Measuring pressure

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 Paroscientific Digiquartz. 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 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 $1450 extra ($ 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.

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

Drift once settled on order 5-10cm If drift is removed, precision probably better than 1cm. Example from PIES (Inverted Echosounder with pressure sensor)

But: - Need very accurate frequency counting electronics - Need to make sure it does not move when deployed

New product