Development of CTD-systems

The birth of the Sea-Bird (The Love Story)

Arthur Pederson (1932-2021)

Meanwhile, on the West Coast of the USA at the Applied Physics Laboratory (APL) of the University of Washington in the mid-1960s Arthur Pederson (1932-2021)  applied Wien bridge circuitry methods to temperature and conductivity measurements in the ocean. Art Pederson came to the University of Washington from college, earning a B.S. in Electrical Engineering with top honors. He later earned an MBA and a Master’s degree in Electrical Engineering from the UW.  He spent 25 years at the APL supporting oceanographic research and during that time patented two scientific instruments. In 1968, Art Pederson introduced the conductivity sensor, based on a PYREX glass tube with a length of 18 cm and a diameter of 4 mm, in which 3 electrodes are axially mounted. The resulting sensor configuration and the connection of its electrodes had to form two resistive cells in parallel. This is shown on the electrical equivalent model.

Pederson’s C-cell electric model

In a Pederson cell, the two external electrodes E2 and E3 are connected together; thus, it is essentially a two-electrode sensor, i.e. V1 and C1 are one electrode, and V2 and C2 are the electrically connected electrodes E2 and E3. The parallel nature of the configured tube and electrodes results in a very low cell impedance, since the two resistive paths are electronically connected in parallel, which reduces the effective resistance of the sensor by half. 

Pederson’s conductivity cell cross-section

In order for Pederson to overcome the low impedance, a glass tube with a small internal diameter and a longer length was chosen. These design solutions led to the creation of the “soda straw” sensor, which had the disadvantage that usually a sampling pump was required to move a seawater sample through the sensor. In addition, the contact resistance of all three electrodes must be stable, since changes in contact resistance cannot be distinguished from changes in the measured conductivity of seawater.

                           SBE CT-duct

To solve the problem of contact impedance, Pederson increased the size and surface area of the electrodes and manufactured them from a precious metal, namely platinum.

To further reduce contact resistance, Pederson applied a platinum-black coating that serves to increase the effective surface area. In a later new development by Pederson, a linked temperature sensor was incorporated into the flow path of the pumped sensor to help reduce the time-constant mismatch between the conductivity sensor and the temperature sensor (CT-duct); this led to improved salinity and density calculations in dynamic mode.

Bird and Art Pedersons, 1958 (Family archives)

     Art took an incentive to retire early from University of Washington along with an exclusive manufacturing license for the Wein bridge T&C sensor from APL and founded Sea-Bird Electronics in 1979. When the time came to choose the name of his company, Art joined together his loves – for the Sea, for his wife (taking her second name Bird), and for  Electronics.

      “Joined by Ken Lawson, the two began by working out of a bedroom in Art’s house. The first CTDs from Sea-Bird Electronics came out in 1982. Over the years, Art and Ken developed exceptionally stable Wein bridge circuitry underpinning the high-accuracy that is a Sea-Bird hallmark. Art also invented hybrid period counting before it was introduced by Hewlett Packard. These innovations formed the basis of the first Sea-Bird CTDs. (N.Larson)

Art Pederson on the trade show SBE booth, 1982 

Despite the difficulties in obtaining sufficient flow through the Pederson sensor, the feature of a completely closed volume, combined with the feature of reducing the misalignment time constant, has led to devices based on the Pederson conductivity sensor becoming the physical oceanographer’s tool of choice. All his life Art Pederson remained proud of the company’s continuing success.  In his free time Art was an avid outdoorsman, traveling all over the globe to trek, hike, and mountain climb. He visited Nepal, Patagonia, Machu Picchu, and summited Mt Rainier, Mt Fuji, and Mt Kilimanjaro. Together with his wife Bird he traveled numerous places in Europe, Asia, Africa and the United States. 
In October 8th 2021 Arthur Pederson passed away. We will remember his outstanding work and remarcable innovations, his exceptional life and great love.