connector (1/2 in. male pipe, catalogue #2672, 

 Thomas and Beits, Elizabeth, New Jersey) which 

 screws into the 1-1/2 in. to 1/2 in. PVC reducing 

 bushing (Fig. 1) and firmly strain-relieves the heavy 

 cable. Teflon tape is used in all of the threaded con- 

 nections to assure a watertight seal. 



ELECTRONICS 



There are many options available in the power 

 supply and the readout circuitry used. That which 

 is described is offered only as a starting point but 

 provides a reasonably inexpensive, efficient unit. 



Fig. 2b indicates the circuitry used, all of which, 

 save the cable and the pressure transducer, is con- 

 tained in a cast aluminum box (BUD #CU-247) as 

 shown in Figs. 1a, 1c, and 1d. The unit is powered 

 by a DC source between 18 and 30 volts. This can 

 be from two to four 9-volt transistor batteries or 

 two larger 12-volt batteries. A DC power supply is 

 desirable for laboratory calibration since it saves 

 battery life. 



The DC source shown in Fig. 2b supplies a regu- 

 lator (LM 34015 National Semiconductor) which 

 maintains a constant 15 volts. This voltage is fur- 

 ther dropped by resistors R3 and R1 to vary the 

 offset voltage of the transducer against which the 

 output voltage of the pressure transducer — itself 

 attenuated by R2 — is measured. Any VOM multi- 

 meter of at least 20,000 ohms/volt will suffice to 

 measure V . Ten-turn small helipots are used for 

 R1 (2K) and R2 (5K). It is desirable to have either 

 the regulator voltage or the battery voltage read- 

 able by means of the two position switch S1. 



Calibration 



Fig. 2c is a curve showing several different ways 

 the device can be calibrated. The voltage measured 

 can accurately indicate (by shifting a decimal) either 

 millimeters of mercury pressure, inches of mercury 

 pressure, or percent saturation, referred to baro- 

 metric pressure (B); a reading of zero volts is equiva- 

 lent to ambient pressure. Approximate helipot dial 

 settings are given for R1 and R2 associated with 

 the three calibration graphs assuming B = 760 mm 

 Hg. These will vary slightly with the altitude and 

 ambient pressures prevailing during use, and it is 

 recommended that the instrument be thoroughly 

 calibrated with a number of different slope and 

 pressure settings prior to use. The pressure trans- 

 ducer itself is sensitive enough to act as a barom- 

 eter and/or altimeter. 



Maximum accuracy is obtained when dV/dP 

 is maximal; that is, maximum voltage per unit 

 pressure. In the experience with the unit thus far 

 it appears most feasible to calibrate in terms of 

 10 mV/mm of mercury so that millimeters of mer- 

 cury can be read directly by multiplying by 100. It 



is of course feasible to integrate into the unit a digi- 

 tal voltmeter for readout. However, this involves 

 considerably greater expense not only for the DVM 

 but also for the more sophisticated power supplies 

 to run it. Applications oriented toward monitoring 

 and telemetry over long time periods may justify 

 such further sophistication. 



Use of the Unit 



Since the measurement of supersaturation 

 always suffers from the uncertainty that a physically 

 unstable situation is being measured, several notes 

 of caution are in order for the use of these instru- 

 ments. As with other units, agitation is necessary 

 to remove bubbles from the silastic tubing to pro- 

 vide the pressure reading. However, when super- 

 saturation is not present, equilibrium occurs without 

 agitation within 8 min. Fig. 2d is a curve showing 

 the time course of pressure buildup in the tube 

 following immersion of the probe in approximately 

 30 ft of supersaturated water in the tower operated 

 by the National Marine Fisheries Service in Seattle. 

 The reading shown in Fig. 2d is 136% saturation and 

 required 7.5 min to reach this value, which was 

 reproduced on three consecutive measurements. It 

 is clear that if the depth of water which is to be 

 measured is at least 10 ft it should be possible to 

 measure supersaturations as high as 33% relative 

 to the surface without having to agitate the probe. 

 It appears that in most field situations it would be 

 sufficient to measure at a depth of 15 ft and, pro- 

 vided the assumption of vertical mixing was valid, 

 agitation should not be necessary. This should give 

 data of greater reliability than that afforded by 

 using existing units in a large sample of water. 

 Further, these units are not efficient because of the 

 large amount of tubing necessary to counteract 

 their large dead space. Since the tubing also acts 

 as growth sites for bubbles, adding more of it to 

 counteract a large dead volume is of questionable 

 benefit. Comparison of the present unit with that 

 commercially available on single samples of super- 

 saturated tap water has shown that the latter can 

 under-read by as much as 40% of the maximum 

 value shown by the unit described here. 



It is important in comparing these devices to 

 other quantitative modes of analysis such as the 

 Van Slyke and/or gas chromatography to keep in 

 mind that, where supersaturation exists in the water 

 being analyzed, tensionometers can underestimate 

 the total dissolved gas content and quantitative 

 methods can overestimate the dissolved gas tension 

 because of the possible presence of microbubbles. 

 Use of either measurement to estimate the other 

 with solubility tables (Weiss, 1970) must be inter- 

 preted with caution. It is also emphasized that the 

 only reliable measurements where supersaturation 



Electronic Monitor 109 



