2810 Revis editoriales 2011

2810 DISSOLVED GAS SUPERSATURATION*
2810 A. Introduction
Water can become supersaturated with atmospheric gases by
various means, heating and air entrainment in spilled or pumped
water being the most common. The primary sign of gas supersaturation is the formation of bubbles on submerged surfaces or
within the vascular systems and tissues of aquatic organisms.
Gas supersaturation can limit aquatic life and interfere with
water treatment processes. Levels of supersaturation lethal to
aquatic organisms have been found in springs, rivers, wells,
lakes, estuaries, and seawater. Gas supersaturation can be produced in pumped or processed water intended for drinking, fish
hatchery supply, and laboratory bioassays. Seasonal and other
temporal variations in supersaturation may occur in surface
waters as a result of solar heating and photosynthesis. Because
the rate of equilibration may be slow, supersaturation may persist
in flowing water for days and excessive dissolved gas levels thus
may persist far from the source of supersaturation.
Gas bubbles form only when the total dissolved gas pressure
is greater than the sum of compensating pressures. Compensating pressures include water, barometric and, for organisms,
tissue or blood pressure. The total dissolved gas pressure is equal
to the sum of the partial pressures of all the dissolved gases,
including water vapor. Typically, only nitrogen, oxygen, argon,
carbon dioxide, and water vapor pressures need to be considered
in most natural waters. Gas bubble disease, of fish or other
aquatic organisms, is a result of excessive uncompensated gas
pressure. A single supersaturated gas such as oxygen or nitrogen
may not necessarily result in gas bubble disease because bubble
formation depends largely on total dissolved gas pressure. The
degree of gas saturation should be described in terms of pressures rather than concentration or volume units.
* Approved by Standard Methods Committee, 2010. Editorial revisions, 2011.
Joint Task Group: 20th Edition—John E. Colt (chair), Larry E. Fidler, John O.
Jensen, John W. Sweeney, Barnaby J. Watten.
2810 B. Direct-Sensing Membrane-Diffusion Method
1. General Discussion
ment of choice will depend on the specific application. All these
instruments are portable so that data collection is completed in
the field.
Test the instrument for leaks according to the manufacturer’s
recommendation. Even a very small leak, difficult to detect and
locate, will result in useless data. Calibrate the pressure-measuring
device with a mercury manometer or certified pressure gauge. If a
manometer is used, include fresh mercury that flows freely in the
tubing. An alternative method for directly testing membrane-diffusion instruments in a small, closed chamber where induced ⌬P
levels can be compared against observed ⌬P levels is available.2
Van Slyke-Neill4 or gas chromatography methods1 are inappropriate for calibration but they may be used to verify results.
These methods measure individual gas concentrations and require further conversion to ⌬P or partial pressure and suffer from
sampling and sample handling problems.5–7
a. Principle: This method requires an instrument with a variable
length of “gas permeable” tubing, connected to a pressure-measuring device. Dimethyl silicone rubber tubing often is used because it
is highly permeable to dissolved gases, including water vapor. At
steady state, the gauge pressure inside the tubing is equal to the
difference in gas pressure (⌬P) between the total dissolved gas
pressure and the ambient barometric pressure. When the water is in
equilibrium with the atmosphere, ⌬P equals zero. If ⌬P is greater
than zero, the water is supersaturated. Conversely, if ⌬P is negative
the water is undersaturated.
b. Working range: The working range of this method depends
on the pressure-sensing device used, but typically will range
from ⫺150 to ⫹600 mm Hg. Dissolved solids in wastewater will
not interfere with this method. The practical depth range for
these instruments is 1 to 10 m.
2. Apparatus
3. Procedure
Several types of membrane-diffusion instruments are available
commercially.* Alternatively, construct a unit from commercially available parts. Several units have been described, including a direct-reading instrument using pressure transducers and a
digital readout,1 an on-line unit that can activate an alarm system,2 and an early model of the Weiss saturometer.3 Each of
these units has specific advantages and limitations; the instru-
At the start of each day, test the instrument for leaks and recalibrate. At a monitoring site, completely submerge the sensing element in the water, preferably below the hydrostatic compensation
depth. This is the depth where the hydrostatic and total gas pressures
are equal and as a result, bubbles will not form on the tubing.
