Conductivity, Temperature and Depth (CTD) • Despite the name, all CTDs actually measure pressure, which is not quite the same thing as depth. • The CTD data can be used to calculate salinity, density, sound velocity, and other parameters of interest. • The term CTD is often used today to describe a package • actual CTD as well as auxiliary sensors • such as dissolved oxygen, pH, turbidity, fluorometers, altimeters, etc.) and a water sampler • Widely used: Seabird instruments
Seabird zoo of sensors…. SBE 9plus: This is the ship-based CTD system, uses ParoscientificDigiquartz. Accuracy 0.015% of full scale, about 1 m for the standard full scale 6800 dbar. SBE 16plus and 19plus: The 16 are the SeaCats (e.g. in Del Mar), the 19 is a profiling version. optional pressure sensors, Strain gauge: 0.1% full scale accuracy, Digiquartz: 0.02% full scale (full scale ranges from 20 to 10500 m). SBE 25: profiling system that is only offered with strain gauge pressure sensors, spec'ed like the SBE19. SBE 37: These are MicroCats. Only available with strain gauge pressure sensors, specs as above. SBE 53: SBE's best-accuracy bottom pressure sensor. It uses a Digiquartz , has additional reference oscillator circuitry, as well as temperature compensation, accuracy of 0.01% full scale. Available up to 6800 dbar. SBE 54: This is SBE's tsunami warning device. It uses a Digiquartz (maybe no reference oscillator) Prices: strain gauge pressure sensor for an SBE 37: about $1450 extra ($5000-700 for the rest of the instrument in a titanium version.) An SBE 16 in plastic has a base price of $12000 (titanium costs $2800 extra). optional Digiquartz listed as $8700.
Shallower than 2000m Seabird uses “Druck” pressure sensors for the strain-gauge, deeper they use “Kessler” pressure sensors since better at high pressure. Both sold as 0.1% accuracy, so for 7000m version they calibrate to 7db. At least for the Kessler, Seabird thinks they CAN achieve 2db accuracy, but that would take a lot of effort for them. Discussions are taking place about this. www.ge-mcs.com/en/pressure-and-level/depth-and-level/ptxpdcr-1830.html
Absolute accuracy: 70cm sealevel equivalent for 7000m version Resolution: 0.007mm for 7000m version Stability: 7ppm/yr in some factory barometers, 5cm/yr for 7000m version
Example from PIES (Inverted Echosounder with pressure sensor) Drift once settled on order 5-10cm If drift is removed, precision probably better than 1cm.
But: - Need very accurate frequency counting electronics - Need to make sure it does not move when deployed
New product www.ge-mcs.com/en/pressure-and-level/depth-and-level/depth-level-terps.html