I 



B. G. D'Aoust 

 R. White 

 H. Seibold 



An Electronic 



Monitor 



for Total Dissolved Gas Pressure 



ABSTRACT 



The environmental and biomedical problem of supersaturation 

 of dissolved gas and the research related to it has produced a 

 need for a more efficient means of measuring and monitoring 

 total dissolved gas pressure than those now in use. A modification 

 of the Weiss saturometer is described which equilibrates within 

 8 min, is portable and can be operated remotely in a recording 

 mode. The basic unit is inexpensive, easily constructed out of 

 available components and allows many options in design so 

 that such units can be custom-made to specific needs. It has 

 been field-tested and is currently in use. 



The environmental problem of supersaturation of 

 rivers, lakes and streams due to naturally or man- 

 induced excess runoff and/or temperature increases 

 has created an urgent need for a less expensive 

 and more efficient and reliable means of monitoring 

 total dissolved gases then is presently available. In 

 this paper we describe the design and function of 

 a basic instrument which involves several modifi- 

 cations of the original Weiss saturometer which has 

 been used and evaluated for the past 3 yr in the 

 northwestern United States. It is based on the use 

 of a watertight plastic tube across which dissolved 

 gases exchange according to their partial pressures 

 and the total pressure is measured manometrically 

 (as described by Enns et al., 1965, in studies of the 

 effect of hydrostatic pressure on dissolved gases). 

 We have replaced the large dead-space Bourden 

 tube gauge with a low dead-space solid state elec- 

 tronic pressure transducer to facilitate both porta- 

 bility and remote monitoring. This ability to monitor 

 a particular location inexpensively should provide 

 more useful and meaningful information at much 

 less public and private expense. 



PRINCIPLE OF OPERATION 



Since the device measures total dissolved gas 

 tension or partial pressure, we propose the term 

 "tensionometer" as a more accurate description of 

 what is actually being measured. The basic prin- 

 ciple uses the fact that most gases and vapors diffuse 

 rather quickly through thin layers of most plastics, 

 one of which (silicon rubber in the form of Dow- 

 Corning medical grade silastic® tubing, catalogue 



#602-105) is particularly suitable for this application 

 since its small diameter provides optimal surface 

 area/volume ratios across a 0.006 in. thickness. 

 With one end of a length of tubing blinded and the 

 other communicating with the diaphragm or sensi- 

 tive element of a low dead-space pressure trans- 

 ducer, the pressure of all gases and vapors in the 

 water contacting the tubing will, at equilibrium, be 

 measured. A most important consideration in 

 design, then, is elimination of dead-space. This 

 involves both the choice of the appropriate pressure 

 transducer and the design of a low dead-space 

 connection of the tubing to the pressure transducer. 



DESIGN 



The unit described has been designed for 

 minimal expense, simplicity of fabrication, and 

 ease of operation. Its basic design also allows 

 individual modifications according to the needs 

 and/or preferences of the user. 



Fig. 1a indicates the complete parts and mate- 

 rials necessary for construction of the unit. Cata- 

 logue sources and numbers are given as the part is 

 described. 



Pressure Transducer and Probe Assembly 



A National Semiconductor® LX 1601 AF inte- 

 grated-circuit pressure transducer which measures 

 from 10 psia to 20 psia appears ideal for the range 

 in dissolved gas pressure likely to be encountered 

 in the field. However, there are many additional 

 models and makes to choose from. The pressure 

 transducer is shown (Fig. 1b) mounted on a 3/16 in. 

 brass Swagelock® O-seal connector fitting (B-300- 

 1-20R) using two O-rings (Parker 1/16 in. -008) 

 instead of the metal ferrules which are normally 

 used. The Swagelock fitting (Fig. 2a) is adapted to 

 the pressure transducer nipple diameter by drilling 

 with a #7 (0.201) drill. In addition it is advisable, 



D'Aoust, White and Seibold: Virginia Mason Research Center, 

 Seattle, Washington. 



706 



