v UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
\ RESEARCH TRIANGLE PARK. NC 27711
'	^ OFFICE OF AIR QUALITY PLANNING AND STANDARDS
Technical Note- Clarifications and Guidance on Residence Time Determination
June 3,2019
Some EPA Regional Offices and Air Monitoring Organizations have interpreted language in the QA
Handbook for Air Pollution Measurement Systems: "Volume II: Ambient Air Quality Monitoring
Program" EPA-454/B-17-001, January 2017, Section 7.3.1 Design of Probes and Manifolds for Automated
Methods, Residence Time Determination differently. Because of this, a better explanation of how to
determine and calculate the residence time of air pollutants within the entire ambient air sampling
system is appropriate to ensure this process is performed correctly and consistently. OAQPS and the
Regions agree the clarifications and guidance described in this technical memo will provide the
necessary information to effectively determine ambient air sampling system residence time. These
clarifications and guidance are effective immediately.
Therefore, the section entitled "Residence Time Determination" (currently page 6 of 17 of section 7 of
the QA Handbook:
https://www3.epa.gpv/ttn/amtic/files/ambient/pm25/qa/Final%20Handbppk%20Document%201 17.pdf
through page 8 of 17, to "7.3.2 Placement of Probes and Manifolds" (of the same referenced section), is
replaced with the following:
Residence Time Determination
No matter how nonreactive the sampling probe material may be, after a period of use, reactive
particulate matter is deposited on the probe walls. Therefore, the time it takes the gas (sample) to travel
from the probe inlet (ambient air) to the sampling device is critical. Ozone, in the presence of nitrogen
oxide (NO), will show significant losses even in the most inert probe material when the residence time
exceeds 20 seconds. Proper sample system designs indicate that a 10 second or less residence time is
easily achievable.
Residence time is defined as the amount of time that it takes for a sample of ambient air to travel from
the opening of the inlet probe (or cane), through the manifold, to the inlet of the instrument and is
required to be less than 20 seconds for reactive gas monitors. The residence time of pollutants within
the entire sampling system is critical. It is recommended that the cumulative residence time of each
component of the sampling system, i.e. the probe, the manifold and sample lines from the manifold to
the back of the instrument be less than 10 seconds (up to a maximum total allowable 20 seconds). It is
important to determine the residence time in each portion of the sampling system. For example, if the
volume of the manifold does not allow this residence time to be achieved, a booster pump/blower
motor or other device (vacuum pump) can be used to decrease the residence time in that portion of the

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sampling system. It is also important to ensure the pressure drop be less than 1 inch of water measured
in the manifold portion of sampling system to prevent the individual sampler pumps (in the monitors)
from being overworked, but also to ensure that ambient air is being sampled.
Residence Time Determination Process
The residence time for a sampling system that contains multiple components (i.e. probe inlet, manifold,
and analyzer sampling lines) can be determined utilizing the following spreadsheet:
I
CARB Res-Time
Example.xls
All the components that make-up the sampling system must be measured individually to obtain the
residence time including inside diameter of the probe/ manifold/ tubing, and lengths of each
component measured. Flow rates in the probe/ manifold and for each instrument must also be
separately measured. When using the spreadsheet above (and attached below), the probe/manifold
flow rate must be measured at the outlet of the booster pump/blower motor. (It is possible to
measure the flow rate elsewhere, but care must be taken to ensure the various flowrates and volumes
of each component of the system be considered properly. The provided spreadsheet cannot be used if
flow of the probe/ manifold is measured elsewhere.)
For the spreadsheet to calculate the sampling system residence time the following steps need to be
performed.
•	Separately obtain the internal diameter of the probe inlet line and manifold in millimeters (mm)
and enter this on the spreadsheet.
•	Separately obtain the length of the probe inlet line and manifold in meters (m) and enter this on
the spreadsheet.
•	Obtain the flow rate from outlet of the manifold blower/booster pump that the probe inlet line
and manifold are connected to in liters per minute (Ipm).
•	For each analyzer connected to the manifold obtain the internal diameter (mm), length (m) of
tubing used between the manifold and back of the analyzer. Note: If there are multiple tubing
diameters and lengths used for a given analyzer sampling line these need to be entered on the
spreadsheet Care should also be taken to consider the volume of any in-line filter holders here as
well.
•	Obtain flow rate (Ipm) for each analyzer and enter this on the spreadsheet.

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PM2 5
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Roof Top
Measure ft a* rate here at blower exhaust for
Vacuum
(•TI-
MET
THC
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Manifold to analyzer (in-
line filter?)
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Span Gas
Internet
PROBE RESIDENCE TIME - Multiple Diameters
Site Name:	 Auditors:	 Date:
Booster pump flow =|	LPM | Pollutant [
Manifold to Manifold to Manifold to Manifold to Manifold to Manifold to
Probe	Manifold	Instrument Instrument Instrument Instrument Instrument Instrument
Material | Teflon | Glass i
Teflon
Teflon
Teflon
Teflon
Teflon
Teflon