Bubble formation on the silicone rubber tubing seriously reduces
accuracy. Compute hydrostatic compensation depth5 as follows:
Z⫽
* Common Sensing, Clark Fork, ID; Eco Enterprises, Seattle, WA; Novatech,
Vancouver, BC, Canada; and Sweeney Aquametrics, Stony Creek, CT.
1
⌬P
73.42
DISSOLVED GAS SUPERSATURATION (2810)/Membrane-Diffusion Method
TGP % ⫽
冋
册
P b ⫹ ⌬P
⫻ 100
Pb
where:
Pb ⫽ true local barometric pressure, mm Hg.
The reporting of total gas pressure as a percentage is not
encouraged.8
b. Component gas pressures: When information on component gas supersaturation is needed, express data as partial pressures, differential pressures, or percent saturation.5,8 This requires additional measurements of dissolved oxygen, temperature, and salinity† at the monitoring site. In a mixture of gases in
a given volume, the partial pressure of a gas is the pressure that
this gas would exert if it were the only gas present.
1) Oxygen partial pressure—Calculate partial pressure of oxygen as follows:
Figure 2810:1 Time response for the membrane-diffusion method.
P O2 ⫽
DO
⫻ 0.5318
␤ O2
where:
where:
PO2 ⫽ partial pressure of dissolved oxygen, mm Hg,
␤O 2 ⫽ Bunsen coefficient for oxygen (Table 2810:I), L/(L 䡠 atm), and
DO ⫽ measured concentration of oxygen, mg/L.
Z ⫽ hydrostatic compensation depth, m, and
⌬P ⫽ pressure difference between total dissolved gas pressure
and the ambient barometric pressure, mm Hg.
The factor 73.42 is the hydrostatic pressure of fresh water at
20°C expressed in terms of mm Hg/m water depth. Because the
variation of hydrostatic pressure with temperature and salinity is
small, this equation can be used for all natural waters.
Dislodge formed bubbles on the tubing by gently striking the
instrument or moving the instrument rapidly in the water. Movement of water across the silicone rubber tubing also facilitates
establishing the equilibrium between gas pressure in the water
and in the tubing.
Operate the instrument “bubble free” until a stable ⌬P is
observed. This may take from 5 to 30 min, depending on the ⌬P,
water temperature, water flow, and geometry of the system. The
time response of the membrane-diffusion method is shown in
Figure 2810:1 for “bubble-free” and “bubble” conditions.
If the instrument is used in heavily contaminated water containing oil or other organic compounds, clean the silicone tubing
with a mild detergent according to the manufacturer’s instructions. Silicone rubber tubing has been used in uncontaminated
natural water for at least eight years without being adversely
affected by attached algal growth.2 The tubing can be damaged
by abrasive grit, diatoms, biting aquatic organisms, certain organic compounds, and strong acids.2
Obtain the barometric pressure with each measurement by
using a laboratory mercury barometer, a calibrated portable
barometer, or pressure transducer. Barometric pressures reported
by weather agencies (or airports) are corrected to sea level and
are unusable.
Bunsen coefficients for marine waters are available.5 The
factor 0.5318 equals 760/(1000 K), where K is the ratio of
molecular weight to molecular volume for oxygen gas.5
2) Nitrogen partial pressure—Estimate the partial pressure of
nitrogen by subtracting the partial pressures of oxygen and water
vapor from the total gas pressure.
4. Calculation
and the nitrogen differential pressure as
P N2 ⫽ Pb ⫹ ⌬P ⫺ P O2 ⫺ P H2O
where:
PH 2O ⫽ vapor pressure of water in mm Hg from Table 2810:II.
This term includes a small contribution from argon and any
other gases present, including carbon dioxide and methane. The
partial pressure of carbon dioxide is negligible in natural waters
of pH ⬎7.0.
