Lab 1: Salinometry

Lab 1: Salinometry#

Lab overview#

Aim: To gain an understanding of how measurement accuracy/precision may affect results.

Learning outcomes: After the session, you will be able to discuss the effect of accuracy and precision on a measurement.

Background: We have various ways to measure salinity in the ocean. The definition of salinity is based on the mass of salt dissolved within the mass of seawater, but ’drying out’ water to measure the residual mass is not a very practical way to make measurements. Instead, we rely on properties of salt water: its conductivity or refractivity. Each sensor type has stated accuracy and uncertainty, and our knowledge of the ocean’s salinity depends on how accurate measurements are. In this lab, you will measure salinity using several different instruments and methods, including

  • A Sea and Sun CTD

  • A microCAT CTD

  • A handheld refractometer

  • A lab-based salinometer with standard seawater

  • Handheld conductivity sensor

At the end, you will make an estimate of the salinity of the water tested, compare the measure- ments from different sensors, and discuss how this might affect our understanding of the salinity in the ocean from e.g., Argo profiling floats.

Directions: In addition to the general requirements to write an experimental design document and report, here you should plan to define terms ‘accuracy’ and ‘precision’, and evaluate what the uncertainty is on a measurement based on measurement uncertainty (the instrument’s accuracy) and on the sampling uncertainty (if repeated measurements are not identical).

Design an experiment#

  1. Get to know your instrumentation

  • For each instrument type, look up its stated accuracy and precision. This will normally be provided in the form of a datasheet, but you may have to look around a bit for estimates from a refractometer and the salinometer.

  • Based on the documentation, which method do you expect to be most accurate?

  • Check the instrument validity ranges. Note that for the salinometer, this will be something near the range of the standard seawater you will be provided with.

  • Make any other notes about the instruments or measurement methods that you think are relevant. E.g., temperature of the room for the salinometer, or anything you’ve learned from the cell thermal mass exercise. What do you need to consider in order for each sensor to provide its most accurate results? Recall that you are writing your experimental protocol as if for gymnasium students–with a fair degree of common sense but no experience handling oceanographic measurements.

  1. Design an experiment

  • In plain language, what are you testing and what do you expect to see? In other words, why are we carrying out this test?

  • Come up with a hypothesis to design an experiment around.

  • What are your variables?

  • What is your dependent variable? The dependent variable is the effect. Its value depends on changes in the independent variable.

  • What is your independent variable?

  1. Outline the methodology

  • How will you set up the experiment, in broad terms. What temperatures/salinities to use, and how to use your sensors?

  • What are the materials required for your experiment? You have access to water, salt, ice, buckets, standard seawater, sensors as above. If you have more requirements, check with instructors.

  1. Revise and finalise

  • Any revisions needed for your hypothesis or experimental design? Can you simplify it?

  • Design the results table/figure. Based on your planned experiment, what form will the results take? Is it a table of discrete values? Is it a graph or a time series plot?

  • Should the experiment be repeated multiple times, or is one time through sufficient? (What about user error?)

  • Imagine you have carried out the experiment and have made a series of measurements. What will you then do to the data to evaluate your hypothesis? How will you quantify uncertainty?

Contents