Analytical Methods--Recommended procedures for calibrating dissolved oxygen meters

                                                March 14, 1979


QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 79.10

Subject: Analytical Methods--Recommended procedures for 
                             calibrating dissolved oxygen meters

Dissolved oxygen meters have been in routine use by the Water 
Resources Division for a number of years for measuring dissolved 
oxygen (DO) concentrations in fresh and saline waters. However up 
to this point there has been no documentation of recommended 
procedures for calibrating these meters. Several calibration 
procedures, based largely on manufacturers' recommendations and on 
instructions given at various WRD training courses,are in current 
use within the Division.  These procedures vary widely from 
district to district.

In an attempt to develop a standardized and recommended DO meter 
calibration methodology the Quality of Water Branch requested the 
California District to conduct a detailed evaluation of several 
calibration procedures. The evaluation was carried out over 
temperatures ranging from 0.5 to 30 deg. C and at altitudes 
ranging from sea level to over 8,000 feet. Based on the results of 
this evaluation the attached method describing four DO meter 
calibration procedures was prepared. The four procedures are: (l) 
air calibration chamber in water, (2) calibration by Winkler 
titration, (3) calibration with air saturated water, and (4) air 
calibration chamber in air. All four procedures will give 
comparable results (within +/- 0.1 mg/L when compared with a 
Winkler DO measurement) provided the procedures are followed 
carefully.

Use of one of the above four procedures is recommended for 
calibration of DO meters used in collection of data for WRD 
programs. Of the four procedures, the air calibration chamber in 
water (procedure l) is preferred. It is the most convenient to use 
and the most accurate under a variety of conditions. In situations 
where this calibration procedure cannot be used, one of the other 
three should be used.

Note 6 on page 5 of the DO calibration method points out a problem 
in using the YSI DO probe model 5739 with the air calibration 
chamber. This probe has a pressure compensating diaphragm which 
will not fit inside the air calibration chamber and can lead to 
erroneous calibration. Quality of Water Branch Technical 
Memorandum No. 79.08(B), dated March 7, 1979, describes a 
modification which can be made to the air calibration chamber to 
accomodate the DO probe and diaphragm, thereby eliminating this 
problem.

Any questions or comments regarding these DO meter calibration 
procedures should be addressed to the Chief, Quality of Water 
Branch, with copies to your Regional Water Quality Specialist.



                           R. J. Pickering
                           Chief, Quality of Water Branch

Attachment

WRD Distribution: A, B, FO, PO




Oxygen, dissolved, electrometric, polarographic probe (I-1576-78)

    Parameters and Codes: Oxygen, dissolved (mg/L): 00300
    Oxygen, dissolved (percent of saturation): 00301

1. Application
1.1 This method may be used to analyze waters and wastewaters 
containing between 0.1 and 20 mg/L of dissolved oxygen. 

2. Summary of method
2.1 Dissolved oxygen (DO) is determined by an electrode system 
consisting of a sensor or probe containing a gold cathode and 
silver anode surrounded by an electrolyte solution of potassium 
chloride. A thin oxygen-permeable membrane is stretched across the 
end of the probe to isolate the electrolyte and sensor elements 
from the environment. When a potential of 0.8 volt is applied 
across the cell, oxygen is reduced at the cathode causing current 
to flow in an amount proportional to the partial pressure of 
oxygen at the external membrane surface. Inasmuch as oxygen is 
consumed at the gold cathode, the oxygen pressure inside the 
membrane is effectively zero, and the force causing oxygen to 
diffuse through the membrane is therefore proportional to the 
absolute pressure of oxygen outside the membrane. A linear 
relationship exists between the cell current and external oxygen 
pressure or concentration of oxygen in the sample solution.
Four procedures for calibrating the oxygen meter are given. They 
are listed in order of preference.

3. Interferences
3.1 The membrane is permeable to gases, other than oxygen. 
Halogens, hydrogen sulfide, sulfur dioxide, and helium interfere. 
Hydrogen sulfide poisons the electrode.

