Figure 11. --Probes with 

 cofferdam and canvas in 

 place . 



switch. The thermocouple temperatures should all record the same ambient stream 

 temperature prior to the freezing operation. The boat for transporting the sample was 

 anchored near the site. 



FREEZING PROCEDURE 



Gloves and goggles were worn by persons working in the immediate area during 

 operation. The C0 2 tanks were fully turned on with the main valve and throttle valves 

 closed. The time was recorded upon opening each of the throttle valves. The first 

 throttle valve was opened 2-1/2 to 3 turns with immediate adjusting of the correspond- 

 ing relief valve to maintain 65 to 70 psig (0.448 to 0.483 MPa) in the system. Above 

 5,000 ft (1 524 m) mean sea level the gage pressure should read at least 70 psig (0.483 

 MPa). This operation was repeated on each of the rows. 



The pressure gages and probe temperatures were checked to determine when the probes 

 were full of liquid C0 2 . When the temperature registered -26°F (-15°C), or less, the 

 last probe had filled. The throttle valve was then turned to within 1/2 to 1 turn from 

 closed. If the temperature reached -40°F (-40°C), or the gage started fluctuating 

 rapidly, or dry ice escaped from the relief valve, the valve was turned off until the 

 gage stopped fluctuating and then reopened 1/2 to 1 turn while observing the probe' 

 temperature. 



The last probe in each row was kept between -26° and -40°F (-15° to -40°C) . We 

 initially set the valve at 1 turn and then gradually closed it to 1/2 turn near the 

 end of the test. This generally provided the proper C0 2 flow. The gages were watched 

 closely and when rapid gage fluctuations occurred, the C0 2 was reduced or turned off 

 temporarily. The relief valves generally required periodic adjustment to maintain the 

 operating pressure at 65 to 70 psig (0.448 to 0.483 MPa). The freezing rate of the 

 sample was checked by the amount of ice forming around the probes where they enter the 

 streambed and by monitoring the temperature probes in the substrate. 



14 