3) Nitrogen:oxygen partial pressure ratio—The ratio of the
partial pressure of nitrogen to the partial pressure of oxygen
(N2:O2) characterizes the relative contribution of the two gases
to the total dissolved gas pressure. In water in equilibrium with
air, this ratio is 3.77.
c. Differential pressures: The differential pressure of a gas is
the difference between the partial pressures of that gas in water
and air. The oxygen differential pressure may be calculated as
⌬P O2 ⫽ P O2 ⫺ 0.20946(P b ⫺ P H2O)
⌬P N2 ⫽ ⌬P ⫺ ⌬P O2
a. Total gas pressure: Preferably report total gas pressure as
⌬P.2,6,8 Express pressure as millimeters of mercury.
Total gas pressure also has been reported as a percentage of
local barometric pressure:
† Methods for these variables may be found in Sections 4500-O, 2550, and 2520,
respectively.
2
DISSOLVED GAS SUPERSATURATION (2810)/Membrane-Diffusion Method
TABLE 2810:I. BUNSEN COEFFICIENT
Temperature
°C
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
FOR
OXYGEN
IN
FRESH WATER
Bunsen Coefficient at Given Temperature (to nearest 0.1°C)
L real gas at STP/(L 䡠 atmosphere)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.04914
0.04780
0.04653
0.04531
0.04414
0.04303
0.04197
0.04096
0.03999
0.03906
0.03817
0.03732
0.03651
0.03573
0.03498
0.03426
0.03358
0.03292
0.03228
0.03168
0.03109
0.03053
0.02999
0.02947
0.02897
0.02850
0.02803
0.02759
0.02716
0.02675
0.02635
0.02597
0.02561
0.02525
0.02491
0.02458
0.02426
0.02396
0.02366
0.02338
0.02310
0.04901
0.04767
0.04640
0.04519
0.04403
0.04292
0.04187
0.04086
0.03989
0.03897
0.03809
0.03724
0.03643
0.03565
0.03491
0.03419
0.03351
0.03285
0.03222
0.03162
0.03103
0.03048
0.02994
0.02942
0.02893
0.02845
0.02799
0.02755
0.02712
0.02671
0.02632
0.02594
0.02557
0.02522
0.02488
0.02455
0.02423
0.02393
0.02363
0.02335
0.02308
0.04887
0.04754
0.04628
0.04507
0.04392
0.04282
0.04177
0.04076
0.03980
0.03888
0.03800
0.03716
0.03635
0.03558
0.03448
0.03412
0.03344
0.03279
0.03216
0.03156
0.03098
0.03042
0.02989
0.02937
0.02888
0.02840
0.02794
0.02750
0.02708
0.02667
0.02628
0.02590
0.02553
0.02518
0.02484
0.02452
0.02420
0.02390
0.02360
0.02332
0.02305
0.04873
0.04741
0.04615
0.04495
0.04381
0.04271
0.04166
0.04066
0.03971
0.03879
0.03791
0.03707
0.03627
0.03550
0.03476
0.03406
0.03338
0.03272
0.03210
0.03150
0.03092
0.03037
0.02983
0.02932
0.02883
0.02835
0.02790
0.02746
0.02704
0.02663
0.02624
0.02586
0.02550
0.02515
0.02481
0.02448
0.02417
0.02387
0.02358
0.02329
0.02302
0.04860
0.04728
0.04603
0.04484
0.04369
0.04260
0.04156
0.04056
0.03961
0.03870
0.03783
0.03699
0.03619
0.03543
0.03469
0.03399
0.03331
0.03266
0.03204
0.03144
0.03086
0.03031
0.02978
0.02927
0.02878
0.02831
0.02785
0.02742
0.02700
0.02659
0.02620
0.02582
0.02546
0.02511
0.02478
0.02445
0.02414
0.02384
0.02355
0.02327
0.02300
0.04847
0.04716
0.04591
0.04472
0.04358
0.04250
0.04146
0.04047
0.03952
