KFDO310 integrated on-line fluorescence dissolved oxygen sensor is
designed and made based on the quenching principle of excited
fluorescence of specific substances in physics. The blue light from
the light-emitting diode illuminates the fluorescent material on
the inner surface of the fluorescent cap. The fluorescent material
on the inner surface is excited and emits red light. By detecting
the phase difference between the red light and the blue light, and
comparing it with the internal calibration value, the concentration
of oxygen molecule can be calculated, and the final value can be
output by temperature automatic compensation.
No electrolytes, no polarization
No need to consume oxygen, not affected by the flow rate
Built-in temperature sensor, automatic temperature compensation
Free from chemicals like sulfides
Small drift, fast response, more accurate measurement
Maintenance-free, long service life, lower use cost
Fluorescent caps are easy to replace
RS-485 interface, Modbus-RTU protocol
| Model number | KFDO310 |
| Measuring principle | fluorescence |
| Range | 0ー20 mg/L (0ー200% saturation, 25 °C) |
| Resolution | 0.01 mg/l, 0.1 °C |
| Precision | ± 2% f.s. , ± 0.5 °C |
| Temperature compensation | Automatic temperature compensation (PT1000) |
| Output mode | RS-485 bus, Modbus-RTU protocol |
| Working conditions | 0ー45 °C, < 0.2 mpa |
| Storage temperature | - 5 ~ 65 °C |
| Installation mode | Immersion mounting |
| Cable length | 5 meters, other length can be customized |
| Power consumption | < 0.05 W |
| Power supply | 12 ~ 24 VDC ± 10% |
| Protection level | IP68 |
| Calibration | Two-point calibration |
| Fluorescent cap life | Guaranteed Use for one year (under normal use) |
| Material for sensor housing | Pom and 316L stainless steel |
The temperature sensing part should be immersed below the liquid
surface to avoid collision with the film head surface. The head
part of the membrane should be free from sediment.
How Can Water be More than 100% Saturated?
Henry’s law determining the dissolved oxygen concentration at 20
degrees C and 100% air saturation (1 kg water = 1 L water)
100% air saturation is the equilibrium point for gases in water.
This is because gas molecules diffuse between the atmosphere and
the water’s surface. According to Henry’s Law, the dissolved oxygen
content of water is proportional to the percent of oxygen (partial
pressure) in the air above it 13. As oxygen in the atmosphere is about 20.3%, the partial pressure
of oxygen at sea level (1 atm) is 0.203 atm. Thus the amount of
dissolved oxygen at 100% saturation at sea level at 20° C is 9.03
mg/L ¹⁰.
The equation shows that water will remain at 100% air saturation at
equilibrium. However, there are several factors that can affect
this. Aquatic respiration and decomposition lower DO
concentrations, while rapid aeration and photosynthesis can
contribute to supersaturation. During the process of
photosynthesis, oxygen is produced as a waste product. This adds to
the dissolved oxygen concentration in the water, potentially
bringing it above 100% saturation ¹⁴. In addition, the equalization
of water is a slow process (except in highly agitated or aerated
situations). This means that dissolved oxygen levels can easily be
more than 100% air saturation during the day in photosynthetically
active bodies of water ¹⁴.
Dissolved oxygen often reaches over 100% air saturation due to
photosynthesis activity during
the day
Supersaturation of water can be caused by rapid aeration from a
dam.
Supersaturation caused by rapid aeration is often seen beside
hydro-power dams and large waterfalls. Unlike small rapids and
waves, the water flowing over a dam or waterfall traps and carries
air with it, which is then plunged into the water. At greater
depths and thus greater hydrostatic pressures, this entrained air
is forced into solution, potentially raising saturation levels over
100%.
Rapid temperature changes can also create DO readings greater than
100%. As water temperature rises, oxygen solubility decreases. On a
cool summer night, a lake’s temperature might be 60° F. At 100% air
saturation, this lake’s dissolved oxygen levels would be at 9.66
mg/L. When the sun rises and warms up the lake to 70° F, 100% air
saturation should equate to 8.68 mg/L DO. But if there is no wind
to move the equilibration along, the lake will still contain that
initial 9.66 mg/L DO, an air saturation of 111%.
