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4.0 REPORTING AND MEASUREMENT OF DISSOLVED GAS LEVELS

4.1 Reporting of Dissolved Gas Levels

Throughout the literature dealing with DGS and GBT in fish, there are a variety of methods used for calculating and reporting dissolved gas levels. Colt (1983, 1984, and 1986) presents a detailed analysis of these methods along with derivations of many of the equations used in computing dissolved gas tensions. The following is a brief summary of the more common methods.

The level of dissolved gas tension in water is most often expressed in terms of the total gas pressure, defined as:

In this relationship, pN2 includes the partial pressure of argon and all other trace atmospheric gases.

Another common form of expressing dissolved gas tension is in terms of the percent total gas pressure, defined as:

When the sum of the partial pressures of all dissolved gases, including water vapour, exceeds atmospheric pressure, the water is supersaturated with dissolved gases (i.e., TGP is greater than pAtm or TGP% is greater than 100%).

Alternatively, dissolved gas tensions can be expressed as the difference between TGP and atmospheric pressure. This difference is defined as the delta P and is given by:

Again, pN2 includes argon and other trace atmospheric gases. When delta P is greater than 0.0 mm Hg, water is supersaturated with dissolved gases.

There is a distinct advantage to using delta P rather than TGP or TGP% as a measure of water dissolved gas tension. The principal advantage comes when assessing the impacts of DGS on aquatic and marine organisms. The major signs of GBT occur as a result of dissolved gases moving from the water phase into intra-corporeal and extra-corporeal bubbles and hollow organs of aquatic and marine organisms. This movement of gases is driven by water delta P (Colt 1984 and 1986, Fidler 1984 and 1988). Furthermore, before bubble growth or over-inflation of hollow organs can begin, a threshold delta P must be exceeded (Fidler 1984 and 1988, Shrimpton et al. 1990a and b). Thus delta P is a direct measure of the potential for aquatic and marine organisms to develop signs of GBT. Another advantage to using delta P is that it can be measured directly with the proper instrumentation (Bouck 1982). On the other hand, TGP or TGP% must be established by measuring delta P or TGP as well as barometric pressure and then calculating TGP or TGP% from the following relationships.

or

The reporting of dissolved gas levels in terms of TGP or TGP% is complicated by the variation of atmospheric pressure with altitude. This is not a problem when dissolved gas tensions are reported in terms of delta P. Figure 7 shows this effect in terms of the thresholds for various signs of GBT. The basis for these thresholds will be examined later in Section 6.1. In the figure it is seen that the thresholds for the GBT signs occur at specific delta P values. Yet, if TGP% is used as a measure of dissolved gas tension, each threshold will vary with altitude. A similar effect is seen when TGP is used for reporting dissolved gas tension.

In the sections and appendices which follow, historical data are presented on DGS levels in various water bodies of Canada and the United States. Many of these data were originally reported in terms of TGP or TGP%, but without corresponding barometric pressure or altitude information. As a result, these data cannot be accurately converted to delta P values. Where, for reference purposes, it is desirable to convert these data to delta P levels, the values cited are only approximate.

4.2 Measurement of Dissolved Gases

Direct measurement of TGP or delta P is the preferred method for determining total dissolved gas tension. Instruments which provide this measurement are described by Fickeisen et al. (1975), D'Aoust et al. (1976), and Bouck (1982). As a class, they have been called "Weiss Saturometers". However, as Colt (1983) indicated, the term is a misnomer since they measure either total gas pressure or delta P and not saturation values. Each instrument has distinct advantages and limitations; however, all employ a gas permeable membrane as the primary mechanism for isolating dissolved gases and water vapour from liquid water. Each instrument consists of a gas permeable silicone rubber tube connected to a pressure measuring device (e.g., a manometer or pressure transducer). The instruments are commercially available from ECO Enterprises, Seattle, Washington; Par All, Salem, Oregon; Common Sensing, Clark Fork, Idaho; and Novatech Designs Limited, Victoria, BC Some of the more sophisticated instruments combine a TGP probe, oxygen electrode, barometer, and temperature transducer into a single instrument with extended data recording, analysis, and transmission capabilities (Common Sensing, Clark Fork, Idaho).

Depending on how the instrument is designed, the dissolved gas parameter measured is either delta P or TGP. If the pressure measuring device is a manometer, the parameter measured is delta P (i.e., the difference between TGP and barometric pressure). If the pressure measuring device is an electronic pressure transducer which has been calibrated to absolute pressure, the parameter measured is TGP. In order to obtain delta P, an independent measurement of barometric pressure is required. On the other hand, if the pressure measuring device is a pressure transducer which has been set to zero at the prevailing barometric pressure, the parameter measured is delta P at the calibration barometric pressure. Measurements made with this type of instrument must be corrected for changes in barometric pressure.

In order to distinguish dissolved oxygen tension from other dissolved gas components, an independent measurement of dissolved oxygen is required. This is usually done with an electronic oxygen probe or by titration methods described in Standard Methods for the Examination of Water and Wastewater (APHA/AWWA/WEF 1992).

Figure 7: Variation of TGP% with Delta P and Altitude

Variation of TGP% with Delta P and Altitude

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