TVAC Techniques

Thermal testing components usually take place in a TVAC (thermal vacuum chamber). The TVAC in use at Magellan Aerospace has the following specifications. There is also another smaller TVAC that Magellan does not use anymore, however, its specifications are unknown.  Please note that clean handling is required for all components that enter the TVAC. Please use IPA, gloves, Kimwips to ensure this! 

I. TVAC Components and Operartion

1. Cold Plate : temperature-controlled test article mounting surface

2. Shroud: outer cylindrical housing of the TVAC

Start the vacuum sequence once the top, side clamps are closed, and hold the bottom clamp closed for ~5 minutes. The bottom clamp should be able to hold itself in place after being held closed for 5 minutes. 

The vacuum procedure should be started in the morning. Once began, air is pumped out of the chamber contineously. Magellan advices that the TVAC to be left at vacuum overnight to achieve stable vacuum conditions. 

To set the cold plate to a temperature, follow instructions on the side of the TVAC door.

The TVAC is only temperature controlled at the cold plate, and not its shroud. Therefore any radiation heat exchange between the test article and the shroud happen at T_shroud = T_room, there is a thermometer in the clean room that reports real-time rom temperature. 

II. TVAC Electrical and Mechanical Interface with Test Article

The TVAC's CAD drawings are found here(1) and here (2).

Mechanical Interface: Figure 2 shows the interior of the TVAC. The silver/white plate is the cold plate. Its exact dimensions are outlined in the drawing, however, the test article should alwys be smaller than the TVAC's dimension to ensure secure mechanical interface. The largest holes shown on the drawing are the cold plate mounting holes. These are not meant to be used as test article mounting holes, however, if they could be used to mount a test article, if that is the only type/style that works. Note that these holes are sunk, use a long bolt! The smaller test article mounting holes should be used to mechanically interface a test article to the cold plate. None of the holes in the plate are through holes, threaded bolts are necessary.

If the test article can be laid flat on the cold plate (similar to the test article in this image), then no bolting is necessary. However, to ensure that the test article does not move during testing, tape the test article down firmly with Kapton tape, the yellow tape pictured in this image (supplied by Magellan Aerospace). Also tape down any electrical interface, to ensure that they do not interfere with closing the TVAC doors. 

Electrical Interface: The TVAC has D-type connectors that allows electrical connections between the inside of the TVAC to the outside of the TVAC. Each internal pin connector has a corresponding external pin connector. By matching the pin number on the inside and outside of a pair of connected D-type connectors, electrical signal can go from the test article to the external data acquisition system. 

Note that since the existing internal to external connection are D-type, it's necessary to have matching adaptors on both sides that connects from the TVAC electrical pins to the test article and DAQ. This requires  crimped pins and a matching D-type connector (available on Digikeys). As of 2024 May, there are 

• size 28-22 female-female contact pins

• size 22 female-male contact pins

• size 28 female-male contact pins

• D-105 adaptors

• crimping tools

To make this adaptor, use vacuum safe electrical wires, strip the tip of the wire and insert it into the female side of a pin, then crimp the pin with crimping tools. Then, insert the crimped pin into the D-105 adaptor, with the male head coming out of its back side. Ensure that pins on the adaptor is correctly matched to pins on the TVAC (see Figure 3).

Alternatively, crimp the wire to the contact pin, without using a D-type connector also worked for a past project.

Figure 4 shows the external connection, electrical interfaces similar to the internal side would be necessary, unless there is an available adaptor, similar to the adaptor in figure 4. 

Fig 3: Contact pin in D-type connector and crimped pin

III. TVAC Data Gathering

The TVAC has three thermistor. Two of them, thermistors 1 and 2, are used to measure the cold plate's temperature. They ensure that the desired temperature is met. Thermistor 3 is extra, which can be used as an additional thermistor on the test article. However, this thermistor's electrical connections are short. In Figure 5, the 3rd thermistor was placed on the back side of the test article. Thermistor 1, 2 and 3's temperature is recorded real time constantly, therefore note down the time of test start is necessary. A Magellan supervisor should be able to forward test data from thermistors 1, 2 and 3.

Author: Josephine Zhao
Date: May 3rd, 2024