0.03861
0.03774
0.03691
0.03611
0.03535
0.03462
0.03392
0.03324
0.03260
0.03198
0.03138
0.03081
0.03026
0.02973
0.02922
0.02873
0.02826
0.02781
0.02737
0.02695
0.02655
0.02616
0.02579
0.02543
0.02508
0.02474
0.02442
0.02411
0.02381
0.02352
0.02324
0.02297
0.04833
0.04703
0.04579
0.04460
0.04347
0.04239
0.04136
0.04037
0.03943
0.03852
0.03766
0.03683
0.03604
0.03528
0.03455
0.03385
0.03318
0.03253
0.03192
0.03132
0.03075
0.03020
0.02968
0.02917
0.02868
0.02822
0.02777
0.02733
0.02691
0.02651
0.02612
0.02575
0.02539
0.02504
0.02471
0.02439
0.02408
0.02378
0.02349
0.02321
0.02294
0.04820
0.04680
0.04567
0.04449
0.04336
0.04229
0.04126
0.04027
0.03933
0.03843
0.03757
0.03675
0.03596
0.03520
0.03448
0.03378
0.03311
0.03247
0.03186
0.03126
0.03070
0.03015
0.02963
0.02912
0.02864
0.02817
0.02772
0.02729
0.02687
0.02647
0.02609
0.02571
0.02536
0.02501
0.02468
0.02436
0.02405
0.02375
0.02346
0.02318
0.02292
0.04807
0.04678
0.04555
0.04437
0.04325
0.04218
0.04116
0.04018
0.03924
0.03835
0.03749
0.03667
0.03588
0.03513
0.03441
0.03371
0.03305
0.03241
0.03180
0.03121
0.03064
0.03010
0.02958
0.02907
0.02859
0.02812
0.02768
0.02725
0.02683
0.02643
0.02605
0.02568
0.02532
0.02498
0.02465
0.02433
0.02402
0.02372
0.02343
0.02316
0.02289
0.04793
0.04665
0.04543
0.04426
0.04314
0.04206
0.04106
0.04008
0.03915
0.03826
0.03741
0.03659
0.03581
0.03505
0.03433
0.03364
0.03298
0.03235
0.03174
0.03115
0.03059
0.03004
0.02952
0.02902
0.02854
0.02808
0.02763
0.02720
0.02679
0.02639
0.02601
0.02564
0.02529
0.02494
0.02461
0.02429
0.02399
0.02369
0.02341
0.02313
0.02286
Based on Benson and Krause.9,10 ␤ ⫽ 9.9902 ⫻ 10⫺4[exp(9.7265 ⫺ 5.26895 ⫻ 103/T ⫹ 1.00417 ⫻ 106/T2)], where T ⫽ 273.15 ⫹ °C.
TGP(%) ⫽ 0.20946 O2(%) ⫹ 0.7902 N2(%)
O 2(%)
⌬P ⫽ 0.20946
⫺ 1 [P b ⫺ P H2O]
100
d. Percent of saturation: In older literature, supersaturation
values have been reported as percent saturation. This method of
reporting component gases is discouraged but can be calculated
as follows:
N 2(%) ⫽
O 2共%兲 ⫽
冋
冋
P N2
册
0.7902 (P b ⫺ P H2O)
P O2
0.20946(P b ⫺ P H2O)
冋
⫹ 0.7902
⫻ 100
册
⌬P ⫽
冋
册
册
N 2(%)
⫺ 1 [P b ⫺ P H2O]
100
DO
(0.5318)(1 ⫹ N 2:O 2) ⫺ (P b ⫺ P H2O)
␤O2
⫻ 100
Use care with these relationships with older data because both
TGP(%) and N2(%) have been differently defined.5
The following relationships are useful conversions:
3
DISSOLVED GAS SUPERSATURATION (2810)/Membrane-Diffusion Method
TABLE 2810:II. VAPOR PRESSURE
Temperature
°C
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
OF
FRESH WATER
Vapor Pressure at Given Temperature (to nearest 0.1°C)
mm Hg
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
4.58
4.92
5.29
5.68
6.09
6.54
7.01
7.51
8.04
8.60
9.20
9.83
10.51
11.22
11.98
12.78
13.62
14.52
15.46
16.46
17.52
18.64
19.81
21.05
22.36
23.74
25.19
26.72
28.33
30.03
31.81
33.68
35.65
37.71
39.88
42.16
44.55
47.05
49.67
52.43
55.31
4.61
4.96
5.33
5.72
