Order of magnitude
Order of magnitude
(OP)
Once upon a time I was designing an HVAC system for a hall, using the relevant British Standards. I went through the process of working out the total heat load due to the occupants, etc etc, to be met, finally, by the following baffling recommendation "The installed cooling capacity should be an order of magnitude greater than this estimate" .
A factor of 10? I'd just spent several hours researching various parameters and then these comedians bung in a fudge factor of ten?
So, what do you think it means when when someone says an order of magnitude?
A factor of 10? I'd just spent several hours researching various parameters and then these comedians bung in a fudge factor of ten?
So, what do you think it means when when someone says an order of magnitude?
Cheers
Greg Locock





RE: Order of magnitude
"The installed cooling capacity should be at least 10% greater than this estimate" ????
and from a previous thread: 10% greater - whatever that means.
RE: Order of magnitude
My guess is that order of magnitude was a factor of 10 - perhaps someone thought you could not estimate the various enthalpies accurately, or perhaps they wished to have reserve capacity for future expansion, etc.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Order of magnitude
Most colloquial speech assumes that it's around a factor of ten, but, I would guess that the root of the expression comes from astronomy, where a integer magnitude change represents a factor of about 2.5 times.
It's a log scale, we just don't know, a priori, what the base is.
TTFN
RE: Order of magnitude
Knew there'd be a definition somewhere!
RE: Order of magnitude
RE: Order of magnitude
If the system were oversized it would cool the air too rapidly without removing any humidity resulting in a higher relative humidity (same absolute moisture content at lower temperature) and thus have moisture condensation problems. An undersized unit would take advantage of the residual heat storage capacity of the interior mass to slow down the rate of temperature rise at peak loads.
To size a unit at 10 times the calculated capacity would be a waste of money and not give the results desired.
But I’m just a dumb civil engineer, what do I know about air conditioning.
I’d definitely question the author of that statement to see what he ment.
Rick Kitson MBA P.Eng
Construction Project Management
From conception to completion
www.kitsonengineering.com
RE: Order of magnitude
Isn't 10% greater simply 1.1 times the given amount? Or maybe I missing something by not being privy to the previous thread.
Jesus is THE life,
Leonard
RE: Order of magnitude
at least, thats how i remember it being explained to me. please tell me if this is just wrong =)
chris
RE: Order of magnitude
i.e. 2.5 x 10^4 is one order or magnitude larger than 2.5x10^3
An order of magnitude is a factor of ten.
Rick Kitson MBA P.Eng
Construction Project Management
From conception to completion
www.kitsonengineering.com
RE: Order of magnitude
RE: Order of magnitude
Since most of us use 10 (i.e., one more than 9) as our base, it would seem to be "understood" that unless a different base is specified the base is 10 and increasing 23 by an order of magnitude would yeild 230.
My guess on the original reference is that some individual writing the spec wanted a fudge factor and didn't want to sound imprecise so he used a term which (in his mind) meant "add a fudge factor". When engineers (especially engineers writing specifications) don't want to admit that they don't know an answer, it leads to all sorts of confusion.
David