Xylem 6-Series Multiparameter - Manual (Page 246)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-8
Teflon membrane. 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 sonde Rapid Pulse system is calibrated using the same basic methods employed for steady state oxygen
sensors. However, the software that controls the calibration protocol is somewhat different depending on
whether the unit will be used in sampling or deployment studies. For sampling studies using either a 650
MDS display unit or a laptop computer, the Rapid Pulse system is allowed to run continuously when the
Calibration mode is activated if “Autosleep” is turned off. Under these software conditions, the user views
the DO readings in real time and confirms the calibration manually after the readings have stabilized.
For studies in which the sonde is deployed and readings are saved less frequently (5 – 60 minutes) to sonde
memory, a computer or data collection platform, an appropriate warm up time is selected for the system in
the Advanced|Sensor menu. Usually 40 seconds is adequate for this parameter, but, in some cases, larger
values may result in more accurate results. Most importantly for deployment studies, “Autosleep” should
be activated. With these software entries in place, the user will input the calibration value (concentration or
barometric pressure), and the unit will automatically calibrate after the selected warm up time.
The description below is designed around deployment applications with “Autosleep” activated.
The two general calibration methods possible with the sonde 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, calibration by either
method results in correct outputs in both units.
If the mg/L method is selected from the sonde Calibrate menu, the oxygen concentration of an aqueous
solution may be determined by several methods:

Winkler titration

Aerating the solution and assuming that it is saturated, or

Measurement with another instrument.
If this calibration method is employed, place the sonde into this known-value solution and wait 5-10 minutes
for equilibration to occur. Then input the value (in mg/L) into the sonde software and begin the calibration