DISCRETESAMPLING
“MAKINGGOODDOMEASUREMENTSWITHTHERAPIDPULSE
DOPROBE”
Preparing a sonde for use in a spot sampling/profiling application is the same as the preparation used in
unattended monitoring. The sonde must be tested and calibrated. In addition the field display (610/650
or PC) must be charged and made ready for the field.
Sampling can be tough on the equipment -- bounced around in small boats, temperature extremes, shock,
and vibration can cause Teflon-membrane DO sensors to experience a calibration shift. The field
technician needs to be aware of this and must be able to document and correct for any DO drift that
might occur during the day.
Other parameters on the sonde -- temperature, conductivity, depth, turbidity, pH, and chlorophyll -- are
generally not affected by the above issues and do not drift during normal sampling use. The best way to
determine if DO sensor drift has occurred is to check the output under a controlled condition each time
you get to a site. By following the steps below you can perform a pre- and post-check at each site and
greatly increase your DO data quality.
Note: These instructions were written primarily for YSI sondes using the Rapid Pulse DO sensor.
Sondes with optical DO probes do not have the warm-up and burn-in requirements.
1. Whenever possible prepare and calibrate your equipment the day before the expected field study.
Make sure that the Auto Sleep RS-232 function has been disabled for sampling applications. This is
found in the sonde’s Advanced Menu under Setup.
2. Transfer of the sonde from the calibration cup to the sensor guard puts the sonde and sensors at risk.
This is when most accidents to the oxygen sensor and membrane occur. To eliminate this risk take
the sonde to the field with the sensor guard installed. Instead of using the cal cup to keep sensors in
saturated air, wrap the sonde in a white towel that has been saturated in tap water. The toweled sonde
can be stored in a cooler or pail. The wet towel is a perfect saturation chamber. In addition it can be
wrapped around the sonde to minimize shock and vibration during transit.
3. When you arrive at the site, the sonde/display will be turned on and started sampling; allow it to
warm up/run for approximately 5 minutes. Check the DO output; it should read saturation for your
local barometric pressure plus or minus the instrument’s tolerance of 2%. Record the sonde’s
reading in your field shield under pre-deployment DO check. If you have a reading that is out of
tolerance, then you would simply recalibrate the DO in the wet towel and then record that sample.
4. Deploy the sonde. Remember to allow the sonde a few minutes to equilibrate to the water
temperature before you take your reading. Once the data has been collected, rinse the sonde with tap
water, shake dry and wrap again in the wet towel. After 5 minutes perform a DO post calibration.
Again, the sonde should return to saturation, plus or minus the tolerance of 2%. If the probe fails to
return to saturation, then the field samples should be retaken after a recalibration.
5. It is strongly recommended that you store this pre- and post-calibration data in the actual site data
file if you are logging the information; or record in your log sheet if manually collecting. This
assures anyone who looks at the records at a later time that the sonde was calibrated and working
correctly. The addition of these QA steps adds very little time to the collection process and can
actually save time when unexpected data is recorded.
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