TD 269 OPERATING MANUAL
OXYGEN OPTODE 4330, 4831, 4835
June 2017
page 69 of 93
Be careful with using compressed air or compressor/vacuum type pumps since these are likely to compress the air/oxygen which will give errors when
calibrating.
Normally the sensor will under-read after such a calibration.
A similar situation will occur if the sensor is calibrated in a “deeper” water tank.
If the air bubbling and the sensor are placed at for example 1 m water depth the over pressure will be approximately 10%.
CCAP10
Q: When calibrating which type of vials/containers should be used?
A: It is preferable to use clean glass vials, instead of plastic, for calibrations and any types of experiments.
There have been examples in which oxygen has either been consumed by substances bound into the plastic container walls or oxygen has diffused through
the walls from the outside.
Glass is preferable for basically all applications that are dealing with dissolved gases.
CCAP11
Q: When sampling the sensors at high frequencies (1-10 s intervals) there appears to be some self-heating of the sensor.
What can be done to minimize the effects of the self-heating and how big is the effect of it?
A: The sensor has linear power regulators which mean that if you supply it with higher voltage (e.g. 8-14V) it will still consume the same amount of Amperes
as at 5V.
The additional energy at higher voltages will be lost as heating which will contribute to the self-heating.
Therefore it is better to supply the sensor with 5V in high sampling frequency applications.
Laboratory testing at 5V has revealed that self-heating of the sensor can introduce a 1µM (giving lower readings than correct) when sampled at a 1 second
sample-interval.
This error drops to 0.2 µM for a 5 second interval. The error of the internal temperature sensor at a 5 s sampling interval is approximately 0.03ºC. At a 1 s
sampling interval it is approximately 0.1ºC. Care should be taken when using the sensor in on-line system applications (e.g. in a ferry box system).
The internal temperature sensor is placed in the “foot” of the sensor (except for the 4330 and 4831 Optodes on which the temperature sensor has been
moved close to the foil). If mounting the sensor in the wall of an on-line system that has high thermal conductivity (e.g. metal walls) with the outside this
might give significant effects on the Optode temperature sensor, which also will lead to errors in the oxygen readings since these temperature readings are
used for the necessary temperature compensation.
CCAP 12
Q. Is there a difference in the sensor response if the foil is wet or dry?
A. Yes the sensor is and should be calibrated in a wet environment and it takes hours for the foil to become completely wet or dry.
Taking a sensor which has been sitting in a dry environment for several hours and introducing it into water to make a spot measurement can lead to an error
of maximum 2%.
Keeping the sensor in a humid environment for at least 12 h will eliminate this error.
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