Copied on Dec 17, 2001 from National Optical Astronomy Observatories site
http://www.noao.edu/ets/gnirs/SDN0008.htm 
 regarding the pre-commissioning tests and preparations for the Gemini Near-Infrared Spectrograph

SYSTEM DESIGN NOTE

SDN0008 - GNIRS Test Dewar Commissioning Tests

Prepared by	Date	Approved by	Date	Rev.	Rev Date
Name removed	4/8/99	Name removed	4/9/99

SDN0008 - GNIRS TEST DEWAR COMMISSIONING TESTS

This is a revised version of the memo of 12 January 1999 describing the commissioning and initial tests of the GNIRS Test Dewar Facility which is being set up in the Assembly Room in the basement of the NOAO Tucson Headquarters.  The planned mechanism tests described in the earlier memo are no longer contemplated and are not included in the present document.

1.  Description

The GNIRS Test Dewar is a large cryostat for testing GNIRS mechanisms under the low-temperature vacuum conditions that will be present in the final instrument.  The cryogenic work surface for mounting mechanisms is approximately 85 cm diameter and the working volume is approximately 85 cm in diameter and 90 cm deep.  The Test Dewar is cooled with liquid nitrogen, so the working temperature will be 77 K, rather than the ~60 K anticipated for the GNIRS mechanical environment.

2.  Safety Items

2.1 Due to the large quantities of liquid nitrogen being used, an oxygen monitor should be installed in the test room [completed 3/1999].

2.2 Install fans to vent the room to the outside to prevent a buildup of nitrogen vapor in the room (Note: the volume of the Assembly Room is equivalent to the STP vapor volume of 270 liters of liquid nitrogen) [completed 3/1999].

2.3 Install lines to vent possible oil mist from the vacuum pump during the initial pumpout of the large Test Dewar.  These lines could also vent nitrogen gas boiloff from the test dewar if the efflux is deemed significant.

2.4 Test the ability of the fans to vent the area.

2.5 Install overpressure safety valve on test dewar in case of cryogen tank failure

3.  Initial Test
 

 Goals: vacuum integrity, establish evacuation procedures
  cryogenic testing
  temperature profiles

 A Test Dewar log book should be set up to record the results of this initial testing.  It would also be used to record use of the dewar for future mechanism tests, but the mechanism tests themselves would be recorded elsewhere.

3.1 Vacuum

 3.1.1 Molecular sieve getter — initial test to be done without getter to evaluate bare dewar performance.  May be installed for later tests.

 3.1.2 Install appropriate vacuum gauges on test dewar.  Two ports are required for vacuum gauge and for leak testing.

 3.1.3 Install valve on pumpout port of test dewar.

 3.1.4 Evacuate test dewar.  Log pressures at pump and in test dewar as a function of time.

 3.1.5 Leak check test dewar after it is sufficiently evacuated.

 3.1.6 (optional) Close pump port valve for a length of time and monitor any pressure rise within the test dewar.

 3.1.7 Monitor test dewar pressure during cooldown, test phase, and warmup.

The result of this experiment should be to set the pressure at which the dewar is considered “pumped out” and to determine roughly how long this process takes for scheduling purposes.

3.2  Cryogenics

 3.2.1 Liquid nitrogen filling — determine techniques for most efficient cooling.

 3.2.2 Determine time for initial fill and amount of liquid nitrogen required.

 3.2.3 Determine if topping off is required after the initial fill (this will depend on the mass of any attachments to the work surface).

 3.2.4 Measure boiloff once equilibrium is reached to establish the daily consumption of liquid nitrogen.

 3.2.5 If refilling once/day is adequate, determine time to refill (this will permit this step to be carried out automatically with a timing mechanism).

The result of this experiment should be to determine the safest and most efficient method of filling the dewar and maintaining the cryogen level during long experiments.

3.3 Temperature Monitoring

 3.3.1 Install temperature sensors in test dewar.  At minimum, two each at the top, middle, and bottom of the work surface, two on inner radiation shield, at least one on outer radiation shield, one on radiation shield at the cryogen tank end of the dewar, and three on the G-10 fiberglass support ring.  As many additional temperature sensors as possible should also be mounted on the work surface to calibrate their readings at liquid nitrogen temperature.

 3.3.2 Automatically monitor a selected number of temperature sensors during the cooldown, once equilibrium is reached, and during the warmup phase.  A current source and strip chart recorder(s) will be adequate during the initial test.  A more automated system capable of logging a larger number of temperature sensors will eventually be required for actual mechanism tests.

 3.3.3 Once equilibrium is reached, manually read the other temperature sensors.

3.3.4 Determine which sensor positions are “permanent”, to be used to measure the test dewar thermal profile during mechanism tests.  During mechanism tests, additional temperature sensors will be installed at critical locations
 

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