Days 31-33

The next day after our visit to Halley, the ship left the ice shelf to head back to the SASSI moorings line. We left the "winterers" behind, the Halley staff who are going to spend the winter there, who were waving goodbye...

...to their friends on the ship, the Halley staff who are coming back with us to civilization.

Even the Emperor penguins came to say goodbye ! I wish them to survive the harsh winter conditions, and hope to see them again next year...

During the transit to the SASSI line, we prepared the equipment for the last mooring, and rigged the PIES (Pressure Inverted Echo Sounder, in white in the picture below) for deployment.

The next day we had reached the SASSI line, and started some CTD/ADCP stations to see where to deploy the last SASSI mooring. As the ship was sailing offshore, we lost sight of the ice shelf for good. I will miss the beauty of it, but hopefully next year I can see it again... The results were quite different from those obtained 3 weeks earlier, with the core of the westward surface current farther offshore. We decided to deploy the last mooring as deep as we could with the available cables, which was in 2600 m depth. The mooring was 2100 m long, its top end reaching 500 m depth, shallow enough for the top ADCP to measure currents up to the sea surface, but deep enough to avoid the instruments being smashed by passing icebergs. We started deploying the mooring around 9 pm, surrounded by complete darkness. It was the first time since we left the Falkland Islands that I saw a dark sky at night !

We finished deploying the mooring around midnight. We then resumed the CTD/ADCP stations to decide where to deploy the PIES. In the meantime I was running a set of tests on the PIES, to check it was working properly, but when checking the vacuum level inside the glass sphere containing the electronics of the instrument, I was horrified to see that there was almost no vacuum at all, contrary to what I had seen 3 weeks earlier ! We could not deploy the instrument in this condition. We had to reseal it first, but we needed a vacuum pump strong enough to make the required vacuum. We asked the ship's engineer whether we could use the ship's pumps, but he said it would not be feasible. We therefore decided to cancel the deployment. I went to bed at 5 am really disappointed. I slept until 3 pm, and checked the altimeter inside the glass sphere again, to find with amazement that the level was back to where it had been 3 weeks earlier ! The only explanation I could see was that because the PIES had been staying outside for some hours before deployment, the temperature inside had dropped, therefore since the volume was constant, the pressure must have dropped too (recall PV=nRT). Therefore the altimeter level had increased, but since at room temperature it was already close to the maximum value it could display, at below freezing temperatures the needle had rotated counterclockwise past the 0 mark, indicating a low altitude and therefore a weak vacuum !! But during the time I slept, the PIES was in the wet lab at 20 degrees Celcius, and the altimeter reading was back to its "normal" position. Very bad design !!! The PIES manufacturers confirmed by email that this is indeed what must have happened. So the PIES was working fine, and we would have deployed it if only I had not checked this damn gauge ! What a frustration... We decided we would deploy it anyway elsewhere, but we were now too far from the SASSI line to turn back, and in too deep water to deploy it now, so we will have to wait until we reach the South Orkney Islands to deploy it.