T1 T2 T3
T1 T2 T3
T1 T2 T3
T1 T2 T3
T1 T2 T3
T1 T2 T3
ID (mm) | | |
1 1
1 1
1 1
1 1
1 1
1 1







Length (m) | | |
1 1
1 1
1 1

1 1





*

*
Flow (Ipm) |	 | |













Time | | |
1 1
1 1
1 1
1 1
1 1
1 1







Total Residence Time =






Residence Time Calculation =
3.14(ID2)x LENGTH x 0.015
FLOW
3.14 * radius (mm) * LENGTH (m) * 60 (sec/min)
FLOW (Ipm) * 1000
Multiple Tubing Diameters Calculation*
'used for entry into the Audit Information System,
where different diameters of tubing are present
((tubing_2I.D./tubing_11.D.)2 * tubing_2 length) + ((tubing_3I.D./tubing_11.D.)2 * tubing_3 length) + tubingM length
Common Sample Line Diameters
Manifolds
Outside Diameter (in.)
1/8"
3/16"
1/4"
1/4"
5/16"
5/16"
3/8"
3/8"
1/2"
1/2"
O.D. (in.)
1.25"
2.0"
Wall Thickness (in.) *
.030"
.030"
.030"
.062"
.030"
.062"
.030"
.062"
.030"
.062"
W.T. (in.)
.25"
.25"
Inside Diameter (mm)
1.6
3.2
4.8
3.2
6.4
4.8
7.9
6.4
11.1
9.5
I.D. (mm)
25.4
44.5
* "thin wall" is typically = 0.030", wtiile "thick wall" is typically = 0.062"
Length (m)
0.25
0.30
Data recorded and verified by:
After the internal diameters, lengths and flow rates are entered on the spreadsheet for each
component, the residence time for that component is calculated and displayed. After the parameters of
the probe inlet line, manifold and each analyzer have been entered on the spreadsheet the total
residence time will be calculated and displayed. This "total residence time" should be less than 10
seconds and must be less than 20 seconds.

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Residence Time Spreadsheet Formulas/Equations
The spreadsheet uses the following formula/equation to calculate residence time for each component of
the sampling system that may be present in any sampling configuration (for example, if a single
sampling line is used from the instrument to collect outside ambient air- only consider the 3rd and 4th
components below):
1)	Residence Time in probe:
(sec) (RT(probe)) = 3.14(ID2) x Length x 0.015
Total Flow
= 3.14 * radius (mm)2 * Length (m) * 60 (sec/min)
Total Flow (Ipm) * 1000
Where:
3.14 = pi
ID = inside diameter of inlet probe, mm
Length = length of probe, m
Total Flow = flow rate of manifold blower or booster pump measured at its exhaust, plus total flow of
each instrument, Ipm:
2)	Residence Time in manifold:
(sec) (RT(manifold)) = 3.14(ID2) x Length x 0.015
Total Flow
= 3.14 * radius (mm)2 * Length (m) * 60 (sec/min)
Total Flow (Ipm) * 1000
Where:
3.14 = pi
ID = inside diameter of manifold, mm
Length = length of manifold, m
Total Flow = flow rate of manifold blower or booster pump measured at its exhaust, plus total flow of
each instrument, Ipm:
3)	Residence Time in sample line (between manifold and instrument:
(sec) (RT(sample line)) = 3.14 (ID2) x Length x 0.015
Sample Flow of instrument/ analyzer
= 3.14 * radius (mm)2 * Length (m) * 60 (sec/min)
Sample flow of instrument/ analyzer (Ipm) * 1000
Where:
3.14 = pi
ID = inside diameter of tubing used from manifold to instrument, mm
Length = length of tubing used from manifold to instrument, m
Flow = flow rate of analyzer, Ipm
• Note: If there are multiple tubing diameters and lengths used for a given analyzer sampling line
these need to be entered on the spreadsheet. Care should also be taken to consider the volume
of any in-line filter holders here as well.

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Total residence time (RT) = RT(probe)+ RT (manifold)+ RT (sample line)
(as described above)
Additional Considerations
It has been demonstrated that there are no significant losses of reactive gas (03) concentrations in
conventional 13 mm inside diameter sampling lines of glass or Teflon if the total sample residence time
(which also includes the RT in the sample line between the manifold and the instrument) is 10 seconds
or less. However, when the sample residence time exceeds 20 seconds, loss is detectable, and at 60
seconds the loss is nearly complete.
The air flow through the manifold must not be so great as to cause the pressure inside the manifold
system to be more than one inch of water below ambient. This is important to prevent the
analyzer/monitor pump from being over worked to maintain proper flows and the system from leaks
that may cause ambient air dilution. The following assessment can be made to make this determination.
Construct the sampling system/manifold. Use a pitot tube to measure the flow of the sample inside the
manifold as close to the blower as possible. At the same time, attach a water manometer to a sampling
port. Turn on the blower and measure the flow rate and the vacuum. (Remember to allow for the air
demand of the instrumentation and confirm the instrument(s) have adequate flow). Adjust the blower
flow rate to fit between these two parameters. If this is impossible, the diameter of the manifold is too
small.
If the manifold that is employed at the station has multiple ports, then placement of the instrument
lines can be crucial. If a manifold like the Figure shown above is used, ambient air flows down the center
tube and then travels up on both sides of the manifold to the analyzer ports. It is suggested that
instruments requiring lower flows be placed towards the bottom of the manifold. The general rule of
thumb states that the placement of the calibration line (if used) should be in a location such that the
calibration gases flow past the instruments before the gas is evacuated out of the manifold. The Figure
above illustrates a potential introduction port for the calibration gas which would enter near the top of
the probe.

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