YSI 6000UPG - Manual (Page 108)

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If this calibration method is employed, place the sonde into this known-value solution and input the
value (in mg/l) into the 6000
UPG software after the reading has stabilized and then, after the reading
has stabilized (2-5 minutes).
If the Percent Saturation method is selected, the sonde is simply placed in a calibration cup which
contains a small quantity of water or a damp sponge. The probe sensor should not be in the water
for this calibration procedure. After time has been allowed for equilibration of the temperatures of
the thermistor and the DO probe (10-15 minutes), the barometric pressure is entered into the
6000
UPG software, and the calibration procedure is begun. Once the actual calibration protocol has
been initiated, the Rapid Pulse system will apply several warm-up pulses to the probe and then read
the last pulse as the calibration parameter. The number of warm-up pulses is determined by the
probe stability and user setup. Thus, the procedure takes a minimum of approximately 44 seconds,
depending on the warm-up time which the user has chosen. This warm-up/read sequence is used in
all Unattended Sampling determinations of dissolved oxygen using the 6000
UPG .
The DO readings of steady state oxygen systems are greatly affected by temperature (approximately
3% per degree Celsius) due to the effect of temperature on the diffusion of oxygen through the
membrane. The Rapid Pulse system exhibits a greatly reduced effect of temperature (approximately
1% per degree Celsius), but this factor still must be accounted for if DO readings acquired at
temperatures different from that at calibration are to be accurate. This compensation is
automatically carried out by the sonde software.
In addition, the relationship between the measured partial pressure of oxygen (percent saturation)
and the solubility of oxygen in mg/L is very temperature dependent. For example, air saturated
water (100 percent saturated) at 20 C contains 9.09 mg/L, but only 7.65 mg/L at 30 C. Both of
these temperature related factors are compensated for by the 6000
UPG software after instrument
calibration. The temperature compensation for the percent saturation reading is empirically
derived, while the conversion from percent saturation and temperature to a solubility in mg/L is
carried out using formulae available in Standard Methods for the Examination of Water and
Wastewater (ed. 1989). See Appendix F for dissolved oxygen solubility tables as a function of
salinity and temperature.
FLOW DEPENDENCE
As noted above, oxygen readings acquired using the Rapid Pulse technology are much less affected
by sample flow than steady state probes. However, there is a finite stirring dependence exhibited by
the Rapid Pulse system if measurements are taken when the probe is being pulsed continuously. Our
tests indicate that, under these sampling conditions, observed dissolved oxygen readings can be 2-3
percent lower than the true readings in very still water. Minimal movement of the water (which
occurs during most environmental measurements) removes this effect.
This small flow dependence of the sensor is greatly reduced in longer term monitoring deployments
where the sampling interval is longer, e.g. 15 minutes. Under these conditions, the sensor is pulsed
for only approximately 60 seconds every 15 minutes, and normal diffusion of oxygen in the
medium re-establishes the oxygen which has been depleted in the previous warm-up/read sequence.