Vents 1997 Leg 1 Readme File

Ship:			NOAA research vessel Ronald H. Brown
Cruise Start:		San Diego, CA, September 12, 1997
Cruise End:		Newport, OR, September 16, 1997
Chief Scientist:	Christopher Fox
System Operator:	Marilyn Roberts

Method:
Infrared absorption of dried gas.  For details of the system see: 
Measurement of fugacity of Carbon Dioxide in surface water and air using 
continuous sampling methods.  Wanninkhof and Thoning, 1993 in Marine Chemistry
44, 189-205, 
And:
Feely, R.A., R. Wanninkhof, H.B. Milburn, C.E. Cosca, M. Stapp, and P.P. Murphy, 
A new automated underway system for making high precision pCO2 measurements 
onboard research ships, Analytica Chim. Acta, 377, 185-191, 1998.

The three standard gases come from CMDL in Boulder and are directly traceable to 
the WMO scale.

Sampling Cycle:
The system runs on an hourly cycle during which 3 standard gases, 3 air 
samples from the bow tower and 8 surface water samples (from the 
equilibrator head space) are analyzed on the following schedule:

Mins. after hour		Sample
4				Low Standard
8				Mid Standard
12				High Standard
16.5				Water
21				Water
25.5				Water
30				Water
34				Air
38				Air
42				Air
46.5				Water
51				Water
55.5				Water
60				Water

Units:
All xCO2 values are reported in parts per million (ppm) and fCO2 values 
are reported in microatmospheres (uatm) assuming 100 % humidity at the 
equilibrator temperature.


Notes for Leg 1:
1. There were problems with valve mistrips that occurred ~ 1.5% of the 
time, causing the wrong gas to be routed through the Licor infrared 
analyzer.  If these occurred during water or air sampling phases, 
they were removed.  When they occurred during standard gas phases, 
the Licor voltage readings of the appropriate standard gas phase 
before and after the bad reading were averaged to produce an 
interpolated reading.
2. Data from the ship's computer system (SST, salinity, wind speed and 
direction) were merged into the system's data file with an 
approximate 3 minute delay factor.  This is to account for the time 
it takes for seawater to travel from the bow intake to the 
equilibrator in the Hydro Lab.
3. The equation used to convert the equilibrator thermistor voltage to 
temperature was incorrect, resulting in values that were 
approximately 0.9 degrees too high.  In addition, relay closures 
affected the voltage readings from the thermistor.  To obtain 
correct equilibrator temperature readings, we assumed that the YSI 
TSG temperature was the same as the equilibrator temperature since 
both sample seawater from the same source in the Hydro Lab.  
However, the YSI TSG temperature readings are known from 
calibration data to be about 0.2 degrees high.  We therefore 
determined a corrected YSI temperature by using calibration data to 
fit a linear equation relating observed YSI temperature to actual 
temperature as given by a Guildline platinum resistance thermometer 
(New YSI T = 1.0026885 * Old YSI T - 0.272854). A new equation 
relating thermistor voltage to the corrected YSI T was then 
determined using all the data from leg 2 of the PACS 1997 cruise (T 
= 6.0955*V^3 -46.862*V^2 + 101.16V - 28.638.
4. Misalignment of the Valco valve resulted in low gas flows in the 
Standard 3, Standard 1, and air phases.  This problem was 
especially bad during the last 2 days of the leg.  To correct them, 
I computed the average Licor voltage in both the Standard 1 and 
Standard 3 phases when gas flow exceeded 30 ml/min.  Whenever 
possible, I interpolated bad values in these phases using the 
previous and next good values in the appropriate phase.  However, 
when more than 2 successive values were bad, I replaced the bad 
values with the computed average value.  In several air phases, I 
deleted the first air value of the group due to incomplete flushing 
of the Licor sample cell.  

For questions or comments contact:
Bob Castle
4301 Rickenbacker Causeway
Miami, FL 33149
305-361-4418
castle@aoml.noaa.gov