Re: [Boost-users] units use and weird cases....suggestions needed

Subject: Re: [Boost-users] units use and weird cases....suggestions needed
From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2009-09-09 14:25:20

Noah Roberts wrote:
> I'm using the units library to make sure values supplied to functions 
> make sense, dimensionally, and are in the units expected.  I've got a 
> strange case I have to work with though.
> 
> For the most part we've decided to standardize on the SI system.  I 
> write my functions to be system independent when possible, but most of 
> what I'm working with is empirically based.  For this I'll take the SI 
> units expected and write the function only for that system (since other 
> systems will use different equations entirely).
> 
> The problem domain is fluid dynamics so enter the Kv and Cv values.
> 
> Technically these are a measure of volumetric flow.  For instance, Kv is 
> defined as:
> 
> the flow of water with temperature ranging 5 - 30 oC through a valve in 
> cubic meters per hour (m3/h) with a pressure drop of 1 bar
> 
> The problem is that engineers see this as a "unitless" value and treat 
> it as having no units at all.  They use Kv in SI based equations, not 
> m3/s as we'd expect.  Thus all equations being fed to us use Kv as if it 
> were a natural part of the equation.
> 
> Most of the time these dimensions do not compute dimensionally anyway 
> but there's rare cases when they do.  I have gotten the engineers (those 
> giving me specs for code I write) to decide on Kv instead of doing both 
> Kv and Cv (it's a basic conversion).
> 
> The options I see are:
> 
> 1. Convert both Kv and Cv into m3/s and require engineers to give 
> equations with that in mind.  This will be very unnatural for them though.

We've picked this course of action.  It turns out that the flow 
coefficient is a volumetric flow over sqrt(pressure).  Now I just need 
to figure out how to represent that in the dimension system of the 
library and all is good.