# How to verify the percentage of outside air in an enclosure

Australia Standard AS1668.2 stipulates the amount of outside air required for the built environment. After completion, how do we verify the amount of outside air percentage going into any enclosure? The video below will describe the a simple way to calculate the percentage of outside air in an enclosure.

In a nutshell

Using temperature measurement
Percentage of outside air = (Treturn – Tsupply)/(Treturn – Toutside air) x 100

or using ppm reading of CO2
Percentage of outside air =
(CO2return – CO2supply)/(CO2 return – CO2outside air) x 100

Derivation of outside air percentage calculation formula.

For those who is interested the below derivation may shed some light for you.

Percentage of outside air = (RAM – SAM)/(RAM – OAM)

RAM = Return air measured
SAM = Supply air measured
OAM = Outside air measured

The measured properties may be temperature, CO2 content in ppm, mosisture content, flow rates and etc but not pressure, relative humidity nor velocity and etc)

We need to go back to fundamental energy balance, to be exact it is mass balance

In this instance you will need measured properties for the below
Return air (R)
Outside Air (O)

These two air streams mixes to gives us
Mix Air (M) = Supply Air (S)

Let assign the measured properties as X hence

Xr = measured property of return air
Xo = measured property of outside air
Xm = measured property of mix air

from conservation of mass
Xm.M = Xr.R + Xo.O

and Xm = Xs and M = S
therefore Xs.S = Xr.R + Xo.O

Divide the above formula with S

therefore Xs = Xr.R/S + Xo.O/S

and assign

fr = R/S and fo = O/S into the formula

therefore Xs = Xr.fr + Xo.fo

We know that S = R + O => 1 = R/S + O/S = fr + fo

therefore fr = 1- fo

so Xs = Xr.fr + Xo.fo will become

Xs = Xr.(1-fo) + Xo.fo

Xs = Xr – Xr.fo + Xo.fo

Xr – Xs = fo.(Xr – Xo)

therefore fo = (Xr – Xs)/(Xr – Xo)

What is the limitation of this outside air calculation percentage?

Conservation of mass (or energy) in ideal situation, in reality and site conditions, there will be may factors that will affect the results. Watch the complete video it covers some of the other limitations as well.

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