Need help to adapt Radiometer ABL510 to measure freshwater samples

[Chow Poh Soon]

Help!

We have been trying for weeks to extend the use of Radiometer ABL510 blood 
gas analyzer to measure pO2 in freshwater samples. The instrument reports 
much higher values than possible for pO2. For example, it reads 180 to 
190 mm Hg for 100 % air-saturated water whereas true values should fall 
between 140 to 150 mm Hg at 37°C. According to the instrument's reference 
manual, a correction factor is used to adjust the differences in measured 
values in gas and liquid samples. I quote the pertinent reference section 
on pO2 measurements:
      ---------------------------------------------------------
pO2 value measured in a sample is calculated from the electrode current, 
I_meas, using the following equation:

   pO2(sample)_meas = {[I_meas - I'_0(t)]/S'} * Gas/liquid ratio

where
I'_0(t) = zero current value, measured during last zero calibration at 
          the same moment of time as the end of the actual measurement on 
          the liquid sample. I'_0(t) = I'_0.
S'      = sensitivity of the pO2 electrode calculated during the last 
          calibration on the pO2(H) gas, I'_0(t)

Gas/liquid ratio describes the ratio between pO2 values measured on gas 
and on liquid due to the fact that the measured pO2 values in gas are 
higher than in the liquid.

For liquid samples:
Gas/liquid ratio = 1 + 1/100 * [-5.837 + sqrt(21.712 + S'/0.48835)]

For gas samples:
Gas/liquid ratio = 1
     ------------------------------------------------------------
Note the coefficients used in calculating the gas/liquid ratio for liquid 
samples. We believed that it's specific to human blood. The company which 
sold us the equipment could not supply us the neccessary information to 
remedy this problem. Could anyone whose has used and adapt this gas 
analyzer for freshwater applications offer some advice? Any form of help 
will be greatly appreciated. Many thanks for responding.
You can contact me at <ZOOCPS at LEONIS.NUS.SG>

Regards,

P.S. Chow
18 July 1995