Water quality

1. DISSOLVED OXYGEN
By -Praful Hanmante

2. Contents
• Basics about Dissolved oxygen (DO)
• Different Measurement technologies
- Electrochemical method
- Optical method
• Factors affecting DO measurements
- Temperature
- Salinity
- Barometric Pressure
• Calibration techniques
• Inferences

3. Dissolved Oxygen
• Oxygen does not react with water.
• DO refers to the level of free, non-compound oxygen
present in water.
• Units : PPM, mg/L, %saturation

4. Electrochemical method
It has three main parts
1. Electrodes
2. Semipermeable
membrane
3. Electrolyte

5. polarographic sensor

6. galvanic sensor

7. • Membrane – Teflon, polyethylene
• Electrochemical sensor consumes oxygen so require
constant stirring.
• Polarographic sensor requires warm time.
• Galvanic sensor has shorter life.

8. Optical method
• DO quenches both the lifetime and intensity of the
luminescence of particular chemical dyes.
• the lifetime and intensity are inversely proportional to the
amount of oxygen present.
• Lifetime has better long term stability over intensity

9. BLUE LED
Dye molecules
excites and
emits radiation
Photodiode
detects

10. • Sensing element
- Polystyrene dye layer
Permeable membrane
- Acrylic Based paint

11. • No stirring required
• Not affected by other gases
• Higher power consumption
• Less maintaince

12. Flow dependence
electrochemical Optical

13. Temperature effect
• Temperature affects DO measurements in two ways.
1. Diffusion through membrane
2. The ability of water to dissolve oxygen
• Warmer water cannot dissolve as much oxygen as colder
water.

14. Salinity
• As the salinity of water
increases, its ability to
dissolve oxygen
decreases.
• Auto calibration using
conductivity
measurement

15. Barometric pressure
• As the barometric
pressure decreases,
solubility of oxygen
decreases
• Auto compensation by
measurement of
pressure

16. 3 methods of calibration
• Winkler titration
• - laboratory method
• - cumbersome and time consuming
• - highly subjective to human error
• Air-saturated water
• - aerate the water for 1hour
• - requires preparation time
• - no assurance of complete air saturation
•

17. Water saturated air
• Easy process
• Place sensor in moist
air
• Temperature and
pressure
compensation

18. Two point calibration
• one point at zero and the
other at full saturation.
• A zero oxygen solution
• -sodium sulfite
• -active dry yeast
-Nitrogen gas
• Water saturated air

19. Possible Interferences
• Oxygen consuming or producing bacteria
• Heavy residue such as oils
• Alcohol and organic solvents