In-Situ RDO PRO Process Optical Dissolved Oxygen Sensor Manual - Manual (Page 18)

6.3.1 Dissolved Oxygen Concentration Equations
DO concentration is internally calculated in mg/L. Conversion to other units is as follows:
µg/L = 1000 * mg/L
Oxygen concentration C
o
(mg/L) is calculated as:
C
o
= 31.9988
× 1E6 × (ρP
o
/k
o
M) (1
− Θ
o
)
× S
c
Where:
P
o
is the partial pressure of O
2
in atmospheres.
P
torr
= 759.999876
× P
atm
S
c
is the salinity correction:
ln S
c
= S(B
0
+ B
1
T
s
+ B
2
T
s
2
+ B
3
T
s
3
) + C
0
S
2
B
0
= -6.246090
× 10 -3
B
1
= -7.423444
× 10 -3
B
2
= -1.048635
× 10 -2
B
3
= -7.987907
× 10 -3
C
0
= -4.679983
× 10 -7
T
s
is the scaled temperature:
T
s
= ln [(298.15 – t) / (273.15 + t)]
t is temperature in degrees C.
S is the salinity in PSU.
k
o
is Henry’s constant:
ln k
o
= 3.71814 + (5596.17/T) – (1,049,668/T
2
)
T is temperature in Kelvin.
Θ
o
is the negative of the second pressure coefficient:
Θ
o
= 0.000975 – (1.426
× 10 -5
t) + (6.436
× 10 -8
t
2
)
t is temperature in degrees C.
ρ
is the density of water in g/cm 3 :
ln
ρ = -0.589581 + (326.785/T) – (45,284.1/T 2
)
T is the temperature in Kelvin.
Molar mass of water: M = 18.0152 g/mole
References:
Benson and Krause, Jr., 1980. The concentration and isotopic fractionation of gases dissolved in
freshwater in equilibrium with the atmosphere. Limnol and Oceanogr, 25(4), 662-671.
Gordon and Garcia, 1992. Oxygen Solubility in Seawater: Better Fitting Equations. Limnol and
Oceaongr, 37(6), 1307-1312.
Page 18 Modbus Registers