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30 

ObservAir

 Operating Manual 

5.

 

Best Practices  

5.1.

 

Filter replacement  

 
As  particulate  matter  deposits  on  the  filter,  the  ObservAir  uses  a  simple 
mathematical relationship to calculate black carbon (BC) concentrations in 
the sample flow as a function of the light attenuation rate through the filter 
(Section  1.1.1).  As  the  aerosol  filter  becomes  overly  saturated  with  BC 
deposits,  however,  this  underlying  relationship  degrades  and  the 
ObservAir’s  BC  concentration  measurements  are  underreported  (lower 
than the true value). This measurement degradation is known as the ‘filter 
loading artifact’, and can be largely avoided by changing the aerosol filter 
tab  when  the  optical  attenuation  (ATN)  values  exceeds  80:  a  commonly 
accepted  ATN  threshold  for  aerosol  photometry  at  880  nm.  While  some 
measurement  error  persists  below  this  threshold,  it  is  usually  small  and 
grows  larger  for  higher  ATN  values.  Therefore,  it  is  best  practice  to 

only

 

operate the ObservAir while the optical ATN is < 80,  and replace the filter 
when this threshold value is exceeded.  

 

 

Given this optical attenuation limit, the filter’s operational life before 

requiring replacement depends only on the average BC concentration and 
flow  rate  of  air  sampled  through  the  sensor.  For  an  average  BC 
concentration of 1 

µ

g/m

3

,  

Table 2 shows that the effective filter life ranges from 6.3 to 1.6 days as the 
sample  flow  rate  settings  increases  from  50  to  200  ccm.  Filter  life  is 
inversely  proportional  to  both  average  BC  and  flow  rate,  and  may  be 
calculated using the equation below. The equation approximates the total 
sampling time required for the aerosol filter to reach an ATN of 80 for the 
given input conditions.  

𝐹𝐿 = 

313.5

𝐵𝐶

)-&

∙ 𝐹𝑅         (5)

 

FL

 = Filter life (days) 

BC

avg 

= Average BC concentration (

µ

g/m

3

FR 

= Flow rate (ccm)  

  

5.2.

 

  Filter loading correction  

 
While sensor operation at low optical attenuation levels largely eliminates 
the filter loading artifact, this may not always be practical and convenient, 
and some BC measurement error necessarily remains. As a result, many 
filter  loading  correction  algorithms  have  been  developed  to  compensate 
aerosol absorption photometers’ BC measurements as a function of optical 
attenuation.  These  empirical  corrections  are  well  documented  and 

 

 

31 

ObservAir

®

 Operating Manual 

generally straightforward to implement. However, the filter loading artifact 
is not static – it depends on the emissions source (e.g. biomass vs. diesel), 
atmospheric  conditions,  seasonality,  and  other  factors.  Therefore,  filter 
loading correction algorithms should be calibrated and validated for each 
particular application of the ObservAir, and periodically updated over long-
term  deployments.  Standard  filter  loading  correction  procedures  for  the 
ObservAir are under development, and will be released in future versions of 
this  manual.  Helpful  resources  on  the  filter  loading  artifact  are  provided 
below.  
 

Filter loading correction resources:  

1.

 

Good, N.; Mölter, A.; Peel, J. L.; Volckens, J. An Accurate Filter Loading 
Correction Is Essential for Assessing Personal Exposure to Black Carbon 
Using an Aethalometer. J. Expo. Sci. Environ. Epidemiol. 2017, 27 (4), 
409–416. 

https://doi.org/10.1038/jes.2016.71

2.

 

Jimenez,  J.;  Claiborn,  C.;  Larson,  T.;  Gould,  T.;  Kirchstetter,  T.  W.; 
Gundel,  L.  Loading  Effect  Correction  for  Real-Time  Aethalometer 
Measurements of Fresh Diesel Soot. J. Air Waste Manag. Assoc. 2007, 
57 (7), 868–873. 

https://doi.org/10.3155/1047-3289.57.7.868

3.

 

Virkkula, A.; Mäkelä, T.; Hillamo, R.; Yli-Tuomi, T.; Hirsikko, A.; Hämeri, 
K.; Koponen, I. K. A Simple Procedure for Correcting Loading Effects of 
Aethalometer  Data.  J.  Air  Waste  Manag.  Assoc.  2007,  57  (10),  1214–
1222. 

https://doi.org/10.3155/1047-3289.57.10.1214

 

5.3.

