Standard Operating Procedures for Surface Water Sampling

STANDARD OPERATING PROCEDURES FOR SURFACE WATER QUALITY SAMPLING4. Use the hand-held altimeter or the barometer in the unit to measure the ambient airpressure (the terms “air pressure” and “barometric pressure” are equivalent). If abarometer is not available or not functioning properly, contact a nearby airport. If thecalibration site is at approximately the same elevation and is not too distant from theairport, the air pressure reading should be usable for calibration purposes.Barometer readings from the Hydrolab or handheld barometer are usually “true”(uncorrected) values of air pressure and can be used “as is” for dissolved oxygencalibration. Weather service readings are usually not “true”, i.e., they are corrected tosea level, and therefore cannot be used until they are “uncorrected”. An approximateformula for this “uncorrection” (where the BP readings MUST be in mm Hg) is: TrueBP = [Corrected BP] – [2.5 * (Local Altitude in feet above sea level/100)]. Also note,25.4 * (inches of Hg) = mm of Hg.If an altimeter is not available or the altimeter on hand is not functioning properly,there is a basic rule of thumb that can be used to obtain air pressure. Standardpressure at sea level is 29.92 inches of Hg. Atmospheric pressure decreases withincreasing altitude. For every increase of 1,000 feet in elevation above sea level, airpressure will decrease approximately 1.0 inch of Hg. This simple formula can beuseful, but may not hold up well in cases where there is a DO exceedance that cannotbe attributed to naturally occurring conditions. Be sure to qualify the DO data if thismethod has to be used.5. Before calibrating the DO probe, record the pre-calibration percent saturation value afterthe unit has stabilized. Using the calibration menu for % saturation, enter the barometricpressure when prompted. Record the new DO percent saturation reading, which shouldbe at or near 100%. The reading should be stable for about 20 to 30 seconds. If DO isbeing measured at a number of sites and at substantially different elevations, the unitshould be calibrated at each site. Most stream and lake sampling will require DOcalibration at each site.1.4.3 YSI CALIBRATION PROCEDURESBefore Calibrating the YSI, connect the Sonde to the unit via cable and turn unit on. Highlightsonde run and press enter. FIGURE 1.4 shows the basic components of the YSI Sonde andSurveyor.REVISED SEPTEMBER 2012 6