4. Apparatus
4.1 Oxygen meter, Yellow Springs Instrument Co., Model no. 54 or 
no. 57, equipped with model 5739 probe, or equivalent.
4.2 The oxygen meter selected should meet the following general 
requirements:
  Range of dissolved oxygen: 0.1 to 20 mg/L. 
  Accuracy: +/- 1 percent of maxiumum range. 
  Operating temperature range: -5 deg. to +45 deg.C. 
  Minimum scale readability: 0.05 mg/L 02
  Temperature-compensated, portable, and rugged for field and 
    laboratory use.
4.3 Calibration chamber, Yellow Springs Instrument Co., Model no 
5075, or equivalent.
4.4 Pocket altimeter-barometer, capable of being read to at least 
the nearest 5 millimeters of mercury. USGS warehouse supply stock 
number 01321, or equivalent. 

5. Reagents
5 1 Sodium sulfite solution: Dissolve 1 gram of sodium sulfite
(Na2S03) and a few crystals (about 1 mg) of cobalt chloride 
(CoC12) in 1 liter of distilled water.
5.2 Reagents for Alsterberg-Azide method: See method I-1575. These 
reagents are required when using calibration procedure 6.3.1B.

6. Procedure
6.1 Prepare oxygen meter for calibration in accordance with the 
manufacturers instructions (Notes 1. 2. 3. 4. and 5).

Note 1. A number of precautions should be observed when using the 
oxygen meter, particularly when the instrument is used for field 
measurements. (l) Check mechanical zero before turning meter on 
and adjust if necessary, (2) turn instrument on to any operational 
position 1 hour before use to insure stability of operation; meter 
should be left on until final measurement for day is completed. 
(3) Check REDLINE and ZERO and adjust as needed; If meter will not 
adjust to REDLINE recharge or replace batteries, (4) Protect 
instrument from temperature changes that may result from exposure 
to direct sunlight or contact with ice or cold wind.

Note 2. The DO probe requires periodic replacement of electrolyte 
and membrane. Follow manufacturers directions but observe these 
points: (l) There should be no bubbles of air behind membrane. 
Electrodes with pressure compensators should have the pressure 
compensation membrane moved in and out with the eraser end of a 
pencil to insure that all bubbles are removed from pressure 
compensation chamber. (2) the membrane should be smooth with no 
wrinkles (3) excess membrane material should be trimmed to within 
2 to 3 mm of the o-ring.

Note 3. O-rings stretch with use and must be replaced before they 
become loose. A common cause of electrode malfunction is leakage 
of electrolyte around the O-ring seal. To check for this problem, 
rinse the probe unit to remove traces of electrolyte remaining 
from membrane replacement. Place the probe in a 250-mL beaker of 
distilled water; Measure the specific conductance with an 
appropriate instrument. An increase in specific conductance 
indicates leakage of the electrolyte around the o-ring. Check the 
probe for physical damage. Replace the membrane and repeat the 
test. It may be necessary to replace the o-ring also.

Note 4. Keep DO probe in water when not in use to prevent drying 
out of the membrane.

Note 5. Periodically check the thermistor in the DO probe against 
a certified mercury thermometer over the normal operating 
temperature range of the instrument. If the two readings are not 
in agreement a correction curve should be constructed or an 
electronic adjustment should be made to the instrument by the 
manufacturer or qualified service center.

6.2 Once daily, check that the instrument will read zero in a 
dissolved-oxygen free solution. This may be accomplished by 
inserting the probe into the sodium sulfite solution and measuring 
the dissolved oxygen. If instrument reading exceeds 0.2 mg/L the 
probe needs repair or the membrane and electrolyte need 
replacement.

6.3 Calibrate the DO meter in accordance with one of the following 
four procedures. Procedure A is the preferred procedure. 
Procedures B, C and D may be used in situations where procedure A 
cannot be used. These four procedures are given in order of 
preference.

6.3.1A Air Calibration Chamber in water : The air calibration 
chamber (Yellow Springs Instrument number 5075, or equivalent) 
permits calibration of the oxygen meter at the temperature of the 
water in which the DO content is to be measured, thereby 
minimizing errors due to temperature differences. Dip the 
calibration chamber into the water; pour out excess water, and 
then insert DO probe into the wet chamber. This insures that the 
air inside the chamber is saturated with water vapor. Caution: Be 
sure that no water can leak into the calibration chamber and that 
the membrane has no droplets of water adhering to it since this 
would reduce the rate of oxygen diffusion through the membrane and 
would produce erroneous results.

