[evlatests] [Fwd: plan to prevent moisture condensation in EVLA feed horns]

[Mark McKinnon]

My apologies to those of you who receive this message twice.

After this morning's VLA test meeting, I thought it would
be useful to distribute this note to a wider audience.

MM



On Wednesday, September 20, 2006, Tommy Montoya, Rudy Latasa, Jim Ruff,
Chuck Kutz, and Mark McKinnon met to develop a plan to prevent moisture
condensation in EVLA feed horns. This note describes the plan and
provides the minutes of the meeting.

1. The meeting started with a description of how moisture condensation
    is prevented at NRAO telescopes.

    a.  At the VLBA antennas, the feed horns are pressurized with air
        provided by a small air compressor. The air is forced through a
        cannister of desiccant prior to entering the horn. A very slight
        amount of air flows through the horn due to its permeability.
        Nitrogen is not used to purge the horns when a receiver is
        installed.

     b. Like the VLBA, the GBT feed horns are pressurized with dry air.
        Nitrogen purging of feed horns is not used on the GBT.

     c. The feed horns on the VLA antennas are not pressurized with dry
        air, nor is the vertex room of a VLA antenna pressurized.
        Desiccant cannisters are installed on the X and K-band feed
        horns, only. Of all the VLA feed horns, only X and K-bands are
        purged with nitrogen when a receiver is changed.

        The VLA K and Q-band horns should not be prone to moisture
        condensation because they are located inside the vertex
        room (i.e. only a small portion of these feeds is exposed to
        ambient air).

        Some VLA C-band horns have had evidence of corrosion, supposedly
        due to moisture condensation. They have been cleaned and
        re-chromated.

     d. On the EVLA antennas, we think desiccant cannisters are installed
        only on the L and X-band horns on antenna 16. The vertex room of
        antenna 13 is pressurized by the antenna's HVAC system in an
        attempt to prevent moisture condensation in its feed horns. To
        date, tests designed to determine the effectiveness of vertex
        room pressurization on antenna 13 have been inconclusive.

        We have some concerns about potential moisture condensation in
        the EVLA L-band horn because a significant portion of it is
        exposed to ambient air.

2. The recent problems we have experienced with moisture in the X-band
    horn on antenna 18 are now attributed to leaks in the horn. The
    O-ring that seals the interface between the horn aperture and the
    horn window-retaining ring was found to be split. The O-ring is
    being replaced. Also, a purge vent at the top of the horn was not
    completely plugged. The vent has since been plugged. If the
    condensation in antenna 18 X-band was due to moist air, then we
    should have seen condensation in other antenna 18 horns and in the
    horns of the other EVLA and VLA antennas. To our knowledge, this is
    not the case.

3. The plan to prevent moisture condensation in the feed horns of EVLA
    antennas is as follows.

    a. The cryogenics group will plug the top purge holes of the K and
       X-band feed horns on EVLA antennas 13 and 16. These antennas were
       chosen because moisture monitors are already installed in them.
       The vertex room of antenna 13 is pressurized, but the vertex room
       of antenna 16 is not.

    b. The cryogenics group will install desiccant cannisters on the
       bottom purge holes of every feed horn in antennas 13 and 16.
       This configuration will allow a feed to "breathe" air from
       the vertex room through the desiccant cannister. In antenna 13,
       the air will actually be forced into the feed because its vertex
       room is pressurized.

    c. We will discontinue the nitrogen purging of the X and K-band
       horns on EVLA antennas.

    d. On future retrofits of EVLA antennas, we will plug the top
       purge holes in the X and K-band horns, and we will replace the
       O-rings at their apertures.

    e. The cryogenics group will monitor the desiccant cannisters in
       antennas 13 and 16. If the desiccant becomes saturated on a
       short timescale (days to weeks), desiccant maintenance will be
       too onerous and we will need to find an alternative solution,
       such as a dedicated dry air system. If the desiccant continues to
       absorb water on long timescales (> 6 months), we will implement
       this plan for the other EVLA antennas. Monitoring antennas 13 and
       16 will allow us to decide if vertex room pressurization is
       necessary.