Drum roll please.....the most significant variable for dissolved oxygen measurements is - temperature. Therefore, it is important to ensure the temperature sensor on the instrument is measuring correctly because temperature affects DO measurements in two ways.
Temperature and Diffusion
First, due to the increase or decrease in molecular activity, diffusion of oxygen through the membrane of an electrochemical probe or sensing element of an optical probe changes with temperature. The change in diffusion rate based on temperature can be up to approximately 4% per degree Celsius depending on the membrane material for steady-state electrochemical sensors, 1% per degree Celsius for Rapid Pulse sensors, and is approximately 1.5% per degree Celsius for optical sensors.
For example, if the temperature of a sample changes from 20°C to 15°C, the probe signal would decrease by varying rates depending on the sensor in use, giving a lower DO % saturation reading even though the % saturation of the water has not changed. Therefore, the sensor signal must be compensated for changes in temperature. This is done by adding a thermistor to the circuit of older, analog instruments. For newer, digital instruments, the software compensates for temperature changes with proprietary algorithms that use the temperature readings from the probe’s thermistor.
Temperature and Oxygen Solubility in Water
The adjustment described so far only compensates for temperature’s effect on the oxygen diffusion rate through a membrane or sensing element. In addition to this effect, temperature also affects the ability of water to dissolve oxygen. It is a scientific fact that the solubility of oxygen in water is directly proportional to temperature; see the Oxygen Solubility Table.
Warmer water cannot dissolve as much oxygen as colder water. For example, in an oxygen saturated sample of water at sea level (exposed to 760 mmHg of barometric pressure), the % saturation value will be 100% regardless of the temperature because it is fully saturated. However, the dissolved oxygen mg/L concentration will change with temperature because the solubility of oxygen in water changes with temperature. For instance, at 15ºC water can dissolve 10.08 mg/L while 30ºC water can only dissolve 7.56 mg/L of oxygen even though the % saturation value is 100% in both samples. Therefore, we must compensate the mg/L concentration reading per the temperature of the sample.
Both of these temperature effects are factored into the conversion of the probe signal to a mg/L concentration. For newer, digital instruments such as the optical ProODO and the traditional Pro20, the software compensates for both of these temperature-related factors after instrument calibration and during readings.
The temperature compensation for the % saturation reading is empirically derived, while the conversion from % saturation, temperature and salinity to a mg/L concentration is automatically carried out by the instrument’s firmware using formulae available in Standard Methods for the Examination of Water and Wastewater. The calculation for converting % Saturation to mg/L and an example is provided below.
Determining DO mg/L from % Saturation
The following explains how to convert % Saturation to mg/L (also referred to as ppm).
In order to perform this conversion, the temperature and salinity of the sample must be known. This is the reason accurate temperautre values must be used in the calculation of mg/L values.
Step one: Determine the % saturation, temperature, and salinity of the sample.
Step two: Multiply the % saturation reading by the value in appropriate column (depends on salinity) and row (depends on temperature) of the Oxygen Solubility Table.
Step one: Sample is measured to have: 80% DO saturation 0 ppt salinity at 20º C
Step two: Multiply .80 (which is the DO %) by 9.09 (value from oxygen solubility table at 0 salinity and 20º C) = 7.27 mg/L.
Result: 7.27 is the mg/L value that corresponds to an 80% DO Saturation reading of a sample with zero salinity at 20º C.
Additional Blog Posts of Interest:
What is Affecting Your Dissolved Oxygen Measurements? Part 4 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 3 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 2 of 4