# Parameters: Units

Model ElementParam_Unit defines the units for the model being defined.

## Description

Param_Unit allows you to specify four types of units: Mass, Length, Time, and Force. The last is obviously a function of the first three and can be automatically calculated for a consistent set of units. However, many unit systems are not "consistent", so it may be necessary to specify a force unit also.

Mass Units | Length Units | Force Units | Time Units |
---|---|---|---|

Gram | Inch | Ounce_Force | Millisecond |

Milligram | Foot | Pound_Force | Second |

Kilogram | Mile | Kpound_Force | Minute |

Megagram | Millimeter | Dyne | Hour |

Pound_Mass | Centimeter | Newton | Microsecond |

Slug | Meter | Kilogram_Force | Nanosecond |

Ounce_Mass | Kilometer | KNewton | Day |

Kpound_Mass | Nanometer | CentiNewton | |

Tonne | Angstorm | MilliNewton | |

Microgram | Yard | MicroNewton | |

Nanogram | MilliInch | MegaNewton | |

US Ton | MicroInch | Poundal | |

Slinch | Mils | NanoNewton | |

Micrometer |

## Format

```
<Param_Unit
{ mass_mult_factor = "
```*real"*
| [ force_unit = "*name"* ]
[ mass_unit = "*name"* ]
[ length_unit = "*name"* ]
[ time_unit = "*name"* ] >
}
</Param_Unit>

## Attributes

`mass_mult_factor`- Defines the multiplication factor to be used so that Newton's second law, Force = Mass * Acceleration, is still valid for the set of units used by the user. mass_mult_factor has a default value of 1.0.
`force_unit`- Defines the units for force. Select from:
- Ounce_Force
- Pound_Force
- Kpound_Force
- Dyne
- Newton
- Kilogram_Force
- KNewton
- CentiNewton
- MilliNewton
- MicroNewton
- MegaNewton
- Poundal
- NanoNewton

`mass_unit`- Defines units for mass. Select from:
- Gram
- Milligram
- Kilogram
- Megagram
- Pound_Mass
- Slug
- Ounce_Mass
- Kpound_Mass
- Tonne
- Microgram
- Nanogram
- US_Ton
- Slinch

`length_unit`- Defines the units for length. Select from:
- Inch
- Foot
- Mile
- Millimeter
- Centimeter
- Meter
- Kilometer
- Nanometer
- Angstrom
- Yard
- MilliInch
- MicroInch
- Mils
- Micrometer

`time_unit`- Defines the time units. Select from:
- Millisecond
- Second
- Minute
- Hour
- Microsecond
- Nanosecond
- Day

## Example

Defining an SI units system:

```
<Param_Unit
force_unit = "NEWTON"
mass_unit = "KILOGRAM"
length_unit = "METER"
time_unit = "SECOND" >
</Param_Unit>
```

- Mass is specified in grams.
The mass scale is 10-3 Kg.

- Length is specified in micrometers.
The length scale is 10-6 m.

- Time is specified in milliseconds.
The time scale is 10-3 s.

- Force is specified in Newtons.
The force scale is 1 N.

A mass multiplication factor is defined as follows:

```
mass_mult_factor = mass_scale * length_scale/(force_scale * time_scale*time_scale)
= 10-3 * 10-6 / (1 * (10-3)2)
= 10-9 / 10-6
= 10-3
```

The Param_Unit blocks is as follows:

```
<Param_Unit
mass_mult_factor = "0.001"
</Param_Unit>
```

- Mass in Kilograms
- Length in millimeters
- Time in seconds
- Force in Newtons

```
<Param_Unit
force_unit = "NEWTON"
mass_unit = "KILOGRAM"
length_unit = "MILLIMETER"
time_unit = "SECOND" >
</Param_Unit>
```

The example below shows the definition of the British units.

```
<Param_Unit
force_unit = "POUNDFORCE"
mass_unit = "POUNDMASS"
length_unit = "FOOT"
time_unit = "SECOND" >
</Param_Unit>
```

## Comments

User units are defined using the `mass_mult_factor` keyword.