function conc = ghobsconv(counts, SN)
% GHOBSCONV- Convert Grays Harbor 2001 OBS data from Counts to g/L 
% using calibration equations specific to each OBS.  
% This includes the datalogger specific counts to volts conversion.
%
% Input: counts - OBS data in counts
%        SN - Serial number of the OBS which collected the data 
%               - must be a S/N used in the Grays Harbor 2001 experiment
%
% Returns conc = -999 for invalid serial numbers
% Returns a value of zero for all negative concentrations
% 
% Laura Landerman, USGS
% December 30, 2002


% Load calibration equations - units for columns are: S/N counts/V counts v/(g/L) volts
EQ = [795    12820 88.553 0.36822 0.04487;  % ND
      796    12820 88.553 0.36822 0.04487;  % ND
      1242   12819 14.257 0.33966 0.01337;  % MD
      1135   12819 14.257 0.36354 0.03607;  % MD
      928    12824 25.898 0.34059 0.06665;  % MS
      1104   12824 25.898 0.35955 0.05486;  % MS
      829    12796 40.452 0.34196 0.02640;  % 231 - MIA 1
      830    12796 40.452 0.34646 0.03119;  % 231 - MIA 2
      924    12834 130.95 0.24210 0.07163;  % 244 - MIA 1
      925    12834 130.95 0.24210 0.07163;  % 244 - MIA 2
      694    12818 8.4416 0.33370 0.05367;  % H40 - MIA 
      794    12818 8.4416 0.34618 0.03274;  % H40 - MIA
      1244   12827 30.806 0.28366 0.07790;  % MIB
      1429   12827 30.806 0.28366 0.07790;  % MIB
      1243   12830 29.489 0.31320 0.03009;  % SD
      1428   12830 29.489 0.32184 0.03978]; % SD
      
[i,j] = find(EQ(:,1)== SN);                 % Find which row contains the necessary calibration

if isempty(i) conc = -999;                  % Return value of -999 if Serial Number can not be found
else
    volts = (counts - EQ(i,3))./EQ(i,2);     % Convert from counts to volts
    conc = (volts - EQ(i,5))./EQ(i,4);       % Convert from volts to g/L
end
conc(find(conc<0))=0;                       % Replace negative value with zero