6.3.2A Place calibration chamber in the water in which the DO 
content is to be measured. Allow 10-15 minutes for the temperature 
of the air inside the chamber to equilibrate with the water.

Note 6. The calibration chamber (model 5075) is designed to allow 
the membrane surface of the DO probe (Model 5739) to be at ambient 
atmospheric pressure while in the chamber, but the chamber is not 
large enough for the entire DO probe and pressure compensator to 
fit inside. Since the pressure compensator must remain at 
atmospheric pressure care must be taken to insure that the 
pressure compensator is not submerged below the water surface if 
method 6.3.1A is used.

6.3.3A Read true atmospheric pressure from pocket altimeter-
barometer to the nearest 5 mm of mercury.

6.3.4A Recheck the REDLINE and ZERO reading on the oxygen meter 
and adjust if necessary.

6.3.5A Measure temperature in the calibration chamber to the 
nearest 0.5 deg. C, using the thermistor in the DO probe.

6.3.6A Using the oxygen solubility table from section 7.2 
determine the DO saturation value at the measured water 
temperature and true atmospheric pressure. Apply a salinity 
correction if appropriate.

Note 7. The solubility of oxygen in water decreases as salinity
increases. Oxygen saturation values may be corrected for salinity
using the correction factors based on either chloride 
concentration or specific conductance. Correction factors based on 
chloride concentration are given in the oxygen solubility tab1e in 
section 7.2. Correction factors based on specific conductance are 
also given in a table in section 7.2. The correction based on 
specific conductance may be more useful since specific conductance 
can be easily determined from a field measurement. The solubility 
of oxygen at 25!C and 760 mm of mercury in water containing 5,000 
mg/L of chloride is calculated as follows using the chloride 
correction factors:

 8.3 mg/L                    5,000 mg/L Cl   x   0.008 =         7.9 mg/L
(Saturation DO at                 100            (Difference per
O mg/L chloride                                  100 mg/L chloride
and 760 mm Hg from                               from section 7.2)
oxygen solubility
table)

If, in the above example, specific conductance of the water was 
15,000 micromhos per cm at 25 deg. C the calculation could be made 
as follows using the specific conductance correction factors:

8.3 mg/L            x           0.951            =     7.9 mg/L
(Saturation DO at      (Correction factor from
O mg/L chloride and    table in section 7.2 for
760 mm Hg from oxygen  specific conductance of
solubility table)      15,000 micromhos and at
                       a temperature of 25 deg. C)

In both of the above examples the DO meter would be adjusted to 
7.9 mg/L rather than 8.3 mg/L.

The YSI model 57 DO meter has a manual salinity correction built
into the instrument. Follow the manufacturer's instructions in its 
use.

The salinity value required for use with the YSI model 57 meter 
may be estimated from a field measurement of specific conductance 
by the following equation:

Salinity, in parts per           [Specific Conductance] - 1.025
thousand                 = 0.677         l000

6.3.7A Select the scale (0-10 mg/L or 0-20 mg/L) to be used for 
the DO measurement and adjust CALIBRATION control until meter 
reads the DO saturation value determined in 6.3.6A. Do not change 
scales without recalibrating meter or verifying that identical 
readings are obtained on both scales.

6.3.8A Remove probe from calibration chamber and place in water to 
be measured. Move probe through water to insure a velocity of at 
least 1 foot per second past the probe (alternatively, a 
submersible stirrer, YSI Model 5491, or equivalent may be used) to 
prevent localized depletion of DO at the membrane-water interface.

6.3.9A Switch meter to TEMPERATURE; after meter reading has 
stabilized record temperature to the nearest 0.5 deg. C.

6.3.10A Switch meter to the proper DO scale; after meter reading 
has stabilized record DO concentration to the nearest 0.1 mg/L.

6.3.1B Calibration by Winkler titration : The DO meter is 
calibrated with distilled or deionized water in which the DO 
concentration has been determined by the Winkler method.

