Pyro Workbench

Definition of Oxygen Units

The oxygen unit can be selected for each channel by the selector Units. The selectable units include % air saturation, % O2, mL/L, µmol/L, mg/L (ppm), hPa (mbar), mmHg (Torr), µg/L (ppb) and dphi.

For measurements in gas phase only the units raw value, % O2, hPa (mbar), mmHg (Torr) and dphi can be selected, whereas for measurements of dissolved oxygen (DO) in a water sample all units except % O2 can be selected. The detailed definition of each unit is given below:

Phase shift: dphi
The phase shift dphi is the fundamental unit measured by the optoelectronics. Please note that dphi  is not at all linearly dependent on the oxygen units, and increasing oxygen levels correspond to decreasing dphi values and vice versa. As a rule of thumb, anoxic conditions will result in a dphi = 53°, whereby ambient air will result in dphi = 20°.

Partial pressure pO2: hPa = mbar
For a calibrated sensor, the partial oxygen pressure pO2 in units of hPa (equivalent to mbar) is the fundamental oxygen unit measured by the device.   
Used in: Gas and water phase

Partial pressure pO2: Torr
Definition: pO2[Torr] = pO2[hPa]  x  759.96 / 1013.25               
Used in: Gas or water phase

Volume percent pV: %O2
Definition: pv = pO2[hPa]  /  patm  x  100%      
Used in: Gas
with patm: Actual barometric pressure

% air saturation A: % a.s.
Definition: A[%a.s.] = 100%  x  pO2 / p100O2 
Used in: Water phase

p100O2 = 0.2095  ( patm –  pH2O(T)  )          
pH2O(T) =  6.112mbar  x  exp ( 17.62  T[°C] / (243.12 + T[°C]))      
pO2: Actual partial pressure        
patm: Actual barometric pressure             
T: Actual temperature  
pH2O(T): Saturated water vapor pressure at temperature T

Dissolved O2 concentration C: µmol/L
Definition: C [µmol/L] = A[%a.s.] / 100% x C100(T,P,S)            
Used in: water phase      
C100(T,P,S): interpolation formula for dissolved oxygen concentration in units of µmol/L at temperature T, atmospheric pressure P and Salinity S.

Dissolved O2 concentration C: mg/L = ppm
Definition: C [mg/L] = C [µmol/L]  x  32 / 1000          
Used in: Water phase

Dissolved O2 concentration C: mL/L
Definition: C [mL/L] = C [µmol/L]  x  0.02241             
Used in: Water phase
Salinity (g/L)
The Salinity (g/L) of the environmental sample is only relevant if a concentration unit for dissolved oxygen measurements is selected (e.g. mg/L or µmol/L). Please select the right medium for your application (salt water, freshwater, physiological). If you do know your exact NaCl concentration, please choose “Custom” and enter the respective exact value. Note: Only if NaCl is the main solved component, the software can calculate the correct oxygen calibration concentration. If NaCl is not the main solved component in your medium, select a partial pressure unit (e.g. hPa) for oxygen and continue. Only after the experiment you can perform the calculation into concentration units externally (for this you need the “Henry coefficient” for your individual medium).

Theoretical Background
The sample salinity needs to be considered, e.g. in case of saline water, because the solved components can strongly affect the oxygen solubility and consequently the oxygen concentration. The oxygen partial pressure is not affected by this. Please see the following table to observe the magnitude of this effect. This table shows the equilibrium oxygen concentration C100(T, P = 1013mbar, S) in units of µmol/L at standard atmospheric pressure of 1013 mbar as a function of water temperature in units of  °C and salinity in units of PSU (“practical salinity unit” ≈ g/L).


Garcia, HE and Gordon, LI (1992)
Oxygen solubility in seawater: Better fitting equations.
Limnol. Oceanogr. 37: 1307-1312

Millero, FJ and Poisson, A (1981)
International one-atmosphere equation of state of seawater.
Deep Sea Res. 28A: 625-629