file:  Calibration.txt
version:  29Jun99
contents:  Description of calibration and quality assurance procedures

David C. Rogers
Colorado State University, Department of Atmospheric Science

    This document describes the calibration and quality assurance procedures 
that were used in support of ice nuclei and aerosol measurements during the 
NASA FIRE Arctic Cloud Experiment, May 1998.  Diagrams and photographs 
showing the equipment and air sampling arrangement can be found at 
http://lamar.colostate.edu/~dcrogers/Arctic.html 

    CSU equipment was installed in a C-130 instrument rack obtained from 
NCAR.  The CSU equipment is designated "CFD," which stands for "Continuous 
Flow Diffusion" chamber.  It includes the chamber, refrigeration system, 
computer for displaying and recording data, air pumps and associated valves 
and tubing, plus electronic interface and sensors for measuring pressure, 
temperature and air flow.  Also, a condensation nucleus counter (TSI Model 
3760), on loan from NCAR, was mounted in the rack, and its counts were 
recorded by the CFD data system.  Two test flights were conducted (April 24 
and 30), and the results were examined to evaluate the performance of all 
components of the CFD system and to inspect comparable measurements from 
other investigators. 

I. CALIBRATIONS AND QUALITY ASSURANCE 

    Basic calibrations were performed on the electronics and sensors in 
March and June 1998.  The calibrations included voltage, temperature, air 
pressure, air flow, and aerosol particle size.  

  a. VOLTAGE
    A precision voltage source was used to calibrate the electronics.  In 
addition, a precision voltage regulator is built into the CFD electronics.  
This voltage is used as the reference for precision thermistors and is 
measured every time the data system starts, as a check on consistency and to 
look for stability and drift.  No significant drift occurred during the 
field project. 

  b. TEMPERATURE
    Temperatures are measured with two thermistors and eleven type T thermo-
couples attached to various parts of the CFD.  Thermocouple temperature is 
derived from a cold junction (CJ) reference in the electronics.  The CJ 
reference was calibrated against a platinum resistance standard.  There are 
also two precision thermistors (YSI 4403a) attached with thermal epoxy 
adhesive to the two chamber walls, adjacent to thermocouples.  Overall 
temperature accuracy is +/- 0.5C over the range +40 to -60C.  The real time 
CFD data system displays temperatures from both the thermocouples and the 
thermistors.

  c. AIR PRESSURE
    Air pressure of the chamber and aircraft cabin was measured with factory 
calibrated Motorola MPX4115 piezo-resistive sensors.  Additional calibrations 
were done at CSU using a Wallace and Tiernan aneroid cell, which we use as a 
standard in our laboratory.  At the start of each flight, before the 
aircraft was moving, pressures from the CFD system were compared with those 
from the aircraft data system.  They were usually within 1 to 2 mb. 

  d. AIR FLOW
    Mass air flow through the CFD chamber is measured with two Honeywell 
Microbridge mass airflow sensors, type AWM5104vn.  Mass flow through the CN 
counter (TSI Model 3760) was measured with a similar sensor, type AWM5101vn.  
All of the mass flow sensors were factory calibrated with dry nitrogen.  
Additional calibrations were performed at CSU with air, using a Gilibrator 
(volume measurement); local pressure and temperature were used to convert 
volume flow to mass flow.  Coefficients derived from the CSU calibrations 
were used in the real-time CFD data system. 

  e. AEROSOL PARTICLE SIZE
    In the CFD chamber, ice crystals nucleate on ice nuclei and grow to 
several micrometers in diameter.  The primary ice nuclei count is based on 
an optical particle counter (Climet) at the chamber outlet that detects all 
particles larger than about 0.5um.  Ice nuclei are identified on the basis 
of particle size as follows.  An impactor at the chamber inlet removes all 
particles larger than 2um, and then particles larger than 3um are identified 
as crystals (nuclei) at the chamber outlet.  High resolution measurements 
of particle size are obtained by sending the Climet analog voltage pulses 
to a 256 channel multichannel analyzer (MCA).  The efficiency of the inlet 
impactor, as a function of particle size, was measured by sampling ambient 
air with and without the impactor.  Before and after the field project, 
polystyrene latex (PSL) spheres 2.0um diameter were used to calibrate the 
Climet.  Response of the Climet to a larger range of particle sizes was 
evaluated in earlier calibration tests in the laboratory, with PSL 0.4 to 
2um and glycerin particles 1 to 12um. 

