Dissolved Oxygen - Xylem EXO Series User Manual

Dissolved Oxygen

2.9
1.7
Sensor Overview
The principle of operation of the EXO optical Dissolved Oxygen sensor is based on the well-documented
concept that dissolved oxygen quenches both the intensity and the lifetime of the luminescence associated
with a carefully chosen chemical dye. The EXO DO sensor operates by shining a blue light of the proper
wavelength on this luminescent dye which is immobilized in a matrix and formed into a disk. The blue
light causes the immobilized dye to luminesce and the lifetime of this dye luminescence is measured via a
photodiode in the probe. To increase the accuracy and stability of the technique, the dye is also irradiated with
red light during part of the measurement cycle to act as a reference in the determination of the luminescence
599100,
599110
Sensor Cap
Sensor without
Sensor Cap
lifetime.
When there is no oxygen present, the lifetime of the signal
is maximal; as oxygen is introduced to the membrane
surface of the sensor, the lifetime becomes shorter. Thus,
the lifetime of the luminescence is inversely proportional
to the amount of oxygen present and the relationship
between the oxygen pressure outside the sensor and the
lifetime can be quantified by the Stern-Volmer equation.
For most lifetime-based optical DO sensors, this Stern-
Volmer relationship
((Tzero/T) – 1) versus O
is not strictly linear (particularly at higher oxygen
pressures) and the data must be processed using analysis by
Specifications
Units % Saturation, mg/L
Temperature
-5 to + 50°C
Operating
-20 to + 80°C
Storage
0 to 500% air sat.
Range
0 to 50 mg/L
0-200%: ±1% reading or 1%
air sat., whichever is greater;
200-500%: ±5% reading
Accuracy
0-20 mg/L: ±1% of reading or
0.1 mg/L;
20-50 mg/L: ±5% reading
T63<5 sec
Response
0.1% air sat.
Resolution
0.01 mg/L
Sensor Type Optical, luminescence lifetime
pressure
2
(continued)
(see pg 12)
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17
pg