6.14
6.58
7.05
7.56
8.09
8.66
9.26
9.90
10.58
11.29
12.05
12.86
13.71
14.61
15.56
16.57
17.63
18.75
19.93
21.18
22.50
23.88
25.34
26.88
28.50
30.80
31.99
33.87
35.85
37.92
40.10
42.39
44.79
47.31
49.94
52.71
55.60
4.64
4.99
5.36
5.76
6.18
6.63
7.10
7.61
8.15
8.72
9.32
9.97
10.65
11.37
12.13
12.94
13.80
14.70
15.66
16.67
17.74
18.87
20.05
21.31
22.63
24.03
25.49
27.04
28.66
30.37
32.17
34.06
36.05
38.14
40.33
42.63
45.04
47.56
50.21
52.99
55.90
4.68
5.03
5.40
5.80
6.22
6.67
7.15
7.66
8.20
8.87
9.39
10.03
10.72
11.44
12.21
13.05
13.89
14.80
15.76
16.77
17.85
18.98
20.48
21.44
22.77
24.17
25.64
27.20
28.83
30.55
32.36
34.26
36.25
38.35
40.55
42.86
45.28
47.82
50.49
53.28
56.20
4.71
5.07
5.44
5.84
6.27
6.72
7.20
7.71
8.26
8.84
9.45
10.10
10.76
11.52
12.29
13.11
13.97
14.89
15.86
16.88
17.96
19.10
20.60
21.57
22.90
24.31
25.80
27.36
29.00
30.73
32.54
34.45
36.46
38.57
40.78
43.10
45.53
48.08
50.76
53.56
56.50
4.75
5.10
5.48
5.88
6.31
6.77
7.25
7.77
8.31
8.89
9.51
10.17
10.86
11.59
12.37
13.19
14.06
14.98
15.96
16.98
18.07
19.22
20.42
21.70
23.04
24.46
25.95
27.52
29.17
30.91
32.73
34.65
36.67
38.78
41.01
43.34
45.78
48.35
51.03
53.85
56.80
4.78
5.14
5.52
5.92
6.36
6.81
7.30
7.82
8.37
8.95
9.58
10.23
10.93
11.67
12.45
13.28
14.15
15.08
16.06
17.09
18.18
19.33
20.55
21.83
23.18
24.60
26.10
27.68
29.34
31.08
32.92
34.85
36.87
39.00
41.23
43.58
46.03
48.61
51.31
54.14
57.10
4.82
5.18
5.56
5.97
6.40
6.86
7.35
7.87
8.43
9.02
9.64
10.30
11.00
11.74
12.53
13.36
14.24
15.17
16.16
17.20
18.29
19.45
20.67
21.96
23.32
24.75
26.26
27.84
29.51
31.26
33.11
35.05
37.08
39.22
41.46
43.82
46.29
48.87
51.59
54.43
57.41
4.85
5.21
5.60
6.01
6.44
6.91
7.40
7.93
8.48
9.08
9.70
10.37
11.07
11.82
12.61
13.45
14.33
15.27
16.26
17.30
18.41
19.57
20.80
22.09
23.46
24.90
26.41
28.00
29.68
31.44
33.30
35.24
37.29
39.44
41.69
44.06
46.54
49.14
51.87
54.72
57.71
4.89
5.25
5.64
6.05
6.49
6.96
7.45
7.98
8.54
9.14
9.77
10.44
11.15
11.90
12.69
13.54
14.43
15.37
16.36
17.41
18.52
19.69
20.93
22.23
23.60
25.04
26.57
28.17
29.85
31.62
33.49
35.44
37.50
39.66
41.92
44.30
46.79
49.41
52.14
55.01
58.02
Based on an equation presented by Green and Carritt.11 This equation is cumbersome to use. The following equation9 is adequate for most applications:
PH 2O ⫽ 760[exp(11.8571 ⫺ 3,840.70/T ⫺ 216,961/T2)], where T ⫽ 273.15 ⫹ °C.5
5. Quality Control
6. Reporting of Results
The quality control practices considered to be an integral part
of each method are summarized in Tables 2020:I and II.
The precision of the membrane-diffusion method depends
primarily on the pressure-sensing instrument. For an experienced
operator it is approximately ⫾1 to 2 mm Hg with an accuracy of
⫾3 to 5 mm Hg.3,6 Air leaks, bubble formation, biofilm development, incomplete equilibration, or condensation produce negative errors while direct water leaks can result in positive errors
in submersible units.
