Aanderaa 4330 Operating Manual page 77

for salinity and pressure changes, to calculate saturation levels and to convert between different oxygen units.
MR 26
Q: What is influencing the O concentration in water?
2
A: In the laboratory how much oxygen that can be dissolved in the water is dependent on the salinity and temperature of the water and on the air pressure in
the room.
If a glass of sterile water is left in a room with constant temperature and constant air-pressure, oxygen in the air will dissolve in the water according to the
common gas law. After some time saturation equilibrium will be reached where no more oxygen can be dissolved in the water. If the water is stirred it will
reach saturation faster. In reality it is difficult to reach equilibrium since temperature and air pressures do not stay constant.
When measuring in natural surface waters which are in contact with the atmosphere the following factors can influence the dissolved oxygen concentrations:
1. Temperature: when water is cooling off it becomes under-saturated and can take up more oxygen from the atmosphere, when it heats up it becomes
oversaturated and releases oxygen. Efficient exchange between water and air takes place when there are waves. 2. Salinity: water with higher salinity can
dissolve less oxygen. 3. Primary production: when e.g. phytoplankton and sea grass grow in the photic zone (where there is light) oxygen will be produced,
this can lead to oversaturation. 4. Consumption/respiration: when there is no light phytoplankton consume oxygen and so do animals (e.g. zooplankton and
fish) living in the water also when organic material is degraded by bacteria oxygen is consumed. 5. Waves: if waves are breaking they will entrain bubbles to
deeper levels which dissolve and create higher oxygen concentrations.
When moving deeper, out of the zone where there is light and waves, oxygen can only be consumed and no oversaturation should be expected. In deeper
waters oxygen changes are mainly related to water movements where water coming from below in most cases contains less oxygen. At the bottom, where
organic material accumulates, oxygen consumption is the highest leading to sharper oxygen gradients when approaching the bottom.MR 27
Q: Does the sensor react to changes in salinity?
A: No, The sensor is measuring partial pressure and does not react to changes in salinity.
This can be verified by having two glasses of air-bubbled water, at the same temperature, next to each other.
One filled with freshwater (0 ppt) and the other with saltwater (e.g. 35 ppt).
When moving the sensor from one glass to the other it should read the same absolute oxygen concentration, in µm, even though the absolute oxygen
solubility in the salt water is lower.
MR 28
Q: Does the % saturation level change with the salinity compensation?
A: No, the % saturation level should be the same.
page 77 of 93
TD 269 OPERATING MANUAL
OXYGEN OPTODE 4330, 4831, 4835
June 2017