November 2012, Vol. 24, No.11

Operator Essentials

What every operator needs to know about dissolved oxygen measurements

 

 Click here to download a PDF version of this section. 

  

Knowledge    Principle    Practical consideration   

Interferences 

 

Dissolved oxygen (DO) measurement is dependent upon both temperature and barometric pressure. 

  

  

Biological activity can interfere with DO measurements. 

  


Some chemicals can interfere with DO measurements. 

 

When comparing readings from two sensors, be sure both are measuring similar temperatures and are compensating for the barometric pressure. Even a small change in temperature can have a significant effect on DO measurement. 

  

Bacteria in a sample will consume oxygen over time. For precise results, measure grab samples immediately or use a portable meter or an on-line sensor directly in the process. 

  

Hydrogen sulfide can poison membrane-based sensors, rendering them inaccurate. Reducing chemicals such as sulfur dioxide and bisulfite also can consume DO. If any of these are present in the sample, use portable or on-line sensors directly in the process to obtain the most precise DO concentration. 

 

Verification and calibration 

 

Know how to compare DO sensors. 

  

  

  

Create a water-saturated air environment. 

 

A general rule of thumb is that two sensors can be considered in sync if they give readings within 10% of each other. Be sure to use sensors with similar methods (e.g. optical compared to optical) since different methods can give different results. 

  

A water-saturated air environment is nature’s DO standard solution. Charts showing the exact DO concentration based upon temperature and barometric pressure can be used to verify or calibrate a sensor. Be sure to keep a stable temperature environment when calibrating sensors by shielding the sample from such heat sources as direct sunlight. 

 

Methods 

 

Today, the most common methods to measure DO are via optical, polarographic, and galvanic sensors.  

Such optical methods as luminescent and fluorescent probes use light to excite a chemical. Oxygen “steals” the energy from the excited chemical, and the rate at which the chemical relaxes is proportional to the DO concentration. 

  

The polarographic method uses a polarizing voltage across two electrodes to destroy oxygen, creating a current (amperage) that is proportional to the concentration of DO. 

  

Galvanic sensors use electrodes and an electrolyte to reduce oxygen spontaneously, creating a voltage that is proportional to DO. 

 

Maintenance 

 

All instrumentation requires maintenance; however, different DO methods have different requirements. Follow the manufacturer’s recommended maintenance schedule to ensure good results. 

 

Optical DO methods require periodic manual cleaning with a towel or rag to remove fouling from the sensor, as well as replacement of the optical chemical every 2 to 5 years. Often, no recalibration is required. 

  

Polarographic and galvanic methods require more maintenance, including cleaning and recalibrating the sensor every 2 to 6 weeks, cleaning the electrodes, and replacing the electrolyte and membrane every 2 to 6 months. 

 

Sample location 

 

Be sure that the sample being measured is representative of the process. 

 

A bad sample is a bad sample. Even the best DO sensor will give poor data with a bad sample.  

  

On-line DO sensors should be installed where the sample is representative — avoid corners, eddies, or nonrepresentative sidestreams. 

  

Likewise, comparison measurements should be made as physically close to the other sensor as possible to reduce the potential effects from aeration, mixing, temperature, etc. 

 

Storage 

 

Improperly storing sensors will affect how well they perform. Be sure to follow the manufacturer’s storage requirements. 

 

Most optical sensors can be stored dry. Most optical process sensors can be left exposed to the elements outdoors for short-term “storage,” but should be stored indoors for longer periods of time. 

  

Membrane sensors must remain moist. Once a membrane dries out, it needs to be replaced, as does the electrolyte behind the membrane. 

 

Bob Dabkowski is a wastewater specialist at Hach Co. (Loveland, Colo.).