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# UnitDispersion

A unit of measure for specific quantities of dispersion.

```
class UnitDispersion
```

## Overview

You typically use instances of [`UnitDispersion`](/documentation/Foundation/UnitDispersion) to represent specific quantities of dispersion using the [`NSMeasurement`](/documentation/Foundation/NSMeasurement) class.

### Dispersion

Dispersion describes the amount of a constituent divided by the amount of all other constituents in a mixture. Dispersion is a dimensionless quantity that is commonly expressed in “parts-per” notation, such as “parts per million” (ppm), to describe small relative quantities.

The [`UnitDispersion`](/documentation/Foundation/UnitDispersion) class defines its [`baseUnit()`](/documentation/Foundation/Dimension/baseUnit()) as [`partsPerMillion`](/documentation/Foundation/UnitDispersion/partsPerMillion).

|Name             |Method                                                                                |Abbreviation|
|-----------------|--------------------------------------------------------------------------------------|------------|
|Parts Per Million|``doc://com.apple.foundation/documentation/Foundation/UnitDispersion/partsPerMillion``|ppm         |

## Topics

### Accessing the Base Unit

[`baseUnit()`](/documentation/Foundation/Dimension/baseUnit())

Returns the base unit.

### Accessing Predefined Units

[`partsPerMillion`](/documentation/Foundation/UnitDispersion/partsPerMillion)

The parts per million unit.


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