YSI 6000UPG - Manual (Page 107)

6-7
METHOD OF OPERATION
Standard Clark dissolved oxygen sensors which are marketed by YSI and other manufacturers are
continuously polarized at a voltage sufficiently negative to cause oxygen (which diffuses through
the Teflon membrane) to be reduced to hydroxide ion at the cathode and silver metal to be oxidized
to silver chloride at the anode. The current associated with this process is proportional to the
oxygen present in the solution outside the membrane. However, as this electrochemical reaction
proceeds, oxygen is consumed (or depleted) in the medium, resulting in a decrease in measured
current (and apparent oxygen content) if the external solution is not stirred rapidly. To minimize
this oxygen depletion, the probe electrodes in the YSI Rapid Pulse system are rapidly and
reproducibly polarized (on) and depolarized (off) during a measurement sequence. The Rapid Pulse
system thus measures the charge or coulombs (current summed over a specific time period)
associated with the reduction of oxygen during a carefully controlled time interval. The coulombs
due to charging of the cathode (capacitance), but not to reduction of oxygen, are subtracted during
integration after the cathode has been turned off. The net charge, like the steady state current in a
standard system, is proportional to the oxygen partial pressure in the medium. Because oxygen is
only being reduced 1/100th of the total measurement time, even if the probe is pulsed in this manner
continuously, oxygen consumption outside the membrane is kept to a minimum, and the stirring
dependence of the system is greatly reduced.
One key to the practicality of Rapid Pulse oxygen system is the fact that the “on time” is very short.
This allows the “off time” to also be relatively short and still maintain the off to on ratio of 100
which is necessary to obtain relatively flow independent measurements. The second important
aspect of the Rapid Pulse technology is the integration (summing of the current) over the total pulse
(on and off). Because the charging current of the electrodes is subtracted in this process, the net
signal is due only to the reduction of oxygen. From a practical point of view, this means that when
there is zero oxygen partial pressure outside the membrane, the Rapid Pulse signal will also be zero;
this in turn allows the system to be calibrated with a single medium (air or water) of known oxygen
pressure.
CALIBRATION AND EFFECT OF TEMPERATURE
The 6000
UPG
Rapid Pulse system is calibrated using the same basic methods employed for steady
state oxygen sensors.
The two general calibration methods possible with the 6000 UPG are “DO mg/l” and “DO %”. The
former method is designed for calibration in solution while the latter utilizes water-saturated air as
the medium. Since the percent saturation (DO %) and concentration (DO mg/l) values are related
by well-known algorithms, calibration by either method results in correct outputs in both units.
If the mg/L method is selected from the 6000
UPG
Calibrate menu, the oxygen concentration of an
aqueous solution may be determined by several methods:
1.) Winkler titration
2.) Aerating the solution and assuming that it is saturated, or
3.) Measurement with another instrument.