Dissolved Oxygen 

Dissolved oxygen is the most widely used indicator of aquatic health, and the parameter most often written into Clean Water Act criteria and NPDES discharge permits. Because oxygen solubility falls as temperature and salinity rise, defensible mg/L reporting depends on measuring temperature and conductivity alongside it.

RAECO Rents supplies optical dissolved oxygen instruments and sensors with NIST-traceable calibration included. Free ground shipping and free return shipping are included on every rental.

YSI ProDSS Multiparameter Digital Water Quality Meter
Frequently Asked Questions
Common questions about dissolved oxygen
How often should I calibrate a dissolved oxygen meter?
YSI recommends calibrating or verifying daily, before sampling starts. Optical DO calibration is very stable, but it should still be checked regularly. The more critical the data — compliance work in particular — the more attention timely calibration deserves. Your data is only as good as your calibration.
What is true barometric pressure, and why does it matter for DO?
True barometric pressure is an uncorrected reading. Laboratory barometers usually give true values and can be used as-is. Weather service readings are corrected to sea level and cannot be used until they are uncorrected. YSI's approximate formula is: True BP = Corrected BP minus [2.5 x (local altitude in feet above sea level / 100)]. Get this wrong and every DO calibration is wrong with it.
Do I need to recalibrate if I move to a different altitude?
No. Dissolved oxygen sensors are calibrated to and measure the partial pressure of oxygen, so after an accurate calibration the sensor compensates for pressure changes automatically. If you are using a DO% Local function, the calibration value stays at 100% regardless of altitude and the internal barometer keeps the saturated value there.
Optical or electrochemical DO sensor — which should I rent?
Optical for almost all field work. There is no membrane to change, no electrolyte, no stirring requirement, and readings stabilize almost immediately after power-on. A polarographic electrochemical sensor needs roughly 5 to 15 minutes to stabilize and its membrane needs changing every 2 to 8 weeks depending on the sampling.
How do I verify a calibration without recalibrating?
Put the sensor back in its calibration environment and compare the reading against the value for the current barometric pressure. If true BP is 750 mmHg, the expected reading is 750/760 x 100 = 98%. Once stable, an optical sensor should sit within about 1% of that figure, and an electrochemical sensor within about 2%.
My percent saturation looks right but mg/L looks wrong. Why?
Almost always a salinity problem. Oxygen solubility drops as salinity rises, and the instrument uses a salinity value to convert saturation into mg/L. If a conductivity sensor is installed and calibrated, that value comes from the sensor. If not, you have to enter it manually — roughly 0 to 0.5 ppt for fresh water, about 35 ppt for seawater.
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YSI ProDSS Multiparameter Digital Water Quality Meter
Water Quality Meter

YSI ProDSS Multiparameter Digital Water Quality Meter

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