One day at lunch Prof. Martin Mork and I were discussing the remarkable steps in temperature and salinity revealed by the Bissett-Berman STD (salinity, temperature, depth) profiler. Each layer appeared to be quite uniform with a thin transition to the next layer. We couldn’t but help wonder what the velocity field might look like. It occurred to me it might be possible to resolve velocity at high vertical resolution using a fast-response hot-film sensor on a free-falling profiler. He liked the idea and invited me to try it out on an upcoming cruise he had scheduled south of Spain that summer (1971). I assembled a simple profiler in a few months and took it as excess baggage with me to his 8-day cruise out of Malaga. The instrument worked and we wrote a technical report on the effort, but we really had no idea how to interpret the measurements. This would be the end of story - except that later that summer while visiting family in Sweden I got an urgent message from Martin to please come and visit him in Bergen. That become the start to what became Yvette.
Martin introduced me to Trygve Gytre, an amazing engineer at the Chr. Michelsen Institute. Trygve had just developed an acoustic current meter and Martin wondered whether it could be used to probe currents at high vertical resolution, what we had tried earlier that summer. We were sold on it as soon as we saw his instrument in action, it was truly amazing. The principle of operation was simple: transmit an acoustic pulse in opposite directions. The difference in time-of-flight would be due to fluid motion along the acoustic path. To implement this concept, he constructed a frame with four 30 cm long vertical rods beneath it with a tiny acoustic pinger at the lower end of each. Each pinger would transmit a pulse and immediately afterward listen for the transmission from the opposite rod along a 40 cm path. To demonstrate its operation, he put the instrument in a pan filled with water, which he tilted ever so slightly to create a tiny seiche. The analogue display of the rocking motion was incredible.
Martin and I immediately contracted with Trygve to build one for us. When I got back to Yale, I contacted Neil Brown at WHOI, who was developing a new CTD (conductivity, temperature, depth) profiler, to ask if I could procure one for the free-falling profiler Martin and I had in mind: it would resolve temperature, salinity, and the velocity field in the same plane, all as a function of depth with decimeter vertical resolution. Sandy Williams, who was working closely with Neil to use a CTD to study saltfingering, offered to supply us with a CTD if we would supply him with a Gytre current meter. Actually, we asked for two CTDs, one for me and one for Martin. And Sandy got two Gytre current meters for his projects, a win-win deal for all of us!
Later, during my leave from Yale at the University in Bergen, I built our free-falling profiler, a 5-meter long tube with the CTD and current meter at the lower end. The current meter would resolve the local velocity structure relative to the long tube. It was a fun build, and when ready we ran successful tests, first in a Norwegian fjord and soon after at sea.
Later that year, 1974, Dave Evans, a graduate student here at GSO, and I were invited to join a cruise Tom Sanford had organized on the WHOI vessel RV Knorr to evaluate a variety of free-falling profilers; a cruise known as the Fine and Micro-structure Experiment, or FAME for short. Things did not go well for us. We ballasted the instrument in St. Georges harbor in Bermuda. This, so that it would sink at about the right speed until it reaches the pressure at which ballast is dropped and it returns to the surface. Unfortunately, we didn’t adequately consider how much fresher the water was in the harbor. As a result, when we released the instrument at sea, south of Bermuda, it wasn’t heavy enough and came to a rest at the bottom of the mixed layer. Unfortunate, but we weren’t worried for the back-up timer would release the ballast. Except that it didn’t. After several frustrating hours hovering right above it - we could hear its pinger, we had to admit defeat and abandon our instrument so others could pursue their work.
The following night word came from the bridge of a flashing light. They wondered whether we should check it out. The scientist on watch agreed. We steamed toward the flasher, and would you believe it, there was our instrument bobbing on the surface. The bridge sent word to me and Dave for we were down below grieving in our bunks over our loss. What had happened was that although the release didn’t work, the thin metal strip that held the ballast weight had completely corroded through in the warm salty ocean. Talk about good luck for this was unbelievable on two counts: first that the steel band had corroded so rapidly, and second that we were still close enough to see the flasher 24 hours later. We retrieved the instrument, repaired it, and went to work. But I forgot to mention that at the beginning of the cruise the bridge asked for short names for each of the instruments to facilitate their log keeping. I suggested we call ours Yvette. Given the wild start to Yvette’s career on the Knorr, the bosun spelled out Yvette: ‘Young virgin expects to travel extensively’!