WARNING:  Improper operation of this equipment can lead to the formation of an ice plug in the neck of the detector's liquid He fill tube.  Severe overpressure can result in a dangerous rupture of the detector.  The procedures listed below as well as the manufacturer’s instructions must be carefully followed.  The safety fixture supplied by the manufacturer shall be used and may not be altered.

 

 

** MANDATORY REQUIREMENTS **

Ø       Operate an infrared detector using pumped liquid He only with the approved safety fixture and pumping manifold, including overpressure relief valves for:
        a) top of safety fixture with 2"long stainless steel tubing extension.
        b) liquidN₂ reservoir fill port.
        c) bolometer detector (guard) vacuum space.
        d) liquid He reservoir pumping manifold.

Ø       Read these procedures, receive training from and show proficiency to an infrared beamline staff member. 

Ø       Ensure that a copy of the manufacturer’s instruction manual and these procedures are at the beamline.  Note that these procedures supercede their equivalents in the manual.

Ø       Take the web-based BNL Cryogen Safety Training (OSH-025): http://training.bnl.gov/demo/courses/courselist.htm

** CAUTION: Wear Proper Eye and Hand Protection When Handling Cryogens.

** CAUTION: Keep clear of the space above the detector's top surface during operation with liquid He.

 

SEE LAST PAGE OF THIS DOCUMENT FOR A DETECTOR DIAGRAM

 

Detector Preparation Procedures:

Note: see attached figure on last page for additional guidance.

1.      Evacuate the detector's guard vacuum space by connecting the side pump-out port (the guard vacuum valve) to an appropriate high-vacuum pump system (e.g., turbo pump).  Pump until the pressure reaches <1x10^-4 Torr as read at the pump.  Then close the guard vacuum valve, turn off the pump and disconnect the pump from the detector.

2.      Check the Teflon washer on the liquid He fill port (top of detector).  If it is dirty, clean it with alcohol.  If it shows excessive wear or no longer fits in the groove properly, replace it. 

3.      Pre-cool the detector interior with liquid N₂.  Fill the liquid nitrogen reservoir (outer port) first.  When the vapor stream from the fill port subsides (indicating that the reservoir has reached temperatures near 77K), fill the liquid He reservoir (top center port) also with liquid N₂ so that it will be pre-cooled to 77K.  Avoid overfilling either reservoir, causing liquid N₂ to spill excessively over the detector's top flange.  Cold vacuum O-rings can leak, causing partial vacuum loss.  Symptoms of this problem are the entire detector container becoming quite cold with condensation appearing on its exterior surface.  If this happens, allow the top flange to warm up and re-evacuate the detector guard vacuum space (step 1).

4.      Allow time (> 20 minutes) for the detector's liquid He reservoir and associated cold plate components to reach 77K.  The vapor stream (due to liquid N₂ boil off) from the liquid He reservoir will subside when this is achieved.  Shining a flashlight down the fill port should reveal a calm liquid (Wear eye protection!).

5.      Drain the liquid N₂ from the liquid He reservoir.  This can be accomplished either by inverting the detector and draining the liquid N₂ from both reservoirs into a proper cryogen storage vessel, or by using an liquid N₂ extraction tube to remove the liquid N₂ from the liquid He reservoir.  Be sure to use either dry nitrogen or helium gas and keep the pressure below 1 psi.  A liquid N₂ hand dewar can be used to recover the liquid.  With either method, be sure that the detector's liquid He reservoir is completely free of liquid N₂ (if you're not sure, a flashlight light can be used to look down the central fill tube - Wear eye protection!).  If the inverted detector method is used, both the liquid He and liquid N₂ reservoirs will be empty.  Be sure to refill the liquid N₂ reservoir with liquid N₂ afterwards.  In either case, cover the liquid N₂ fill port with its combination cap and 0.5 psi relief valve. (Note:  if you need to transport the bolometer at any time, cover the liquid N₂ fill port with aluminum foil during transport.  Once at the beamline, then screw on the cap and relief valve). 

During cold operation, check the detector's liquid N₂ reservoir level periodically (~8 hrs) and fill as needed.

Prepare liquid He Transfer Line

6.      Purge the liquid He transfer line with He gas.  Check that you have a compression fitting to form a seal around the transfer line when it is inserted into the liquid He storage dewar.  Vent the liquid He storage dewar through its vent valve and close the relief valve.

