For many experiments, calibrating the Salinity Sensor is not required. We store a calibration equation on each sensor before shipping it, which is used as a default by our software.
For the most accurate measurements with this sensor, we recommend calibration. It is a simple process that takes only a few minutes.
Making Standard Calibration Solutions
If you choose to calibrate the Salinity Sensor, you will want an accurate standard solution. Vernier sells a 35 ppt standard solution (order code SAL-ST). To prepare your own 35 ppt standard solution using solid NaCl, use a container with accurate volume markings (e.g., volumetric flask). Add 33.03 g NaCl to enough distilled water to make 1 liter of solution. Use reagent grade NaCl.
Note: The Practical Salinity Scale 1978 was developed relative to a KCl solution. Standard Methods for the Examination of Water and Wastewater states that “A seawater with a conductivity at 15°C equal to that of a KCl solution containing a mass of 32.4356 g in a mass of 1 kg of solution is defined as having a practical salinity of 35.”
Automatic Temperature Compensation
Your Vernier Salinity Sensor is automatically temperature compensated between temperatures of 5°C and 35°C. Note that the temperature of a solution is being read by a thermistor that extends into the space between the graphite electrodes. Readings are automatically referenced to a salinity value at 25°C. Therefore, the Salinity Sensor will give the same reading in a solution that is at 15°C as it would if the same solution were warmed to 25°C. This means you can calibrate your probe in the lab, and then use these stored calibrations to take readings in colder (or warmer) water in a lake or stream. If the probe was not temperature compensated, you would notice a change in the reading as temperature changed, even though the actual ion concentration did not change.