 

Flow rate setting: Filter life vs. BC resolution 

 
The  filter’s  operational  life  decreases  at  higher  flow  rates,  so  it  may  be 
tempting  to  set  flow  rate  at  the  lowest  possible  option  (25  ccm)  and 
minimize  sensor  maintenance.  However,  effective  BC  measurement 
resolution also depends on flow rate: Since the filter loads up with BC more 
rapidly  at  higher  flows,  the  time  rate  of  light  attenuation  is  more  readily 
detectable, and BC baseline noise decreases. BC baseline noise represents 
the  sensor’s  effective  measurement  resolution  and  is  shown  for  various 
sampling intervals in Table 4 for a flow rate of 100 ccm. BC baseline noise 
is inversely proportional to the sample flow rate. For example, 1-minute BC 
noise is around 0.025 

µ

g/m

3

 at 200 ccm (half that shown in Table 4 for 

100 ccm). In this way, higher flow rates provide measurements with higher 
temporal resolution, but at the expense of filter life. The optimum flow rate 
setting maintains adequate BC measurement resolution for the monitoring 
application  while  maximizing  filter  life  such  that  sensor  maintenance 
remains  convenient  and  practical.  The  procedure  below  outlines  the 
calculation and selection of an appropriate ObservAir flow rate setting.  

Содержание ObservAir

Страница 1: ...rational limits and warnings 11 3 Operating Instructions 13 3 1 Hardware overview 13 3 2 Interactive LED button Sensor display and control 14 3 2 1 Sensor startup 15 3 2 2 Default LED mode Pollutant c...

Страница 2: ...pport WiFi and USB communication protocols and include a 16GB removable SD card for onboard data storage Units may also be supplemented with an LTE LoRa or SigFox communication module and a GPS unit f...

Страница 3: ...collection area is 3 mm in diameter After the first filter the air flow passes through a second reference filter assembly that is identical to the first Since the air is filtered devoid of PM the int...

Страница 4: ...plication Operate the 7 ObservAir Operating Manual ObservAir for 12 to 24 hours in an environment that is free of the analyte gas es and average the voltage differential data collected over the entire...

Страница 5: ...technical specifications are summarized in the tables below All electrochemical cells are sourced from Spec Sensors Gas measurement performance specifications are adapted from data provided by the ma...

Страница 6: ...intervals 2 3 Operational limits and warnings Environmental limits Only operate the ObservAir within the conditions listed in Table 1 Temperature and relative humidity must remain between 5 to 40 C an...

Страница 7: ...nd may result in permanent damage to the unit and or harm to the user Please contact DST Technical Support at info dstech io regarding any problems with your unit that cannot be resolved with the inst...

Страница 8: ...llutants For every new start up event a new data file is created on the SD card txt file and is assigned a filename that contains the sensor s ID number and the start up time date Section 3 5 1 3 2 2...

Страница 9: ...f the ObservAir hold the LED button to cycle through the menu until the first red flash and release The sensor will flash red two more times to confirm that it is shutting down 3 2 6 Sensor alarms and...

Страница 10: ...ront panel 19 ObservAir Operating Manual 3 5 Data collection from onboard SD card While the ObservAir is operating data is written to the onboard SD card in real time as both a primary means of data c...

Страница 11: ...e ADC s raw count Full scale value is 8388607 d ATN Optical attenuation unitless e BC Black carbon in g m3 f T Temperature in C g RH Relative humidity in h FR Sample flow rate in ccm i VBAT Battery vo...

Страница 12: ...tings Figure 12 6 With the Serial Monitor configured ObservAir data is displayed in real time at the time interval set by the user Commands can also be sent to the sensor using the dialog box at the t...

Страница 13: ...8 to 1 2 Set flow rate calibration coefficient Table 7 ObservAir serial commands and descriptions 25 ObservAir Operating Manual 3 7 WiFi connection Provisioning WiFi credentials to the ObservAir allo...

Страница 14: ...1 Zero calibration of pollution sensors Each ObservAir comes with a zero calibration sheet but it is good practice to periodically verify the sensor s baseline performance especially before and after...

Страница 15: ...but clearly the calibration is only as good as the reference measurements collected 4 4 Leak check The ObservAir incorporates a monolithic design architecture that is more robust than traditional ins...

Страница 16: ...loading artifact is not static it depends on the emissions source e g biomass vs diesel atmospheric conditions seasonality and other factors Therefore filter loading correction algorithms should be c...

Страница 17: ...ing can be optimized to meet your needs For example if greater BC measurement resolution is desired at the expense of filter life the flow rate can be increased past the minimum value Conversely flow...

Страница 18: ...exhaust stacks vehicle tailpipes and other pollution sources When sampling emissions directly do not exceed the ObservAir s maximum temperature relative humidity range and pollution concentration rat...

Страница 19: ...If BC measurements are proportionally lower than the reference value then it is also possible but less likely that the ObservAir is leaking Given these potential error modes it is important to valida...

Страница 20: ...38 ObservAir Operating Manual Figure 14 Hard reset using the magnetic cover...

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