7.      Close the storage dewar vent valve, open its top valve, and carefully insert the liquid He transfer line.  The compression fitting screws onto the threaded top fitting of the storage dewar.  It should be tight enough to hold the pressure necessary for transferring liquid.  Insert the transfer line slowly to avoid excessive (more than 3 psi) pressure build-up in the storage dewar.  You should open the vent valve if the pressure exceeds 5 psi and close it once the pressure has dropped to 3 psi or less.

8.      Gently insert the transfer line until it is reaches the bottom of the liquid He storage dewar, and then back it off slightly to avoid picking up ice that may be at the bottom.  If a plug develops (indicated by a lack of gas or liquid coming through the line), then vent the storage dewar, remove the transfer line and purge it with He gas or allow it to warm up completely before trying again (return to step 6). 

9.      Allow helium to flow and cool the transfer line interior before inserting the line into the detector liquid He fill port.  A milky plume indicates that liquid is reaching the far end.  If the pressure in the liquid He storage dewar falls to near zero, attach a helium gas line to the liquid He storage dewar vent line and open the vent valve.  Use helium gas from a regulated gas cylinder set to deliver about 2 psi. 

Fill the Detector liquid He Reservoir

10.  Insert the transfer line into the detector liquid He reservoir fill tube.  On a bolometer, be sure to turn on the pre-amplifier electronics if not on already.  This prevents the first stage JFET from getting too cold to operate.  Adjust the detector height so that the transfer line reaches near the liquid He reservoir bottom (while the transfer line's other end remains near the bottom of the liquid He storage dewar). 

11.  Fill the detector's liquid He reservoir.  Initially, a large plume of gas will evolve from the detector's liquid He fill port.  Once the detector's internal components reach liquid He temperature, liquid will begin to collect and the plume will drop dramatically while becoming more densely white.  Continue transferring liquid until the detector reservoir is full.  This is indicated by a change in the plume - it will appear very dense and tend to spill across the top of the detector.

12.  Close the liquid He storage dewar vent valve and disconnect the helium gas line from the storage dewar vent line connection.  Turn off the flow of helium gas at the gas cylinder. 

13.  Vent the liquid He storage dewar completely through the vent line.  Loosen the transfer line compression fitting and remove the transfer line from both the storage dewar and detector liquid He reservoir.  Close the storage dewar vent and top valves.

14.  Open up the over-pressure relief valve to the liquid He storage dewar.   This is a good time to visually inspect the other storage dewar's medium and high-pressure relief valves.

Prepare the Detector for Operation

15.  Insert the liquid He safety fixture into the liquid He fill port, and firmly screw it onto the liquid He fill port fitting (hand tight).  This safety fixture has a small stainless steel tube that reaches past the liquid He fill tube's narrow neck and into the top of the liquid He reservoir, thus providing 2 separate vent paths from the reservoir.  For a pumped detector, the safety fixture has its overpressure relief valve at the end of a 2" length of stainless steel tubing.  Inserting this fixture slowly into the liquid He fill port helps to conserve liquid helium.

16.  If you do not intend to initiate pumping at this time, you should immediately attach a draped hose to the side port of the liquid He safety fixture.  Otherwise connect the pumping manifold hose to the liquid He safety fixture's side pump port (NW-25 connection).  Check that all the connections are tight, and all the valves on the manifold are closed.  

17.  Turn on the vacuum pump and open the "fine pumping valve" about 1/4 turn.  The vacuum pump's "gurgling noise" should go up in pitch as helium gas flows.

18.  The liquid He safety fixture will frost up on the outside as cold He gas passes through.  This is normal.  However, if a portion of the vacuum hose or the safety fixture relief valve begin to show frost, then the pumping rate must be reduced.  Close the fine pumping valve a bit.

19.  Occasionally, a particular pumping rate causes an acoustic resonance inside the detector's liquid He reservoir and rapid boil-off of the liquid He.   This also causes excessive frost on the safety fixture and vacuum hose.  The resonance can sometimes be heard or felt in the vacuum hose.  Adjust the fine pumping valve (for a higher or lower rate of pumping) to eliminate the resonance. 

20.  As the liquid He vapor pressure is reduced (as monitored on the pumping manifold vacuum gauge), the pumping speed can be increased.  This is done by occasionally opening the fine pumping valve a small amount.  As the pressure drops to below 40 Torr, the fine pumping valve can be opened completely and the coarse pumping valve opened about 1 turn.  Usually the coarse pumping valve can be opened completely when the pressure has dropped to around 20 Torr.

