DQRID : D981223.2
Start DateStart TimeEnd DateEnd Time Data Quality Metric
06/19/1998183010/08/19981925Suspect
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Subject:
SGP/AOS/C1 - Intermittent leak in AOS spare sampling line and thermal sensitivity
DataStreams:sgpaosC1.a0
Description:
Starting on 980619, periodic fluctuations in several of the measured AOS parameters (e.g., 
Aerosol light absorption coefficent, TSI nephelometer relative humidity, condensation 
particle counts, etc.) were observed. The start of these fluctuations coincided exactly 
with the shutdown of the AOS for power washing of the wooden stairs and stack platform.  
Numerous attempts at troubleshooting were conducted via telephone with technicians at the site.
 
These concentrated on 1) trying to find an electrical noise problem or 2) determining 
whether our stack heating unit was somehow producing particles. Tests were devised to check 
both of these possibilities, and both were determined to be non-problems.  The 
troubleshooting of this signal fluctuation problem was compounded and delayed by the concurrent 
unrelated electronic ball valve problem (see AOS DQR#980731.1).

Pat Sheridan and Jim Wendell from the NOAA Aerosols Group went to the SGP Central Facility 
to diagnose and repair the problem.  All electromagnetic noise possibilities, including 
bad grounds and RF fields were checked. These were not causing the fluctuations, which 
varied in period from ~15-30 minutes. Upon dismantling the insulated air intake assembly 
inside the trailer, a minute crack was found in one of the spare sampling lines at its 
connection to the manifold.  This line was one of the original installed by the previous mentor. 

Repair of this line break essentially repaired the fluctuation problem, although this 
extensive troubleshooting revealed a slight temperature dependence for some of our reference 
signals (more on this below).

Our explanation of this problem is as follows.  When the AOS stairs were power washed, 
either the activities associated with power washing (e.g. people or compressors on the 
platform) or lowering the stack caused enough vibration to cause a hairline break in the spare 
sampling line at the connection with the manifold.  We believe this tubing is 
high-density polyethylene, which is semi-rigid, and it has now been replaced with less brittle 
material.  The routine daily system zero checks established by the previous mentor would not 
have detected this leak problem, because overpressurization of the manifold with 
particle-free air showed up as good zero checks in all instruments and some of this clean air 
would have simply flowed out of the spare line tubing crack.  Since the crack was beneath at 
least two layers of foam insulation which itself was covered with foam-backed metallic 
tape, we believe that free air flow of trailer air to the crack was not likely.  However, 
during periods when the large air conditioning unit was on, pressurization of the trailer 
caused air to slowly leak into the spare sampling line and back into the manifold and 
other sampling lines.  Upon arrival at the site, we found the fan for the A/C system 
operating only when the A/C compressor came on.  We were informed by Dan Nelson, Site Engineer, 
that this fan is supposed to stay on all the time.  Our data suggest that this fan has 
been off since before the problem began.  This is fortuitous, because the trailer was not 
continually pressurized and room air was not sampled continuously over this period.  
Thus, only a portion of the data are of questionable quality.  This leak was not immediately 
obvious (to AOS technicians or to us) because of its concealed location and because some 
aerosol measurements (e.g., particle number concentrations, aerosol light absorption 
coefficents, etc.) did not change dramatically during these periods (i.e., the aerosols 
brought inside the trailer through the cooling system were often not much different in 
concentration or optical properties than were aerosols sampled at the top of the stack).  Thus 
data during the A/C periods will be labeled as questionable.

We will institute a new leak check/zero check procedure which should catch leaks of this 
type.  The old system zero check devised by the previous mentor was insufficient to find 
this leak. 

The A/C unit in the AOS trailer is more than adequate to cool that space.  The cold air 
that comes out of the vents is probably >20F cooler than trailer air.  This cold air causes 
slight fluctuations in the lamp voltages and reference signals of several of our 
instruments.  We expect this problem to be more severe in the summertime because of increased 
air conditioner use and indoor temperature swings.  Fortunately, these signal fluctuations 
are not observed in the final processed signals from these instruments.  They do, 
however, suggest that a gentler cooling of the trailer should be considered.

