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Ref. No. [UMCES] CBL 2016-015
ACT VS16-06
6
the Stern-Volmer equation. For most lifetime-based optical DO sensors, this Stern-Volmer
relationship
(Tzero/T) – 1) versus O 2 pressure
is not strictly linear (particularly at higher oxygen
pressures) and the data must be processed using analysis by polynomial non-linear regression.
Fortunately, the non-linearity does not change significantly with time so that, as long as each
sensor is characterized with regard to its response to changing oxygen pressure, the curvature in the
relationship does not affect the ability of the sensor to accurately measure oxygen for an extended
period of time.
Xylem EXO2 DO sensor specifications:
Units
% Saturation mg/L
Temperature
Operating
Storage
-5to+50°C
-20to+80°C
Range
0to500% airsaturation
0to50mg/L
Accuracy
0-200%: ±1% reading or 1%air sat., whichever
is
greater; 200-500%:: ±5% reading 0-20 mg/L:
±1%
ofreadingor0.1mg/L;20-50mg/L;±5%reading
Response
T63<5sec
Resolution
0.1%airsat
0.01mg/L
SensorType
Optical,luminescencelifetime
Variables that could affect dissolved oxygen measurements include temperature, salinity,
and barometric pressure. Temperature and salinity are compensated for during instrument
calibration and field use with the use of additional sensors and/or instrument software settings.
Barometric pressure relates to the pressure of oxygen in the calibration environment, and barometric
pressure changes due to a change in altitude or local weather. Generally the effect of barometric
pressure is overcome by proper sensor calibration to a standard pressure. However, if the user
measures dissolved oxygen in something besides per cent saturation, then the EXO2 DO sensor can
store a local barometric reading put into the KOR software (DO % local) or the EXO2 handheld
can take a live barometric reading with its internal barometer (ODO % EU).
ODO % Sat = Raw DO reading corrected with temperature and local barometric pressure at the
time of calibration. (Local pressure/760 mmHg x 100 = % Sat.)
ODO % Local = Raw DO reading corrected with temperature and % Sat output fixed to 100%
regardless of barometric pressure entry. (The entered local barometric pressure is
used by KOR software for mg/L calculations.)