This image shows the SNAP-19 ETG unit serial number 108 being tested at Martin Marietta. The group that conducted the testing was the Power Sources Group at Martin Marietta. One of the challenges of testing the RTGs was there was no "off" switch, so when it was not being used for
testing, it was necessary to disconnect the power from the RTG to the electronics test interface and plug in a shorting plug to prevent damage. Another challenge was that once an RTG was fueled, there was a constant flow of energy (heat) that needed to be regulated. In order to do this, coolant lines were used with liquid coolant flowing at all times during until launch. This was necessary to keep the system from overheating from the waste energy of the RTGs. Once the craft was launched the coolant lines were cut and the temperatures started to go up. But once the shroud came off, the heat transfer to deep space permitted it to cool down. Thermal teams were included in this test design and thermal switches designed for that purpose. One of the critical tests measured the waste (heat) and the diffusion into the heat rejection fins. You can see the fins in this image as well as the heat sensor element that attached to the fins and test unit and measured the output and temperature of the fins. Testing tried to recreate the environment that was expected (range of temperatures) using a simulation to get the temperature on the fins to the ranges needed to test. The fins were covered in insulation, which can be seen in other images.