Possibilities for this could include:
1)	Installation of a smaller air conditioning unit
2)	Changing the ratio of outside air mixed in with the cooled air
3)	Venting the exhaust from the cabinets into the air recirculating 
        system to preheat the cooled air.

We realize that these suggestions may not be in line with the notion of efficient cooling 
of the trailer.  However, we feel that the root cause of the observed temperature 
fluctuations is that the air conditioning unit has far more capacity than is needed by the AOS 
trailer.  A smaller air conditioner, properly matched to the heat load of the trailer, 
would probably be a more efficient solution.

Since the cooling of the trailer generally was faster than the warming-up period, a 
majority of data should be valid.  Also, not all measurements appeared questionable (i.e., the 
data did not appear to change when the A/C unit came on).  We have flagged all aerosol 
data as "questionable" during A/C "on" periods because of the likelihood that at least some 
mixing of trailer air with ambient air occurred.
Suggestions: 
An algorithm for flagging the questionable data during the periods the air 

conditioner was running has been developed at CMDL.  Basically, we use a filtering program to flag 
data as questionable when the air conditoner was on.  This is most easily determined by 
looking at the room temperature sensor data (RoomTemp in sgpaosauxC1.a0 datastream).  The 
room temperature sensor is located directly under an air conditioning vent, and so it 
should provide extremely reliable and fast-response data.  When the first derivative of the 
room temperature sensor data is negative, the trailer is being cooled.  Data are flagged 
until the room temperature first derivative changes sign.
Measurements:sgpaosC1.a0:
  • GrnBScatCoef_1um_LRH
  • pa800_p1000conc
  • GrnTScatCoef_10um_LRH
  • RedBScatCoef_1um_LRH
  • RedBScatCoef_10um_HRH
  • RedTScatCoef_10um_LRH
  • RedBScatCoef_1um_HRH
  • Bap_I_1um
  • pa14_p16conc
  • GrnBScatCoef_10um_LRH
  • pa30_p35conc
  • RedTScatCoef_10um_HRH
  • GrnBScatCoef_10um_HRH
  • pa12_p14conc
  • pa140_p160conc
  • pa400_p500conc
  • CPCPartConc
  • pa80_p90conc
  • pa60_p70conc
  • pa18_p20conc
  • GrnBScatCoef_1um_HRH
  • pa180_p200conc
  • BluBScatCoef_1um_LRH
  • pa130_p140conc
  • pa90_p100conc
  • pa35_p40conc
  • pa26_p30conc
  • BluBScatCoef_1um_HRH
  • pa16_p18conc
  • Bap_I_10um
  • pa260_p300conc
  • pa500_p650conc
  • Pa1000Conc
  • pa10_p12conc
  • pa200_p230conc
  • BluTScatCoef_1um_HRH
  • pa230_p260conc
  • Ozone
  • BluTScatCoef_1um_LRH
  • pa650_p800conc
  • pa300_p350conc
  • pa50_p60conc
  • Bscat530nm
  • pa350_p400conc
  • pa40_p45conc
  • pa70_p80conc
  • pa160_p180conc
  • GrnTScatCoef_1um_LRH
  • BluTScatCoef_10um_HRH
  • GrnTScatCoef_1um_HRH
  • pa100_p130conc
  • RedTScatCoef_1um_LRH
  • pa20_p23conc
  • RedTScatCoef_1um_HRH
  • BluTScatCoef_10um_LRH
  • RedBScatCoef_10um_LRH
  • pa23_p26conc
  • GrnTScatCoef_10um_HRH
  • BluBScatCoef_10um_LRH
  • pa45_p50conc
  • BluBScatCoef_10um_HRH
more

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