6.3.2B Place 1 to 2 liters of distilled or deionized water in a 
large container. The water should preferrably be near saturation 
with respect to DO and the water temperature should be reasonably 
close to the ambient temperature. These conditions should insure 
that the DO concentration in the distilled water remains constant.

6.3.3B Place DO probe in the distilled water. Maintain a velocity 
of at least 1 foot per second past the DO probe with a magnetic 
stirrer or other means.

6.3.4B Check ZERO and REDLINE, and adjust if necessary.

6.3.5B Switch meter to the appropriate scale and adjust reading to 
the approximate DO concentration with the CALIBRATION control.

6.3.6B After DO meter has stabilized determine the DO 
concentration of two aliquots of the distilled water by the 
Winkler titration procedure (Method I-1575). If results of the two 
Winkler titrations do not agree within 0.1 mg/L, the titration 
should be repeated.

6.3.7B Without delay recheck ZERO and REDLINE then adjust 
CALIBRATION control until DO meter reads DO concentration 
determined in step 6.3.6B. Meter is now calibrated and ready for 
use.

6.3.8B Place DO probe in water to be measured. Allow sufficient 
time for the probe to equilibrate with the water temperature. Move 
probe through water at a velocity of at least 1 foot per second 
(alternatively, a submersible stirrer, YSI Model 5491, or 
equivalent may be used) to prevent localized depletion of DO at 
the membrane-water interface.

6.3.9B Switch meter to TEMPERATURE; After meter has stabilized 
record temperature to nearest 0.5  deg. C.

6.3.10B Switch meter to the appropriate DO scale. After meter has 
stabilized record DO concentration to nearest 0.1 mg/L. The 
alternate salinity correction procedure (See note 7) should be 
applied if appropriate.

6.3.1C Calibration with Air Saturated Water: The DO meter is 
calibrated against water which is saturated with oxygen at a known 
temperature and true atmospheric pressure. Obtain approximately 
1 liter of distilled water or water from the water body to be 
measured. The temperature of the water used for calibration should 
be close to ambient temperature so there is no heating or cooling.

6.3.2C Place DO probe and the calibration water in a large beaker 
or open-mouth container and allow it to come to thermal 
equilibrium. Shield beaker from direct sunlight and wind to 
minimize temperature variations.

6.3.3C Aerate the water for 5-10 minutes. This is most readily 
accomplished with a small battery-operated aquarium pump or minnow 
bucket aerator and a short section of tubing. A gas diffusion 
stone should be attached to the end of the tubing and placed at 
the bottom of the beaker of calibration water.

Note 8. It is extremely important to insure that the water is 
exactly 100 percent saturated with oxygen. An indication of 100 
percent saturation can be determined as follows. Switch DO meter 
to the 0-10 mg/L scale and adjust meter reading to approximately 8 
mg/L with the CALIBRATION control. Observe meter while aerating 
calibration water. When no change in DO reading on meter is 
observed for a period of 4-5 minutes the water can be assumed to 
be saturated.

6.3.4C Read true atmospheric pressure from pocket altimeter-
barometer to the nearest 5 mm of mercury.

6.3.5C Recheck the REDLINE and ZERO reading on the oxygen meter 
and adjust if necessary.

6.3.6C Measure temperature of the calibration water to the nearest 
0.5 deg. C.

6.3.7C Using oxygen the oxygen solubility table from section 7.2 
determine the DO saturation value at the measured calibration 
water temperature and true atmospheric pressure. Apply a salinity 
correction if appropriate (Note 7).

6.3.8C Select the scale (0-10 mg/L or 0-20 mg/L) to be used for 
the DO measurement. Turn off aerator and adjust CALIBRATION 
control until meter reads the DO saturation value determined in 
6.3.7C. Do not change scales without recalibrating meter or 
verifying that identical readings are obtained on both scales.

Note 9. It is essential that calibration be completed with the 
temperature of the calibration water at the value measured in 
6.3.6C, otherwise the calibration water may be undersaturated or 
oversaturated with oxygen. If the temperature changes or later 
recalibration of the meter is necessary the calibration procedure 
must be repeated beginning with step 6.3.2 C.

