Gross vs. Net Room Volume
and its Affect on Air Change Rate
M. T. Weeks, CIH
F.W. Boelter, CIH, PE
C. E. Simmons, CIH
American Industrial Hygiene Conference and Exposition
Philadelphia, Pennsylvania – June 5, 2006
Introduction
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ASTM Method E741 is a standardized method used
to measure the air change rate of a single-zone
space using a tracer gas
Using this method, the volume of the space is not a
factor in determining air change rate
Other methods for determining air change rate
require the estimation of the volume of the space
How important is it to accurately estimate room
volume when calculating air change rate for a
space?
Air Change Rate – ASTM E741
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Standard Test Method for Determining Air Change
in a Single Zone by Means of a Tracer Gas Dilution
(2000)
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Allows direct measurement of air change rate
Concentration Decay Method used
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Background measurements taken
Tracer gas introduced
Uniform concentration established
Concentration measured over time
Linear regression performed; air change rate is equal to
slope of regression line
ASTM Method E741-00, Standard Test Method for Determining Air Change in a Single Zone by Means of a
Tracer Gas Dilution. American Society of Testing and Materials, 2000.
Sulfur Hexafluoride Concentration vs. Time
Inside Test Chamber During Test
30
Concentration SF6 (ppm)
25
20
15
10
5
0
10:54
11:53
12:53
13:53
14:53
15:53
Time
Air Changes per Hour = 0.1
16:53
17:53
18:52
Air Change Rate - Traditional
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Volume of room determined –
length x width x height
Volume of air coming into and leaving room is
determined
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Using measurements (e.g., air velocity)
Using other available information (e.g., fan cfm)
Simple calculation
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e.g.:
Q (cfm) x 60 = Air changes per hour
V of room (ft3)
Occupied Space
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Room space can be occupied by:
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Production equipment or machinery
Ductwork
Piping
Office or other furniture
Other work settings may present different
objects
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Bench equipment at a laboratory hood or other
similar workstation
Our Study
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Fourteen tests performed in a test chamber
(20’ x 20’ x 9’ = 3600 ft3)
Air machines set to move 0, 2, 6, and 10 air
changes per hour of total (i.e., unoccupied) room V
Air change rates determined using:
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ASTM method E741-00 (tracer gas)
Calculation based on velocity of ventilated air
Volume of space occupied at 0%, 10%, and 25%
Air mechanically mixed using box fans
Equipment
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Tracer gas – sulfur hexafluoride (SF6)
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Innova 1412 Multi-Gas Monitor
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Extremely low background levels (10-20 ppb)
Easily detectable with proper equipment
Non-toxic (PEL = 1000 ppm) and inert
Measures SF6 at ppb level
Occupied space – 4’ x 4’ x 8’ boxes (128 ft3)
Two air machines – approx. 600 cfm
Air Change Rates
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Measured – ASTM E741
Calculated – Air Change Rate = Q*60/V
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Total Room Volume of 3600 ft3 (VT)
Effective Room Volume after occupying space
(Veff)
Q calculated using velocity measurements of
exhausted air
Expected ACH values were calculated based
on tracer gas result with no occupied space
and change in room volume
Results – No Occupied Space
Target Air
Change
Rate
0 ACH
Measured
ACH –
Tracer Gas
0.3
Calculated
ACH – VT
(3600 ft3)
0
Calculated
ACH – Veff
(3600 ft3)
0
2 ACH
0.9
2.3
2.3
6 ACH
5.0
6.0
6.0
10 ACH
7.7
10.4
10.4
Results – 10% Occupied Space
Target Air
Change
Rate
2 ACH
6 ACH
10 ACH
Expected
ACH Value
1.0
5.6
8.6
Measured
ACH –
Tracer Gas
Calculated
ACH – VT
(3600 ft3)
Calculated
ACH – Veff
(3216 ft3)
1.0
2.0
2.2
0.7
2.0
2.2
1.1
2.0
2.2
5.5
6.1
6.8
8.3
9.9
11.1
9.3
9.9
11.1
7.8
9.9
11.1
Results – 25% Occupied Space
Target Air
Change
Rate
Expected
ACH Value
Measured
ACH –
Tracer Gas
Calculated
ACH – VT
(3600 ft3)
Calculated
ACH – Veff
(2576 ft3)
0 ACH
0.4
0.05
0
0
2 ACH
1.2
1.2
2.2
3.1
10 ACH
10.8
11.5
9.9
14.1
Conclusions
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Tracer gas air change rates mirrored expected
values for changing volumes (average difference
9%), so Veff does affect air change rate
Calculated air change rates ignoring volume change
56% above expected values
Calculated air change rates accounting for volume
change 82% above expected values
Using Veff to calculate air change rate produced 1042% higher result than using VT, increasing with
amount of occupied space
Recommendations
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In all cases, direct measurement of air change rate
using a tracer gas method provides most accurate
results
When calculating air change rate for a space, taking
effective volume into consideration will have a
significant effect on your result
Using the effective volume of a space may or may
not bring you closer to the truth – professional
judgment should be used
When modeling, consider your method of calculating
air change rates very carefully
Gross vs. Net Room Volume
and its Affect on Air Change Rate
Thank you!
Michael Weeks, CIH
Boelter Associates, Inc.
Park Ridge, Illinois
(847) 692-4700
mweeks@boelterassociates.com
www.boelterassociates.com