range of a million to one in pressure, and the micropascal sits at the bottom of it.

Underwater acoustics uses the same unit but a different reference. Sound in water is referred to 1 micropascal rather than 20, because the threshold of human hearing is meaningless in the sea. That difference matters enormously: a level quoted in decibels underwater is not comparable to one in air, and the gap between the two conventions is about 26 decibels before any other correction. Sonar figures, whale-song measurements and shipping-noise studies all carry the re 1 µPa qualifier for that reason.

Outside acoustics the micropascal appears in vacuum science. A good high vacuum is around 100 micropascals, and ultra-high vacuum, the regime used for surface physics and particle accelerators, runs from a few micropascals down to nanopascals. Pumping a chamber to that level takes hours of baking to drive adsorbed gas off the walls.

The unit also shows up in radiation pressure and in the very small pressure differences that laboratory instruments resolve. Anything measured in micropascals is a measurement rather than a force anyone would feel, which is exactly what makes the reference-level convention so useful.

One micropascal equals 0.000001 pascals, 0.001 millipascals, 1,000 nanopascals, or about 0.000000000145 pounds per square inch.

Information about the Pascal (Pa)

The pascal is the SI unit of pressure, written Pa. It is defined as one newton of force spread over one square metre, and it is named after Blaise Pascal, whose experiments with barometers in the 1640s established that air has weight and that its pressure falls with altitude. The General Conference on Weights and Measures adopted the name in 1971.

It is a remarkably small unit. Atmospheric pressure at sea level is 101,325 pascals, so the pressure everyone lives under is a six-figure number, and almost every practical use of the unit therefore carries a prefix. That awkwardness is not a design fault but a consequence of coherence: the newton and the square metre were fixed first, and the pascal is whatever falls out of dividing one by the other.

Acoustics is the field where bare pascals are natural. The reference pressure for the decibel scale is twenty micropascals, taken as the quietest sound a healthy young ear can detect, and ordinary conversation is around 0.02 pascals of sound pressure. The threshold of pain sits near 63 pascals, so the entire range of human hearing spans about six orders of magnitude in this unit.

Everyday objects give a sense of the scale. A sheet of office paper lying flat presses on the desk with about 0.8 pascals. A gentle breeze exerts a few pascals on a wall, and the pressure difference that drives ventilation through a building is typically between ten and fifty. Anything a person can feel as force is already in the thousands.

Prefixed forms carry the real work. Weather uses hectopascals, which are numerically identical to the older millibars, so a forecast reading of 1013 needed no relearning. Engineering uses kilopascals for tyre and fluid pressures, and materials science uses megapascals and gigapascals for strength and stiffness, where one megapascal is one newton per square millimetre.

The pascal also appears wherever a stress rather than a pressure is meant, since the two have the same dimensions. Young's modulus, yield strength and shear stress are all quoted in pascals or their multiples, which is why a single unit spans the pressure in a tyre and the stiffness of steel.

One pascal equals 1 newton per square metre, 0.01 millibars, about 0.0000099 atmospheres, or about 0.000145 pounds per square inch.