Re: [Boost-users] Feedback on units use

Subject: Re: [Boost-users] Feedback on units use
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-08-27 19:06:16

AMDG

Noah Roberts wrote:
> Is there a better way to do the following?  The purpose is to provide 
> a conversion from a pressure to a head given the density of the fluid 
> the head should be in.  Head is "hydrostatic pressure" and is a 
> measure of the length a fluid can be pushed up a column at pressure 
> X.  The value is a function of the fluid and of the pressure acted 
> upon that fluid.
>
> Object is to do this in any system.  Part of the equation is the 
> constant "g", which is acceleration due to gravity.  We're modeling 
> objects on earth so the value of g is 9.80665 m/s^2.  The conversion 
> equation is very basic:
>
> h = p / (d * g)
>
> where h is the headloss in length, p is pressure, d is mass density, 
> and g is the gravitational constant.
>
> #include <boost/units/quantity.hpp>
> #include <boost/units/physical_dimensions.hpp>
>
> template < typename T, typename System >
> struct acceleration_due_to_gravity;
>
>
>
> template < typename T, typename System >
> boost::units::quantity< 
> boost::units::unit<boost::units::length_dimension, System>, T >
> pressure_to_head( boost::units::quantity< 
> boost::units::unit<boost::units::pressure_dimension, System>, T > press
>                 , boost::units::quantity< 
> boost::units::unit<boost::units::mass_density_dimension, System>, T> 
> density)
> {
>   return press / (density * 
> acceleration_due_to_gravity<T,System>::value());
> }
>
> #include <boost/units/systems/si.hpp>
> template < typename T >
> struct acceleration_due_to_gravity<T, boost::units::si::system>
> {
>   typedef boost::units::si::system System;
>
>   static boost::units::quantity< boost::units::unit< 
> boost::units::acceleration_dimension, System>, T>
>     value() { return boost::units::quantity<boost::units::unit< 
> boost::units::acceleration_dimension, System>, T>::from_value(9.80665); }
> };

9.80665 is the value in meters/second^2.
You need to use
  return quantity<unit<acceleration_dimension, System>, T>(9.80665 * 
meters/second/second);
to get the correct value for any system.

In Christ,
Steven Watanabe