  f. SAMPLING FILTERED AIR 
    There are no such things as reference standard ice nuclei, so there is 
no way to perform ice nuclei calibrations.  There is, however, a standard 
for zero particles.  The zero reference in the CFD system is a HEPA filter 
that can be selected with a valve to remove all particles from the air
sample.  This filter was used many times for brief periods in all the
flights to estimate what the zero response was when the air contained
no particles.  The position of the valve ("Filter" or "Sample") was
automatically monitored by the CFD data system.  The filtered air could
be routed to the CFD or to the CN counter with another valve.

  g. PARTICLE DILUTER FOR CN MEASUREMENTS
    CN number concentrations were calculated by counting the number of 
pulses from the CN counter and dividing by the air mass flow rate.  The 
counter chip in the CFD data system is a 16-bit counter (0-65535).  It was 
interrogated approximately 5 times per second, giving the CN concentration 
in a volume of ~3.3cm^3.  A particle diluter was used upstream of the CN 
counter to reduce the CN concentration for two reasons.  First, the counter 
can roll over for CN >~20,000/cm^3, producing ambiguity in the derived 
concentration.  Second, coincidence errors become significant in the TSI 
3076 for concentrations >10,000/cm^3.  The particle diluter was a cartridge 
filter punctured through the center with a small brass capillary tube.  Most 
of the air flow (75%) went through the filter media, where particles were 
trapped.  The remaining 25% of the flow, along with the particles, went 
through the capillary tube.  The dilution ratio, 4.0, was measured before, 
during and after the project.  CN data in the archive files are corrected 
for this dilution. 

  h. RESPONSE TIME CORRECTION 
    The ambient air was sampled with a 0.5 inch forward facing orifice.  
This expanded to 1 inch metal tubing which was brought into the C-130 cabin,
expanded again to 2 inch diameter, and then was exhausted overboard.  The 
ventilation rate was ~1000 LPM to achieve approximately isokinetic sampling 
at the 0.5 inch inlet.  The residence time was <0.1s, based on the tubing 
volume and flow rate.  The CFD and CN system sampled from the 2 inch tube at 
the location of the CFD instrument rack through 0.25 inch i.d. tubing.  This 
smaller tubing was ~150cm length, including various branches and valves.  
Before and during the field project, travel times from the start of the 
small tubing to the CN counter were estimated by sampling cabin air or 
particle free air.  Travel times through the CFD were estimated in the same 
manner.  The response time was ~4 to 6 seconds.  The archive data have been 
corrected by subtracting this time.  Thus, the clock times of CFD and CN 
data correspond to the air sample entering the 0.5 inch inlet.

  i. PARTICLE LOSSES IN TUBING 
    Diffusion particle loss in the tubing, including tees and valves, was 
estimated with the CN counter by sampling cabin air with and without the 
tubing.  This correction factor was 1.40, based on several tests before and 
during the field project.  The archive CN data were corrected by multiplying 
the indicated concentration by 1.40.  The greatest diffusion losses probably 
occur for particles ~10 to 100nm in size, which are also usually the most 
abundant.  The factor 1.40 is only a first order empirical correction.  A 
more accurate estimate could include particle size, pressure, temperature, 
electrical charge, and other effects.  For the ice nucleating particles, 
however, no information is available on their size, and no corrections were 
applied to create the archive data.

  j. DATA SYSTEM CLOCK CORRECTION 
    The C-130 aircraft data system time was distributed through a real-time 
RS-232 data stream.  At the start of each flight, the CFD data system clock 
was set 2 seconds ahead of the RS-232 data stream time, based on advice from 
NCAR that the serial data were delayed about 2 seconds.  A more nearly 
correct "real time" clock appeared on the aircraft video camera displays, 
and several times during each flight, the CFD clock time was compared 
visually with that display.  Differences were entered into the CFD log book. 
There were systematic differences between the aircraft and CFD clocks.  The 
CFD gained ~10 s during a 10 hour flight.  This clock difference was 
corrected by software that generated the CFD archive files. 

