When testing for dissolved oxygen in the field, an important thing to consider is the type of dissolved oxygen (DO) meter to use. There are several different types of DO sensing technologies, most of which fall under the category of optical or electrochemical sensors. All of these technologies have advantages and disadvantages which must be considered including maintenance, the measurement process, and initial cost.
Electrochemical sensor meters have an anode, cathode, and oxygen permeable membrane, so there most obvious disadvantage is in regards to maintenance. Because of the chemical reaction that constantly takes place, a buildup of oxidized material will begin to form either on the anode or in the electrolyte solution. This buildup must be removed every two to eight weeks to keep the sensor at peak performance. Optical sensor meters, on the other hand, are low maintenance with the only regular maintenance typically required being the yearly replacement of the sensing element.
Another vital specification to keep in mind is the response time of the sensor. The response time for an electrochemical DO sensor is 8 to 18 seconds, while an optical sensor response time is around 40 seconds. This is not a huge difference when taking a couple of samples, but those seconds can add up over the course of a day. After 100 samples, the time difference is almost an hour!
In addition to faster response times, the optical dissolved oxygen technology does not consume oxygen during its measurement process. The optical sensor does not require the user to stir the sample as they take their measurements. Stirring the sample can be difficult when taking down well measurements or when turbidity is a concern.
The cost of an instrument is always something that has to be considered. While a 25′ DO meter with an electrochemical sensor costs around $800 to $900, a 25′ optical DO meter can cost $1500 or more. Depending on accuracy requirements and time constraints, an electrochemical DO meter can provide a lower cost option for companies on a tight budget.
Now that optical DO technologies have been approved by the EPA, there are many more choices out there when it comes to DO meters. Considering these advantages and disadvantages will lead to a better decision when it comes to choosing a meter and give peace of mind for many years to come.
Note: With both types of sensors you need to consider whether or not the unit will be used down-hole (in a well) or above ground. Knowing this will help you determine the cable length you need and the correct probe diameter for your application. Also choosing a unit that has a detachable cable and sensor allows you to replace those items without sending the unit in for service. Durability, such as whether or not a unit is water proof or water resistant, and warranty should also be taken into consideration.
Environmental Specialist II