The Hydros-21 sensor already does internal temperature compensation of the depth data, so you shouldn’t have to provide your own compensation. The sensor outputs the depth readings in millimeters of water above the sensor, so you don’t even have access to the raw pressure readings like you might with a basic pressure transducer, so you’d have to do any corrections on the sensor’s already-corrected depth output. You could record temperature with a separate external temperature sensor, but we’ve found that the temperature reported by the CTD sensor is accurate, except in cases of extreme rapid temperature changes, in which case it takes anywhere from 5 to 15 minutes for the CTD temperature to match a DS18b20 sensor in the same water (like when we poured refrigerated water on a room-temperature sensor). The new Generation 2 Hydros-12 sensor had a quicker response to the temperature change than the old generation.
We do see a variability of about 5mm to 10mm in the depth reading if the sensor is in a constant depth of water but the water temperature changes from about 10C to 30C over the course of a day. However, that variability depends on the individual sensor, since we tested 10 different sensors all from the same production lot and found that some were more constant across temperature changes than others. So if micro-scale variations in depth or conductivity matter to your experiment, I’d recommend doing some controlled testing with your sensor in a lab environment where you can change the temperature of the water while recording sensor output to get a profile of how your individual sensor responds to changing temperatures. And since the conductivity readings of the Hydros-21 sensors are specific conductivity, then temperature sensitivity affects those readings as well, so check your conductivity readings over a range of temperatures as well. Some of the sensors are more accurate than others, but all of them have been within the design specs of the manufacturer’s stated accuracy levels, but it’s important to find out how precise your sensor is since it varies for each one.