II. DATA REDUCTION 

    Calibrated data were recorded as binary records on the CFD data system.  
The data reduction process involves unpacking these records, calculating 
derived parameters, applying correction factors and writing the results to a 
sequential ASCII text file.  Over a period of time, different versions of  
data logging and unpacking software have been developed.  The unpacking 
software checks the consistency of these programs.  Plots are then created 
and reviewed by an analyst.  By looking at these and referring to the log 
book notes, the analyst selects time periods when the data are of acceptable 
quality.  Time periods with filtered air samples are automatically excluded 
from the archive data.  

    The multichannel analyzer data contain high resolution measurements of 
particle size at the outlet of the CFD.  These are recorded at 10s intervals 
and can be accessed for additional studies.  The MCA data files are separate 
from the continuous CFD records, although they have a common time stamp. 

    Data files were re-processed in June 1999 to subtract the background or 
blank count.  The "blank" response is the average count obtained when
sampling filtered air.  It can be a fractional value such as 0.25.  During
data reduction, this blank value is updated whenever a filtered air sample
is obtained.  The blank value is subtracted from the total integer count
of subsequent data, and the net value is rounded off and listed in the
data files.  Thus, if this net value is less than 0.5, zero is reported.

    For calculating the average IN concentration, ten second accumulations 
were made, using values before any rounding is done.  The concentration 
is the total net counts divided by the total sample volume.  Due to the 
rounding, some of the numbers may appear inconsistent.  For example, 
during a ten second period, there could be net 0.25 counts each second, 
and this would be reported as ten integer zeroes.  The concentration,
however, would be based on 2.5 counts, and when it is divided by the
typical 10s sample volume, 0.16 liters, the average concentration is
15.63 per liter.

    Another caveat concerns the raw IN count.  The value in the data 
archives is based on particles (crystals) that produce an analog voltage 
pulse larger than the fixed threshold.  This thresold corresponds to 3um 
diameter when the total flow rate is 10LPM.  However, the pulse height 
varies with flow rate, hence the threshold "size" also varies with flow 
rate.  There is a way to correct for this dependency, by deriving the 
crystal count from the MCA pulse height distribution data, combined with 
flow rate data.  This correction has not been done yet. 

III. KNOWN PROBLEMS

    Several problems with the instrumentation were encountered during the 
field season.  First, a leak was discovered in the CSU CN counter, as 
indicated by its failure to show zero concentration when a HEPA filter was 
placed on the inlet tubing.  The amount of leak according to the pressure 
difference between the aircraft cabin and the CN counter, which was 
approximately at ambient pressure.  Thus, the leak was worse at high 
altitudes.  At low altitudes, less than about 500m, the leak was not 
observed, and the CN counter would zero properly on filtered air.  In the 
CSU archive data set, CN values are reported throughout all flights.  Data 
users should be aware that the CSU CN data are probably okay at low 
altitudes but not at high altitudes. 

    The CFD data system hard disk failed on three flights, May 4, 11 and 18.  
When this occurred, the data logging program halted.  When the program was 
restarted, the hard disk usually operated okay, although there were 
sometimes additional failures during the same flight.  On one occasion, the 
hard disk did not recover.  A new disk was installed on May 9, but it also 
failed, and we surmised the vibration levels in the instrumentation rack 
were the cause.  These failures occurred during ferry flight periods enroute 
from Fairbanks to the SHEBA ice camp.  Gaps in the data could be as long as 
several hours.  This problem did not cause us to lose all data because we 
also had a backup data logging procedure that used diskettes, and it did not 
suffer from the vibration failure problem.  Overall, some data were lost on 
three flights enroute to the SHEBA ice camp, but no data were lost in the 
vicinity of the ice camp.