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.