For accurate work, measure water temperature to the nearest
⫾0.1°C.
In reporting results, include the following data:
• Sensor depth, m,
• Barometric pressure, mm Hg,
• Water temperature, °C,
• Dissolved oxygen, mm Hg or mg/L,
• Salinity, g/kg, and
• ⌬P, mm Hg.
If component gas information is needed add:
• Partial pressure of oxygen, mm Hg,
• Partial pressure of nitrogen, mm Hg, and
4
DISSOLVED GAS SUPERSATURATION (2810)/Membrane-Diffusion Method
• Nitrogen:oxygen partial pressure ratio
2. BOUCK, G.R. 1982. Gasometer: an inexpensive device for continuous monitoring of dissolved gases and supersaturation. Trans. Amer.
Fish. Soc. 111:505.
3. FICKEISEN, D.H., M.J. SCHNEIDER & J.C. MONTGOMERY. 1975. A
comparative evaluation of the Weiss saturometer. Trans. Amer.
Fish. Soc. 104:816.
4. BEININGEN, K.T. 1973. A Manual for Measuring Dissolved Oxygen and Nitrogen Gas Concentrations in Water with the Van
Slyke-Neill Apparatus. Fish Commission of Oregon, Portland.
5. COLT, J. 1984. Computation of dissolved gas concentrations in water
as functions of temperature, salinity, and pressure. Spec. Publ. 14,
American Fisheries Soc., Bethesda, Md.
6. D’AOUST, B.G. & M.J.R. CLARK. 1980. Analysis of supersaturated
air in natural waters and reservoirs. Trans. Amer. Fish. Soc. 109:
708.
7. PIRIE, W.R. & W.A. HUBBERT. 1977. Assumptions in statistical
analysis. Trans. Amer. Fish. Soc. 106:646.
8. COLT, J. 1983. The computation and reporting of dissolved gas
levels. Water Res. 17:841.
9. BENSON, B.B. & D. KRAUSE. 1980. The concentration and isotopic
fractionation in freshwater in equilibrium with the atmosphere. 1.
Oxygen. Limnol. Oceanogr. 25:662.
10. BENSON, B.B. & D. KRAUSE. 1984. The concentration and isotopic
fractionation of oxygen dissolved in freshwater and seawater in
equilibrium with the atmosphere. Limnol. Oceanogr. 29:620.
11. GREEN, E.J. & D.E. CARRITT. 1967. New tables for oxygen saturation
of seawater. J. Mar. Res. 25:140.
12. WEITKAMP, D.E. & M. KATZ. 1980. A review of dissolved gas
supersaturation literature. Trans. Amer. Fish. Soc. 109:659.
13. COLT, J. 1986. The impact of gas supersaturation on the design
and operation of aquatic culture systems. Aquacult. Eng. 5:49.
or
• ⌬PO2, mm Hg, and
• ⌬PN2, mm Hg.
7. Interpretation of Results
The biological effects of dissolved gas supersaturation depend on the
species, age, depth in water column, length of exposure, temperature,
and nitrogen:oxygen partial pressure ratio.12 Safe limits generally are
segregated into wild/natural circumstances, where behavior and hydrostatic pressure can modify the exposure by horizontal and vertical
movements away from dangers, and captive environments such as
aquaria, hatcheries, or laboratories, where conditions not only preclude
escape but also include other significant stresses. Of these two realms,
captive circumstances are more likely to cause illness or mortality from
gas bubble disease and will do so sooner and at the lower ⌬P levels.
In wild/natural circumstances, the limit of safe levels of gas
supersaturation depends on the depth available to the species
and/or species behavior, but this limit usually occurs at a ⌬P
between 50 and 150 mm Hg. Under captive conditions, the ⌬P
should be as close to zero as possible. For sensitive species and
life stages, sublethal and lethal effects have been observed at ⌬P
of 10 to 50 mm Hg.13
8. References
1. D’AOUST, B.G., R. WHITE & H. SIEBOLD. 1975. Direct measurement
of total dissolved gas partial pressure. Undersea Biomed. Res.
2:141.
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