On 7/21/2011 3:16 PM, Michael Powell wrote:
Okay, here's what we need to get at, for starters. And maybe an illustration or three and a little exchange will go a long way towards helping my better comprehend units.
I'm starting with a set of SI calculations for oil and gas constants calculations. Eventually we will need to accommodate US units as well. But not quite yet.
We need to get at a calculation involving Pressure Gradient, which ends up being metric::bar/si::meter (bars over meters) in specific units, or I suppose si::pressure/si::length might also work.
Then we need to get after Fluid Density, which ends up being si::kilogram/si:meter^3 (kilograms over cubic meters) in specific units, or I suppose si::mass/si::meter^3 (I don't know what this looks like in terms of boost::units, maybe one of the volumes?), or perhaps make use of mass_density?
Yes, it's mass_density.
We take all that and divide Pressure Gradient by Fluid Density to arrive at what we hope will be the the conversion factor: 0.0000981. Which we could specify that as a constant, but I like proving it through the software first (plausibly once) when we ask for it.
So, by your description: ML^-3 * (ML^-1T^-2 * L^-1)^-1 = L^-1T^-2. This latter part is the dimension your factor is in. using namespace boost::units; typedef derived_dimension<length_base_dimension, -1, time_base_dimension, -2>::type funky_factor_dimension typedef unit<funky_factor_dimension, si::system> funky_factor; quantity<funky_factor> factor(quantity<si::pressure> p, quantity<si::length> l, quantity<si::mass_density> d) { return (p / l) / d; } Alternatively: template < typename System > quantity<unit<funky_factor_dimension, System>> factor( quantity<unit<pressure_dimension,System>> p , quantity<unit<length_dimension,System>> l , quantity<unit<mass_density_dimension,System>> d ) { return (p / l) / d; }
Similar type calculations would follow for US units involving gallons, cubic inches, inches, and inches per foot, along these lines.
As long as you use a coherent system, the template version above should work (assuming I got all the types correct).