Table 4 Data collection by hydrographic sensors

For a summary of CTD data collection please see the Supplementary material: Temperature and salinity cast records.

i. Pressure (Depth) and Temperature

The nominal accuracy of the pressure transducer is approximately 20 m for the entire range of temperature and pressure it is designed to sample (from 0 m to 2000 m and from -40 to +80 °C). In practice the temperature range is much less than these limits, and it has been found that the error in surface pressure following a deep dive can be about 0.5-1.5 m rather than zero. To minimize the pressure measurement error as much as possible, the instrument updates the zero-offset, whenever the wet/dry sensor reads “dry”. The exact value of the zero-offset will vary according to the hydrographic conditions, which is why the instrument is programmed to update this value whenever the sensor reads “dry”. The pressure reading is deliberately shifted away from zero to avoid interference from electrical noise generated by the tag.

Depth within a dive is calculated as follows: 1) pressure is collected, 2) the zero-offset is subtracted from the pressure measurement, 3) the resulting value is converted to depth, and 4) pressure is rounded to the nearest dbar (or nearest 2 for deep diving species) for the purpose of constructing a temperature vs depth or salinity vs depth profile. The resulting depth measurements are processed by the instrument at its internal resolution of 10 cm for behavioural samples such as the dive shape buffer.

Upcasts performed by the current generation of CTD-SRDLs yield 17 values each for temperature and conductivity. Temperature is collected at a resolution of 0.001 K within a valid range of -3 to 40 °C (see Supplementary material: Temperature and salinity cast records).

The minimum and maximum temperature within an upcast are sent with high precision, and each of the 17 temperature values is expressed as a percentage of the range (0 = Tmin, 100 = Tmax), in units of 1/1000. The resolution of the temperature measurements will be 0.001 K if the temperature range of the profile is less than 1 K, or (Tmax-Tmin)/1000 if the range is greater than 1 K. For CTD-SRDLs accuracy of the temperature measurement is at least 0.01 K.

ii. Salinity

Salinity is treated in the same way as temperature (see Supplementary material: Temperature and salinity cast records). Conductivity and temperature are used to calculate salinity, then conductivity is discarded. Salinity is stored at a resolution of 0.001 within a valid range of 10 to 50 psu. Only minimum and maximum salinity within an upcast are sent and each of the 17 salinity values is expressed as a percentage of the range (0 = Smin, 100 = Smax), in units of 1/250. Minimum and maximum salinities are sent with resolution 0.02 psu, which will limit the resolution of the salinity measurements when the salinity range of the profile is less than 5 psu. If the range is greater than 5 psu the resolution is (Smax-Smin)/250.

Additional information on the post processed accuracy of the CTD data can be found in [10] and at [25].