Temperature Sensor
The temperature sensor consists of a pt 100 platinum element, housed in a stainless steel sheath
for robustness and corrosion resistance. The temperature sensor requires little maintenance,
however the temperature measurement is used for calculating the dissolved oxygen in mg/l and for
temperature correction of the conductivity sensor, so it is important that the temperature sensor is
properly calibrated.
Dissolved Oxygen Sensor
Dissolved Oxygen is measured using an active type membrane covered sensor. The sensor itself
consists of silver and lead electrodes and a 25um PTFE membrane and is filled with a 1.0M
potassium hydroxide. A constant flow of water passes the sensor, maintained by a stirrer located
on the bottom section of the probe.
When the silver and lead electrodes are connected through the external circuit, electrons pass from
the lead electrode to the silver electrode. When oxygen is present at the surface of the silver
electrode, it reacts with electrons to produce hydroxyl ions.
At the lead electrode the loss of electrons produces lead ions. The lead's electrons combine with
hydroxyl to precipitate lead hydroxide on the lead electrode.
The rate of transference of electrons via the external circuit from the lead to silver electrode ie that
is the current flowing in the external circuit, is the measure of the rate of cell reaction and thus the
rate at which oxygen reaches the silver electrode. The current flowing in the external circuit is
directly related to the oxygen concentration in the sample being measured by the electrodes.
The Dissolved Oxygen sensor may periodically require a new membrane and electrolyte. A unique
knurled nut is used to hold the sensor membrane in position without overstressing the membrane.
This gives long term stability and allows easy replacement. The sensor can be removed from the
probe for servicing. A replacement probe is ready for use immediately after installation and
calibration.
Conductivity Sensor
The conductivity is measured using a 4 electrode bridge. The four electrode system uses automatic
compensation to overcome any build up of contamination on the electrodes. The electrodes are
made from fine platinum and are coated with platinum black to enhance the long term stability and
sensitivity of the sensor. The coating should last for a long period of time if it is not mechanically
removed, however, the coating can be replaced by returning the sensor to YEO‐KAL Electronics Pty
Ltd.
Turbidity Sensor
The turbidity sensor is located in the hole which runs through the bottom section of the probe and
is lined with a glass tube. Turbidity is measured by the nephelometric method which uses a light
source and a detector measuring the light scattered at 90 degrees to the incident light beam. A
pulsed infra‐red light source is used. A Turbidity screen has also been installed which has apertures
to reduce bounce off the glass wall and remove refraction from samples high density (sea water).