SIO Paleomag Lab blog

The 2G magnetometer (FLO) is temporarily down. During annual maintenance of the adsorber, the brass hose line began to fail. A loose o-ring in the connector may have been the cause of this failure. Later during this maintenance the system was exposed to atmospheric pressure. The first instant the system reached atmospheric pressure there was a cracking/popping noise that came from the coldhead. The noise may have been ice forming instantly inside the coldhead sleeve. After this, the coldhead motor was kind of running but the piston didn't seem to be moving at all. 12 hrs later, the pressure in the liquid helium reservior had reached about ~8psi (safety valve pops at 10psi) and the vent valve was completely frozen over. The extreme buildup of pressure in the initial 12 hours may have been due to the coldhead was running, trying to pump helium in though unable to recycle any out. After turning the cold head off, defrosting the ice around the vent valve, and depressurrizing back to ~2.5psi the system began to boil off liquid helium at a safer rate. About 65 liters of liquid helium boiled off in 8 days. Best case scenario for the 2G is that only the coldhead will have to be replaced. Then we can make a new vacuum, use the cryocooler to precool the reservoir, and fill her up with liquid helium.

Normal operating pressure for the CTI 8200 Compresor is about 270-290 psig and static pressure should be around 245-255 psig. If the normal operating pressure increases above 295 the pressure needs to be lowered. In order to bleed the pressure down, follow these steps.

1.  Turn off the compressor
    
2.  If the static pressure is higher than 250 psig,  SLOWLY open the
    helium gas fitting and charge valve.  The pressure will begin to 
    slowly decrease.  Bleed the pressure down to 250.  

3.  Turn on the compressor nominal operting pressure should be about
    270-290 psig)

Filled BUBBA with about 33 liters of liquid helium. There were no problems and the fill took about 20 minutes. During the last fill a partial ice block was noted. This may have been caused from a humid day. To prevent ice blocks, pressurize the transfer line very slowly and insert the line into the magnetometer before it completely liquifies. :>