Diurnal Dissolved Oxygen Monitoring:
Optical versus Membrane Sensors
Thomas Amidon
Omni Environmental LLC
8th Water Monitoring Summit
West Trenton, NJ
December 2nd, 2011
Acknowledgements and Outline
Acknowledgements
Joe “JJ” Schwarz, Omni Field Sampling Manager
Managed all field activities and assembled data
Glen Petrauski, SRVSA
Client for whom comparative data were obtained
United States Geological Survey
Continuous water quality data in Raritan River @ Manville
Outline
Why is it important?
Ways to measure DO
A few interesting comparisons
Conclusions
Why is Diurnal DO Monitoring Important?
DO, pH, temperature, as well as conductivity and
turbidity, are frequently measured in-situ and diurnally in
surface waters
In-situ: measurements performed in the field
Diurnal: measurements performed throughout the day
DO and pH in particular can be used to assess the net
impact of biochemical processes on surface water quality
Photosynthesis
Respiration
Decay (decomposition)
Levels and diurnal swings of DO and pH are useful in
assessing nutrient impacts
pH
Dissolved Oxygen
Temperature
7/7 12 AM
7/6 6 PM
7/6 12 PM
7/6 6 AM
7/6 12 AM
7/5 6 PM
7/5 12 PM
7/5 6 AM
7/5 12 AM
7/4 6 PM
7/4 12 PM
7/4 6 AM
7/4 12 AM
Dissolved Oxygen (mg/L) or pH (s.u.)
14.0
30
13.0
28
12.0
26
11.0
24
10.0
22
9.0
20
8.0
18
7.0
16
6.0
14
5.0
12
4.0
10
Temperature (° Celsius)
Example Diurnal Swing
Ways to Measure DO
Winkler Method
Preserve sample in a bottle and titrate later in lab
The oldest tried and true method
Sensor methods
Used in portable meters as well as in-situ meters
2 types of technology
Membrane (aka Clark cell or amperometric method)
Optical (aka luminescent sensor)
Advantages and Disadvantages
Winkler method
Nobody is going to argue with the results
No meter calibration – simplest method by far
Disadvantage – only one sample at a time!
In-situ Sensor Methods
Membrane Sensor
Certified method!
Method has been in use for many years and is accepted
Optical Sensor
No consumbles (membrane and filling solution)
No stirring required and no known interference
Not yet certified for all uses in State of New Jersey
Both Sensors Perform Very Well
Mainstem Raritan River
16
14
DO (mg/l)
12
10
8
6
4
2
07/27/10
07/29/10
DO (Optical)
07/31/10
08/02/10
DO (Membrane)
08/04/10
DO Grabs (Optical)
08/06/10
08/08/10
DO Grabs (Winkler)
08/10/10
DO (Optical)
DO (Membrane)
DO Grabs
6:00 AM
12:00 AM
6:00 PM
12:00 PM
6:00 AM
12:00 AM
6:00 PM
12:00 PM
6:00 AM
12:00 AM
6:00 PM
12:00 PM
6:00 AM
DO (mg/l)
Both Sensors React Similiarly
Millstone River
16
14
12
10
8
6
4
2
Optical DO Sensors Capture Peaks
Mainstem Raritan River
16
14
DO (mg/l)
12
10
8
6
4
2
06/25/10
06/27/10
DO (Optical)
06/29/10
DO (Membrane)
07/01/10
07/03/10
DO Grabs (Optical)
07/05/10
DO Grabs (Winkler)
07/07/10
Optical Sensor Reproducibility
12
12
11
11.5
10
11
9
10.5
8
10
7
9.5
6
9
5
8.5
4
8
3
7.5
2
08/06/10
7
08/08/10
DO (USGS Optical)
08/10/10
08/12/10
DO (Omni Optical)
08/14/10
DO Grab
08/16/10
pH (USGS)
08/18/10
pH (Omni)
08/20/10
pH Grabs
pH (s..u.)
DO (mg/l)
Raritan River at Manville
Conclusions
Both methods can produce excellent in-situ results
If regulatory approval is required, be sure to specify
method in quality assurance plan
For diurnal deployment, optical sensor enjoys a few
key advantages
Less susceptible to biofouling and other interferences
Better performance in low DO environments
Might capture peaks better under certain conditions
Better performance in low velocity environments
Usually holds calibration longer
Optical sensor must be conditioned prior to deployment
Important part of meter calibration
Requires overnight (plan ahead!)
Questions and Discussion
www.Omni-Env.com
Thomas Amidon
Senior Associate
Water Resources Manager
609-924-8821 x128
[email protected]
Joseph W. Schwarz
Senior Engineer
QA/QC Field Manager
609-924-8821 x119
[email protected]