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 12:22:21

Matthias Schabel wrote:
>> 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).
> 
> It's a little hard to see where the problem is without a concrete 
> example. How about one equation and the corresponding function, with 
> ancillary information about the expected units of inputs/output?

I'll try, but I'm not an engineer.

I believe the "flow coefficient" (Kv) is mostly used in calculating flow 
through valves and fittings.  It has a relationship with a "resistance 
coefficient" (K) in that:

K = C1 * pi^2 * d^4 / Kv^2
Kv = C2 * pi * d^2 / K^.5

d is the valve diameter.  C1 and C2 are really large numbers.

The volumetric flow (Q) can then be found:

Q = C * Kv * ( dP / p )^.5

Where C is a constant, dP is a pressure, and p is a density.

That equation might look a bit different for SI (that's in some set of 
US units, not necessarily a true system) but the point is that Kv will 
still be in cubic meters per hour.  TP410 states, "At the time of 
preparation of this paper there is no agreed international definition 
for flow coefficient in the terms of SI units."  That was 50 years ago 
or something and it's not changed.

It should be possible to convert Kv into m3/s but the equations the 
engineers work in, and thus will be giving me and are in our sources, 
simply do not use this unit for that variable even if everything else is 
in SI.