WATER QUALITY--New tables of dissolved oxygen saturation values
In Reply Refer To: May 8, 1981
EGS-Mail Stop 412
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 81.11
Subject: WATER QUALITY--New tables of dissolved oxygen saturation
values
Quality of Water Branch Technical Memorandum No. 79.10 described
approved methods for measuring dissolved oxygen (DO) using an
instrument system with a polarographic-type probe. Included with
the method description were tables of saturation DO values in
distilled water as a function of temperature and barometric
pressure as well as tables of salinity correction factors with
which the user can correct the saturation DO values for the effect
of dissolved solids having knowledge of the specific conductance
and temperature of the water being measured.
The Branch has received numerous requests for new original tables
as the quality of copies from the originals has deteriorated. In
response to these requests, a computer program, K321A, and
catalogued procedure, DOTABLES, have been prepared and placed on
file for general use. This procedure will print tables of
saturation DO values and/or salinity correction factors over
ranges of temperature, pressure, and specific conductance selected
by the user. The maximum temperature is 40!C and maximum specific
conductance is 76,000 umhos'.'
To execute procedure DOTABLES the user should submit a job request
similar to the example shown below. The proper user name, ID, and
account number should be inserted where appropriate. The procedure
requires one input data card for each table requested. There is no
maximum limit to the number of data cards that can be submitted. A
class D run for several tables is less than $2.00.
Sample card deck
//useridxx JOB (accountno,K321A,5,5),'username',
// CLASS=D
//PROCLIB DD DSN=WRD.PROCLIB,DISP=SHR
// EXEC DOTABLES
//SYSIN DD *
data cards; 1 card needed for each table desired
//
Data-card format
In column 10 code the type of table desired (required)
1 dissolved oxygen saturation table to 0.1 mg/L output
2 dissolved oxygen saturation table to 0.01 mg/L output
3 salinity correction factors table
For table types l or 2 these options are available:
(Note: Options must be right justified or include a decimal
point.)
Columns 11-20, starting barometric pressure in mm Hg.
If left blank, table will start at 760 mm.
Columns 21-30, barometric pressure increment in mm Hg.
If left blank, the pressure will decrease in increments of 10 mm.
Columns 31-40, starting temperature in degrees C.
If left blank, the table will start at 0 degrees C.
Columns 41-50, temperature increment in degrees C.
If left blank, the temperature will increase in increments of
1 degree C.
The table will include 31 temperature increment lines and 20
pressure increment columns. The default table goes from 0 degrees
C to 30 degrees C and 760 mm to 570 mm.
For table type 3 these options are available:
(Note: Options must be right justified or include a decimal
point.)
Columns 11-20, starting specific conductance in umho/cm @
25 degrees C. If left blank, table will start at O umho.
Columns 21-30, specific conductance increment in umho/cm @
25 degrees C. If left blank, the table will increase in
increments of 2,000 umhos.
Columns 31-40, starting temperature, same as type 1 or 2 above.
Columns 41-50, temperature increment, same as type 1 or 2 above.
The table will include 31 temperature increment lines and 17
specific conductance increment columns. The default table goes
from 0 degrees C to 30 degrees C and O umho to 32,000 umhos.
This is the first time that saturation DO values listed to the
second decimal place have been made available by the Branch. This
is done in anticipation of future developments in instrumentation
which may make DO measurements with precision +/- .01 mg/L
possible, at which time saturation DO values accurate in the
second decimal place will be needed to take full advantage of that
instrumentation. At the present time we cannot justify reporting
of DO values to that level of accuracy.
Many readers will recognize that the saturation DO tables in Memo
79.10 were calculated by Whipple and Whipple (1911, J. Am. Chem.
Soc., 33:362-365) from gas absorption measurements by Fox (1909,
Trans: Faraday Soc., 5:68-87). These values were accepted as the
standards by water agencies for decades. However, they were shown
in the 1960's to be inaccurate by as much as 0.14 mg/L. For this
reason, the values available from the catalogued procedure,
DOTABLES, are based on an equation by Weiss (1970, Deep-Sea Res.
17:721-735) which fits the data by Carpenter (1966, Limnol.
Oceanogr. 11:264-277) with a maximum difference of -0.04 mg/L.
Carpenter's values are, at the present time, widely accepted as
the most accurate determinations of saturation DO available. For
complete consistency, both saturation tables obtained from the
DOTABLES procedure are calculated from Weiss' equation, and the
salinity correction factors are obtained from a modification of
Weiss' equation. The derivations follow:
The equation by Weiss reads:
ln DO = Al + A2 100/T + A3 ln T/1OO + A4 T/1OO (1)
+ S [B1 + B2 T/100 + B3 (T/100)2]
where
Al = -173.4292 T = temperature degrees K
A2 = 249.6339 (273.15 + t degrees C)
A3 = 143.3483
A4 = - 21.8492 S = salinity in g/kg (o/oo)
Bl = - 0.033096
B2 = 0.014259
B3 = - 0.001700
_______________________________________________________________
The DO solubility is calculated in milliliters per liter
(ml/L) which must be multiplied by the constant 1.4276 to
convert to milligrams per liter (mg/L).
With salinity (S) equal to zero, the second factor on the right
side drops out leaving an equation for saturation DO in distilled
water. The barometric pressure correction is calculated by the
usual equation
DO' = D0! (P-u/760-u) (2)
where DO' is the saturation DO at barometric pressure P, D0! is
saturation DO at barometric pressure 760 mm Hg, and u is the vapor
pressure of water calculated from an empirical equation derived
from the Handbook of Chemistry and Physics (Chemical Rubber
Company,(Cleveland, Ohio, 1964)
log u = 8.10765 - (1750.286/ (235+t)) (3)
where t is temperature in degrees C.
From equation (1), we may write two equations for DO in the
exponential form obtaining D0! at S = O and D!s at salinity S.
Dividing DOs by D0! we obtain an expression for the salinity
correction factor identical to the second factor on the right side
of equation(l) in the exponential form. To express the salinity
correction factor in terms of specific conductance, an approximate
relation as follows (4) is substituted into the equation,
Salinity (o/oo) = 5.572 x 10-4 (SC) + 2.02 x 10-9 (SC)2 (4)
where SC is the specific conductance in micromhos/cm. The error in
using the approximate relation (4) as opposed to a more complex
but accurate one is, at most, 8% in the low salinity range. This
error affects the correction factor in the fourth place and is not
a significant error for most work.
Comparison of tables from the DOTABLES procedure and those in Memo
79.10 will show differences of as much as 0.1 mg/L in the
saturation DO values and of 0.004 in the salinity correction
values. For most present-day routine field work, these differences
are not significant. Thus the tables in Memo 79.10 are still
accepted as valid for routine use. But the Branch encourages a
gradual replacement of those tables with the ones available from
the DOTABLES procedure.
As a further reminder, the tables in Memo 79.10 or those that are
available from the DOTABLES procedure are the only ones approved
by the Geological Survey for routine field use. Tables provided by
instrument manufacturers or from any other source should not be
used unless they are the same as the USGS tables.
Any questions or comments regarding the dissolved oxygen
saturation tables or salinity correction factor tables should be
directed to Wes Bradford of my staff.
R. J. Pickering
Chief, Quality of Water Branch
WRD Distribution: A, B, S, FO, PO
Key words: Water quality, data handling, dissolved oxygen.
This memorandum expands upon QW Branch Technical Memorandum
No. 79.10.