6.3.9C Remove probe from calibration container and place in water 
to be measured. Move probe through water at a velocity of at least 
1 foot per second (alternatively, a submersible stirrer, YSI Model 
5491, or equivalent may be used) to prevent localized depletion of 
DO at the membrane-water interface.

6.3.10C Switch meter to TEMPERATURE; after meter reading has 
stabilized record temperature to the nearest 0.5 deg. C.

6.3.11C Switch meter to the proper DO scale; after meter reading 
has stabilized record DO concentration to the nearest 0.1 mg/L.

6.3.1D Air Calibration Chamber in Air: This procedure is similar 
to the procedure described in 6.3.1A, except that calibration is 
performed with the air calibration chamber in air rather than in 
water. This procedure can only be used with probes that have the 
temperature sensing thermistor located adjacent to the membrane. 
The probe must be capable of automatically compensating for 
temperature changes. However, it is recommnded that this procedure 
not be used if the air temperature differs from the water 
temperature by more than 10 degrees. Also, the calibration chamber 
must be shielded from direct sunlight and wind to avoid large 
temperature fluctuations.

6.3.2D Wet the inside of the calibration chamber with water; pour 
out the excess water and insert the probe into the wet chamber. 
This insures 100 percent humidity. Allow sufficient time (10-15 
minutes) for the DO probe and air inside the calibration chamber 
to equilibrate. Observe precautions stated in 6.3.lD.

6.3.3D Read true atmospheric pressure from pocket altimeter-
barometer to the nearest 5 mm of mercury.

6.3.4D Recheck the REDLINE and ZERO readings on the oxygen meter 
and adjust if necessary.

6.3.5D Measure temperature in the calibration chamber to the 
nearest 0.5 deg. C using the thermistor in the DO probe.

6.3.6D Using oxygen the oxygen solubility table from section 7.2 
determine the DO saturation at the measured temperature and true 
atmospheric pressure. Apply a salinity correction if necessary. 
(Notes 7)

6.3.7D Select the scale (0-10 mg/L or 0-20 mg/L) to be used for 
the DO measurement and adjust CALIBRATION control until the meter 
reading is at the DO saturation value determined in 6.3.6D. Do not 
change scales without recalibrating meter or verifying that 
identical readings are obtained on both scales.

6.3.8D Remove probe from calibration chamber and place in water to 
be measured. Move probe through water at a velocity of at least 1 
foot per second (alternatively, a submersible stirrer, YSI Model 
5491, or equivalent may be used) to prevent localized depletion of 
DO at the membrane-water interface.

6.3.9D Switch meter to TEMPERATURE; after meter reading has 
stabilized record temperature to the nearest 0.5!C.
6.3.10D Switch meter to the proper DO scale; after meter reading 
has stabilized record DO concentration to the nearest 0.1 mg/L. 

7. Calculations

7.1 Read the concentration of dissolved oxygen in milligrams per 
liter directly from the meter of the instrument.

7.2 The following table of values for the solubility of oxygen in 
water exposed to water-saturated air should be used to calibrate 
the DO meter. The table may also be used to determine dissolved 
oxygen as percent saturation at the measured temperature and true 
atmospheric pressure as follows.

DO (percent saturation) = Measured DO (mg/L)/saturation DO (mg/L)  
x 100

8. Report 

8.1 Report dissolved oxygen concentrations to the nearest 
0.1 mg/L. 

8.2 Report dissolved oxygen as percent saturation to the nearest
percent.

9. Precision 

9.1 Precision data are not available for this method.

                          References

American Public Health Association and others, 1976, Standard 
methods for the examination of water and wastewater (14th ed.): 
New York, Am. Public Health Assoc., Inc., p. 446

Brown, Eugene, Skougstad, M. W. and Fishman, M. J., 1970, Methods 
for the Collection and Analysis of Water Samples for Dissolved 
Minerals and Gases: Techniques of Water-Resources Inv. of the U.S. 
Geological Survey, book 5, ch. Al, p. 129.

Green, E. J. and Carritt, D. E., 1967, New Tables for Oxygen 
Saturation of Seawater: Journal of Marine Research, p 140.