4) Trim any excess membrane material with a pair of scissors.
5) Inspect the membrane for air bubbles or wrinkles. If any exist, go back to step one.
6) Do not leave it exposed to air for long periods. Place a few teaspoons of water in the
storage cup and secure to the sonde.
It is best to let a freshly prepared DO sensor age for 24 hours before it is calibrated, as the
sensitivity of the sensor decreases by two or three percent or more during the first 24 hours, with
most of the decrease occurring during the first 12 hours. This is caused by a reduction in
membrane permeability, due partly to a relaxation of stresses in the membrane and partly to the
response of the membrane material to exposure to water. After about 24 hours, the sensor
becomes stable.
It is perfectly all right to use a freshly prepared probe immediately, however the probe should be
calibrated once after allowing the membrane to relax for approximately four (4) hours..
Here are the steps to water-saturated air calibration:
1) Make sure your circulator is turned off.
2) Fill your calibration cup up to the level of the DO membrane with tap water, DI water, your
conductivity standard, or a pH standard.
3) With a paper towel, make certain the membrane is dry and free of water droplets.
4) Place the cal cup cover upside down over the calibration cup.
5) Make sure your instrument’s Barometric Pressure setting is accurate.
6) Wait a few minutes for the air to become fully saturated and the temperature to
equilibrate.
7) Follow the Manta 2 Control Software’s calibration instructions.
A number of customers are moving away from the time-honored water-saturated air calibration to
the air-saturated water calibration. Here are the steps to water-saturated air calibration:
1) Put a half-liter of tap water in a liter jar and shake the jar vigorously for one minute. Take
the lid off the jar (assuming you put one on in the first place) and let the water stand for
about five minutes to let the air bubbles float out.
2) Make sure your circulator is turned off.
3) Fill your calibration cup until your aerated water covers the DO membrane by a
centimeter or so.
4) Make sure your instrument’s Barometric Pressure setting is accurate.
5) Wait a few minutes for the temperature to equilibrate.
6) Follow the Manta 2 Control Software calibration instructions.
E7 Dissolved Oxygen, Optical Sensor
The optical dissolved-oxygen sensor comprises a blue light source, a sensing surface, and a red
light receiver. The sensing surface is an oxygen-active compound stabilized in an oxygen-
permeable polymer, usually silicone. The oxygen-active compound fluoresces – that is, it
absorbs energy in the form of blue light and then emits energy as red light. The red-light receiver
measures the amount of red light emitted as a result of the blue light’s energy. However, this
fluorescence is quenched by oxygen – that is, the emission of red light is reduced if oxygen
molecules are present to interfere with the oxygen-active compound.
As the oxygen presence grows, the red light emitted falls. When the sensing surface is exposed
to water (or air, for that matter), oxygen diffuses into the sensing surface according to the amount
(partial pressure) of oxygen in the water. Thus, the amount of red light received by the sensor is
directly relatable to the amount of oxygen in the water.
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