Tuesday, September 12, 2017

Example of a Large Vacuum Chamber Used at NASA

Facility 238 is a large, vertical, cylindrical thermal vacuum chamber which is used for thermal vacuum and thermal balance testing, and baking out spacecraft hardware. Test articles are normally loaded through the top of the chamber using the building crane; however, small payloads can be transported through the personnel entrance. Ports for electrical feedthroughs, liquid/gas feedthroughs, and viewing are located around the perimeter of the chamber. A clean tent at the chamber entrance provides class 10,000 cleanliness conditions.

Mode of Operation


With the chamber dome rolled back, the overhead crane is used to lower the payload onto the support fixture. In most cases, special fixturing must be designed due to the uniqueness of the test article support system. Once installed, the payload is instrumented and connected to the ground support equipment via feedthroughs. Access to the chamber is throught a clean tent. The use of cleanroom procedures and the wearing of clean garments are required when working in the chamber.

Initial chamber evacuation is provided by two rotary piston mechanical pumps, with four closed cycle cryopumps for high vacuum pumping. Each cryopump is isolated from the chamber by a sliding gate main valve to allow off-line cool down and regeneration.

Parameters

  • Test Pressure: 5 x 10-7 mmHg
  • Shroud Temperature: GN2 mode -90°C to +90°C , LN2 mode -190°C
  • Chamber Pump: 4 cryopumps
Physical Characteristics
  • Test Volume: 12' x 15'
  • Payload Support: Floor level - 4' square platform
  • Side Wall: Hardpoints at 6' and 12' levels
  • Crane Capacity: 5 tons Viewports: 9" diameter
  • Standard Electrical Feedthroughs: 36 - 37 pin connectors (RF feedthroughs available on request)
Integral Instrumentation
  • Pressure: Capacitance manometer - Atm to 10-3 mmHg
  • Ion Gauge: 10-3 mmHg to ultimate
  • Payload Temperature: 324 channels of thermocouple or thermistor channels
  • Contamination Monitor: TQCM, coldfinger, residual gas analyzer

Friday, September 8, 2017

Custom Heating Element for Lab Mice Stabilization During MRI

Custom electric heating element for laboratory
Custom electric heating element developed for keeping
mice warm during MRIs.
Cancer researchers in the UK have developed a special electric heating system to keep lab mice warm during MRI's without electromagnetic radiation effecting the quality of the MRI.

Compatibility between MRI machines and resistive, electric heaters has been a challenge to manufacturers of heating devices. MRI's and electric heaters don't like each other because of the electromagnetic field given off by DC and standard AC voltages. That E/M field disturbs the image resolution and quality of the MRI.

In research and lab testing, mice and other rodents are examined via MRI technology. In order for the rodents to be compliant with the testing procedure, the mice are given anesthesia which lowers their body temperatures and can send them into hypothermia. In the past, this situation was corrected by warming the air around the animal sufficiently to warm their body. Air heating requires a significant amount of valuable space, so a different heating source that would both not conflict with the MRI output and also provide a more compact, economical solution was required.

While DC voltages and standard AC voltage caused problems with the MRI, the solution turns out to be a copper wire element, embedded in heating blanket, and powered by AC voltage in a frequency ranging between 10-100 kHz. 

Conclusion

High frequency electrical heating provides a simple means by which stable body temperatures can be maintained in the mouse. The space requirement for the heating apparatus around the mouse is minimized and the system can be extended to use arbitrarily shaped resistor systems. Image and spectral quality are not adversely affected by the presence of the AC used in the heater so MRI performance is not compromised. As such a new MRI-compatible mouse heating system has been developed and validated.

To read the entire article, An MRI-Compatible High Frequency AC Resistive Heating System for Homeothermic Maintenance in Small Animals, visit this link.

Wednesday, September 6, 2017

Polyimide Thermofoil Heaters Used for Space Applications

Thermofoil heaters of an all polyimide (adhesive-less) construction are used for high reliability space applications. They are ideal for extreme temperature fluctuations and have high tear and tensile strength. The specifications for the design and manufacture of these heaters are rigidly controlled, and rightly so. These heaters often perform critical heating application on satellites and other space craft.

Construction

A polyimide (Kapton) is used as the heater base material, also known as the substrate or mounting surface. A second polyimide layer over the heating element provides a protective enclosure.

Heater Element

The heating element is made of a Ni-600 Inconel (nickel-chromium-iron) alloy. Single or dual resistive elements may be used in the design, but the element must be must be of a single layer in cross-section. The element are normally an etched foil design with uniform in cross-section, and they have a minimum trace width of 0.010 in. (0.0254 cm) by design. Spacing between foil traces is tightly controlled and must not be less than 0.010 in. (0.0254 cm). The spacing between the outer foil trace and the heater edge (border trim) also cannot be less than 0.010 in. (0.0254 cm).

Lead Wires

Lead wires must be a minimum of 26 gauge when using high strength copper alloy conductors, and 24 gauge for all other copper conductors. Lead wire insulation must consist of polyimide (Kapton), polytetrafluoroethylene (PTFE/Teflon), or ethylene-tetrafluoroethylene (ETFE) materials.

Lead Termination


The termination of the lead wire has to be welded and contain a minimum of two weld points between the lead wire and landing bond pad. Lead wire terminations also need to be enclosed in a hardened Hysol epoxy potting, in order to secure lead wires to the heater so that lead pull stresses are not transmitted to the weld joint.
Power Rating

Polyimide thermofoil heaters used for space have a maximum power rating of 4.5 W/in2 (0.7 W/cm2) when suspended in still air at 25°C, although this specification is for test purposes only and is not indicative of the maximum power rating in application (with heater mounted to a heat sink). Actual rated power (or voltage) are specified in each application.
Visit this web page to view or download the entire NASA General Specification for Thermofoil Heater, All- Polyimide, Space Applications.