EXO Measurement Units – Dissolved Oxygen

EXO Measurement Units – Dissolved Oxygen

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EXO Measurement Units – Dissolved Oxygen

This video covers each available unit for dissolved oxygen measurements and when to use them.

Timestamps of specific moments of the video

  • 0:00 – Overview of Dissolved Oxygen Measurement Units
  • 0:39 – Unit: mg/L
  • 1:15 – DO% Saturation Options
  • 1:44 – Unit: % Sat
  • 2:48 – “Local DO”
  • 3:38 – Unit: % CB
  • 4:20 – Unit: % RTB

Video Transcript

In this video, we’ll discuss the EXO Dissolved Oxygen parameter and units. EXO uses optical technology to measure DO in percent saturation and milligrams per liter. I’ll be referring to this parameter and its units in Kor Software, but you can also use the EXO Handheld or the Kor Mobile app.

In Kor, you’ll see four unit options for DO. One is for milligrams of oxygen per Liter, and the other three reflect a percentage measurement of oxygen in the water.

Milligrams per liter is one of the most commonly reported units for dissolved oxygen, and it is equivalent to ppm, or parts per million. This unit represents the milligrams of gaseous oxygen dissolved in a liter of water. The EXO Sonde automatically calculates this value using formulas found in the Standard Methods for the Examination of Water and Wastewater. The DO percent saturation, temperature, and salinity readings are used in the calculation. The latter two inputs are provided by the EXO Conductivity and Temperature Sensor.

The DO percent saturation represents the pressure of oxygen that is dissolved in a sample. You’ll notice three options for reporting percent saturation in Kor: percent sat, or saturation, percent CB, or Calibrated Barometer, and percent RTB, or Real-Time Barometer.

It is important to note that no matter which percent units are selected, the milligrams per liter will be the same.

Percent Saturation is the direct DO measurement provided by the EXO Dissolved Oxygen Sensor. The solubility of oxygen in water is influenced by the barometric pressure. For example, at sea level, where barometric pressure is 760 millimeters of mercury, a fully saturated environment will calibrate to 100 percent. If your elevation increases, or if air pressure decreases, water can hold less oxygen due to the partial pressure of oxygen in the atmosphere decreasing. For example, at a higher altitude, say Denver, Colorado, with a barometric pressure reading of 640 mmHg, the DO will calibrate to 84.2% in a fully saturated environment. At this pressure, dissolved oxygen saturation cannot be greater than 84.2% as long as water and air are in equilibrium. This is the value that the percent sat unit will calibrate to.

Our two remaining DO percent units, %RTB and %CB, are sometimes referred to as Local DO, where the calibration value is 100% regardless of the barometric pressure at the time of calibration. The water sample in Denver at 640 mmHg may not be able to hold as much oxygen as the water at sea level, but it is still holding as much as it can where it is located… so “locally” it is fully saturated.

Some prefer this way of monitoring DO in water as it is specific to the water at the measurement location. So why are there two different units for Local DO? The difference is based on the availability of real-time barometer readings to account for changes in pressure.

% CB, or Calibrated Barometer, represents the Local DO value calculated from the barometric pressure value entered at the time of calibration. This unit should be used when you want to measure Local DO, but real-time barometric pressure is unavailable while monitoring. So if you are deploying your sonde for unattended monitoring, %CB is the unit you should use to measure Local DO.

Extreme changes in pressure may affect the readings over the course of a deployment; however, as long as the sonde is deployed at roughly the same elevation as it was calibrated, the % CB readings will be representative for most applications.

%RTB, or Real Time Barometer, represents the Local DO value calculated from the barometric pressure readings that are provided from the handheld during measurement. For example, if you were to calibrate the sensor to 100% at sea level, and then take that sensor to Denver, where air pressure is lower, the DO %RTB measurement in a fully saturated water body would still read 100% with the barometer input from the EXO Handheld. %RTB should only be used for measurement when the sonde can receive real-time barometric pressure input from the handheld, like for sampling and profiling applications.

If the sonde cannot receive barometric pressure input during measurement, such as during an autonomous deployment, then % RTB cannot be calculated and % CB should be used for monitoring Local DO.

That covers all of the units available for the EXO Dissolved Oxygen Sensor. For more information, please check out our other EXO University videos. Thanks for watching!

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