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There are several parameters that affect the DO measurement accuracy and reliability, they are temperature, salinity, atmospheric (barometric) pressure and flow (stirring). ----- are we gonna discuss about stirring?Temperature, salinity and pressure are discussed below.
Temperature
Temperature is the most significant variable for the measurement accuracy. Therefore it should be ensured that the temperature sensor on the probe is working correctly. Temperature can influence the DO measurement in two ways [6]:
- Diffusion of oxygen through the membrane (electrochemical) or sensing element (optical) on the probe increases/decreases with higher/lower temperature due to change in molecular activity (up to 4% difference per °C).
With digital sensors, the effect of temperature can be compensated with software, as the temperature is known; with analog sensors, compensation is done by adding a thermistor (a temperature-sensitive resistor) into the circuit. [6] - Ability of water to dissolve oxygen is directly proportional to temperature. Warmer water dissolves less oxygen than colder water. Consequently, with same saturation rate, warmer water contains less oxygen in absolute terms. The absolute (mg/L) concentration must be therefore compensated according to the temperature of the sample. [6]
Salinity
Similarly with temperature, increasing water salinity decreases its ability to dissolve oxygen. Some of the DO sensors measure also conductivity, and the value is used for calculating salinity and, based on that, oxygen concentration. If built-in conductivity sensor is available, it is important to ensure that it is calibrated and working correctly. If the conductivity is measured with separate sensor, the salinity value must be entered by the user. [6]
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Electrochemical sensors are more prone to drift and require more frequent calibrations than optical sensors. In principle, teady-state galvanic and polarographic sensors need calibration daily when in use. If the measurements, however, are reliable also with less frequent calibrations, calibration frequency can be reduced. [6]
Optical sensors are more stable than traditional electrochemical sensors. It has been shown that optical sensors can hold their calibration for months. It is still recommended to calibrate the sensor regularly to obtain most correct measurements. The calibration is done by measuring known sample and comparing the measured value to the known real value. [6]
Three main methods for calibrating DO sensor are
- Winkler titration
- Air-saturated water
- Water-saturated air [6]
Cleaning and Maintenance
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The following points should be emphasized when carrying out the cleaning and maintenance process . Source: [1].
- The sensor must be disconnected from the meter. When the sensor is connected and submersed in the cleaning solution, no chemical reaction takes place between the solution and the oxidized reference electrode surface; instead, the cleaning solution may become electrolyzed!
- Use the cleaning or electrolyte solution suitable for the particular sensor as stated in the operating manual! A solution that is suitable for silver electrodes cannot regenerate lead electrodes!
- Only the gold cathode should be polished; the counter electrode is merely wiped clean with a soft cloth to wipe away easily removable salt deposits! A spotty coating after regeneration of the lead or silver electrodes does not impair measurements!
- When polishing the gold electrode, only use the moistened EID abrasive film that has a special grain that polishes and do not scratch!
- It is also recommended to use a new membrane head since the used membrane cannot necessarily guarantee that the membrane fits correctly against the gold cathode which is ensured by a spacing lattice on the inside of the membrane. Baggy clothing don't fit either!
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