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# 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?

Cheers

Greg Locock

### RE: Order of magnitude

Just a guess.

"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

Greg,

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

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### RE: Order of magnitude

It varies.

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

I think we all know the correct definition of an order of magnitude.  I gather the question is trying to understand what the user meant when they said "order of magnitute", which is probably not base on the true definition.

### RE: Order of magnitude

I always thought that an air conditioner was to be sized as close to or slightly smaller than the maximum expected heat load. That way the cooling would be accomplished with dehumidifying the air.

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

Cajun,
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

afaik, order of magnitude is defined as teh next preamble. Mega --> Giga and so on. this would mean, an order of magnitude is factor 1000.

at least, thats how i remember it being explained to me. please tell me if this is just wrong =)

chris

### RE: Order of magnitude

Order of magnitude is the next power of ten

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

It is commonly thought thar orders of magnitude represent the powers of 10 multiplied by the quantity in hand. But some dictionaries say these could also be interpreted as small multiples of that quantity. Some may, for example, interpret it as the powers of 2. Thus, in GregLocock's particular case it could mean doubling the calculating results.

### RE: Order of magnitude

Isn't it amazing how confused things get when people are imprecise in their use of terms?  The reference jmw found explains it the way I learned the term - an order of magnitude is an increase of one in an exponent of your base number.  If you are working in base 2 (like the computer folks sometimes do) then an order of magnitude is twice.

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

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