There are many factors that can affect your dissolved oxygen meter measurements. In this series, we are focusing on the affects from temperature, salinity, barometric pressure and flow dependence.
Steady-state electrochemical sensors, Clark (Dr. Leland Clark pictured) polarographic and galvanic, consume oxygen during measurement and therefore require sample movement or the readings will be artificially low. This is commonly referred to as flow dependence since the sensor is dependent on flow or water movement across the membrane in order to obtain accurate readings.
Optical sensors (ProODO, ProDSS, EXO), however, use a non-consumptive method for dissolved oxygen measurements resulting in a sensing method with zero flow dependence or stirring requirement. However, YSI scientists have confirmed response times on optical sensors can be improved with flow. Accuracy doesn't change - just the amount of time to get to a reading which may be an advantage in the laboratory measuring many BODs.
The graphs below illustrate this advantage of the optical sensor. The first is a graph of data measured with a steady-state polarographic sensor in an air-saturated water sample where adequate sample movement was provided by a mechanical stir bar. When the stirring mechanism was turned off, the readings began to fall resulting in artificially low dissolved oxygen measurements.
The second is a graph of data measured with an optical sensor in the same air-saturated water sample where sample movement was provided by a stir bar. When the stirring mechanism was turned off for the optical measurements, the readings remained constant and accurate proving the optical sensor is not dependent on flow. This is a considerable advantage of the optical sensor especially for low flow applications or applications where probe stirring or sample movement is difficult such as down well.
For steady-state electrochemical sensors, the membrane material and thickness dictates the degree of the sensor’s flow dependence. For example, polyethylene membranes, frequently notated as PE, require less movement or flow than traditional Polytetrafluoroethylene (you know it as a more common name but we're not allowed to use it for legal reasons...) membranes as illustrated by the graph below.
In this graph, three different sensors were placed in fully aerated water with a stir bar. Once the stirring was ceased, the steady-state electrochemical readings began to fall. Notice how the Polytetrafluoroethylene covered sensor fell further and more rapidly then the PE covered sensor.
The stirring dependence of each sensor and membrane type, along with the recommended stirring rates, is listed in the Membrane Comparison Guide.
Additional Blog Posts of Interest:
What is Affecting Your Dissolved Oxygen Measurements? Part 3 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 2 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 1 of 4