Why and how should I align data from a 911plus CTD?
The T-C Duct on a 911plus imposes a fixed delay (lag time) between the temperature measurement and the conductivity measurement reported in a given data scan. The delay is due to the time it takes for water to transit from the thermistor to the conductivity cell, and is determined by flow rate (pump rate). The average flow rate for a 9plus is about 30 ml/sec. The Deck Unit (11plus) automatically advances conductivity (moves it forward in time relative to temperature) on the fly by a user-programmable amount (default value of 0.073 seconds), before the data is logged on your computer. This default value is about right for a typical 9plus flow rate. Any fine-tuning adjustments to this advance are determined by looking for salinity spikes corresponding to sharp temperature steps in the profile and, via the SBE Data Processing module Align CTD, trying different additions (+ or -) to the 0.073 seconds applied by the Deck Unit, until the spikes are minimized. Having found this optimum advance for your CTD (corresponding to its particular flow rate), you can use that value for all future casts (change the value in the Deck Unit) unless the CTD plumbing (hence flow rate) is changed.
Oxygen and other parameters from pumped sensors in the same flow as the CT sensors can also be re-aligned in time relative to temperature, to account for the transit time of water through the plumbing. A typical plumbing delay for the SBE 43 DO Sensor is 2 seconds. However, the DO sensor time constant varies from approximately 2 seconds at 25 °C to 5 seconds at 0 °C. So, you should add some advance time for this as well (total delay = plumbing delay + response time). As for the conductivity alignment, the Deck Unit can automatically advance oxygen on the fly by a user-programmable amount (default value of 0 seconds) before the data is logged on your computer. However, because there is more variability in the advance, most users choose to do the advance in post-processing, via the SBE Data Processing module Align CTD. For additional information and discussion, refer to Module 9 of our training class and the SBE Data Processing manual.
Note: Alignment values are actually entered in the 11plus Deck Unit and in SBE Data Processing relative to the pressure measurement. For the 9plus, it is sufficiently correct to assume that the temperature measurement is made at the same instant in time and space as the pressure measurement.