This has not been attempted previously to CP3 or CP4 though it is not a new or original idea.  

Both CP3 and CP4 are thought to be experiencing the same issue, i.e. an obstructed level sensing line.  This could be due to the accumulation and clumping of "snow flakes" of water ice.  It may be that the relative buoyancy in LN2 of these "snow flakes" is concentrating them near the bottom of the LN2 pump which is also where the lower sensing line enters the pump and samples the LN2.  With an increasingly dense concentration over time it is easy to imagine these "snowflakes" interlocking and forming clumps which eventually restrict the ID of the sensing line (the "cholesterol" theory!) to the point of complete obstruction.    

In the past we had applied UHP N2 (various pressures/durations -> Max. of 180 psi) to these clogged lines in an attempt to "push" this obstruction back into the 80K pump.  These attempts gave no indication of a resulting movement of the obstruction nor flow of the applied UHP N2.  Today, while changing the most recently depleted cylinder of UHP LN2 (leak into the room), I decided to reconnect the sensing line to the local differential pressure gauge.  I noticed that it indicated a differential pressure of 5" of water.  I repeated the process of decoupling the line from the gauge and exposing it to atmosphere for a period then reconnecting it to the gauge.  Each time the pressure difference increased by ~0.5" of water.  This got me thinking..... 

Considering the relatively large mass of water that can be extracted from food in the freeze drying process while exposed to vacuum and in such a relatively short amount of time, I discussed with John W. the risks and possible benefits of applying vacuum to our clogged CP4 line.  The hope is to encourage the sublimation of the "ice plug".  It was decided that applying a vacuum to the sensing line would be low risk at this point.   

So, late in the day, I hooked up a small diaphragm pump and pumped on the sensing line for a few minutes.  After reconnecting it to the differential pressure gauge and CDS level transducer I found that the indicated pressure difference had increased.  Repeated cycles produced repeated benefit (see attached image).  Of particular note is that an audible release of built-up gas could be heard each time the sensing line connection was loosened to do the next round of vacuum pumping.  This suggests that the CDS indicated pressure difference is due to a real pressure difference and that, perhaps, the vacuum pumping has established a flow path through the plug!  Or, that the ice plug has been pulled into a warm area.

Gerardo and I will continue tomorrow.  Chandra R. will be back after the new year.