21.  The infrared detector can be turned on for the entire pumping process.  With the preamplifier already "on", turn on the detector bias and then the input switch.  The detector signal should steadily increase as the liquid He temperature is reduced.  As the minimum temperature is approached, the detector signal will level off.  This indicates that the detector is ready for operation.  The pressure on the pumping manifold will be less than 10 Torr.  Total pump-down time is about 90 minutes.

The detector has reached its base temperature (~ 1.5K) and is now ready for use.

See below for ice plugs and possible overpressure dangers.

Refilling a Detector Using Pumped liquid He

22.  It is assumed that the detector contains liquid He at a reduced temperature (< 4.2K).  Thus the system cannot be left open to air, even with a draped hose, since the liquid He vapor pressure will be lower than 1 atm.  Begin by closing both pumping valves (coarse and fine) on the pumping manifold.  Attached a purged He gas line (e.g., from a regulated He gas cylinder) to the manifold bleed-up connection.  The gas pressure should be set to about 1 psi.  Slowly open the bleed-up valve.  The detector liquid He reservoir gas pressure (read on the pumping manifold gauge) should increase toward 760 Torr (1 atm).

23.  Prepare the liquid He storage dewar and transfer line, following steps 6 through 9 above. 

24.  Close the manifold bleed-up valve, remove the detector liquid He safety fixture, and immediately insert the delivery end of the liquid He transfer line.  Note that the long central tube and copper radiation baffles of the safety fixture should be clean and free from ice.  If not, you should stop the liquid He transfer and inspect the detector's central liquid He fill tube (Wear eye protection!).  Any ice collected on the sides can be scraped off using a length of stainless tubing.  The ice will fall to the bottom of the reservoir where it is not a problem.  Ice in the detector's fill tube indicates either an improperly configured safety fixture (not screwed on tight, leaking relief valve) or failure to install it promptly after filling with liquid He.  You should immediately resume the liquid He transfer after clearing any ice.

25.  Complete the liquid He transfer, then securely re-attach the safety fixture and vacuum pump hose to the detector. 

26.  The detector may be left in this configuration (full reservoir at 4.2K, hose attached with relief valves on both the safety fixture and the pumping manifold), or may be pumped down to the 1.5K operating temperature following steps 17 through 21. 

 

See below for ice plugs and possible overpressure dangers.

 

Warm Up

27.  When finished using the detector, it is good practice to turn off the detector's input, bias and preamplifier power switches.  The detector may be left with the pumping hose attached and the manifold pumping fine and coarse valves open.  Assuming the manifold has the proper relief valve, one may also close both pumping valves on the manifold and allow the system to warm up to 4.2K. 

28.  Under no circumstances should the detector's liquid He reservoir be left open to air when there is any chance that it contains  liquid He at reduced (< 4.2K) temperature.  A detector containing liquid He at 4.2K and atmospheric pressure must be handled according to applicable rules (i.e., safety fixture with full-length tube, connected to either a 24" long draped hose or a check/relief valve).

29.  The time for a detector to warm up completely varies, but waiting 24 hours after the liquid N₂ reservoir is empty is usually sufficient.

WARNING.   IMPORTANT HAZARD INFORMATION:  CLEARING ICE PLUGS

If an ice plug forms in the neck of the liquid He reservoir, the result is extreme pressure buildup and eventual deformation and rupture.  The liquid He safety fixture has been included with the detector to prevent this from occurring.  The safety fixture must be used at all times. Identifying an ice plug.  An ice plug (blockage of the liquid He reservoir fill tube) prevents the vacuum pump from pumping on the liquid He reservoir contents.  Thus, the pump manifold pressure will drop to near zero even though there is liquid He inside the reservoir.  If you suspect that an ice plug has formed, contact beamline staff for immediate assistance.  If unavailable, discontinue pumping, put on eye protection and gloves, bleed-up the detector's liquid He reservoir (with He gas through the manifold) and remove the safety fixture.   Keep clear of the detector's top surface!  Use a metal rod to probe the fill tube for any blockage.   An ice blockage/plug can be cleared with a length of copper tubing carrying warm He gas directly to the blockage.    Note that the reservoir pressure also drops to zero when the detector no longer contains any liquid He.  This does not indicate an ice plug, and can be recognized by an increased boil-off rate from the liquid N2 reservoir along with the detector's responsivity dropping quickly (or non-existent).   In some cases the electrical resistance of the bolometer element can be used to deduce the internal temperature.

 

 

Attachment 1 - PUMPED (T<4.2K) IR-LABS  DETECTOR OPERATIONS CHECKLIST

 

Attachment 2 - SYSTEM CONFIGURATION