Our first day started out the usual way, with introductions and a review of the syllabus. I gave an introductory lecture on the scientific method, Global Positioning Systems (GPS) and water column properties. These properties include things like salinity, temperature, pH, dissolved oxygen, light and others that describe the physical, chemical and biological conditions in the water. After my lecture, which hopefully didn't put too many of the students to sleep, we took a variety of equipment out to the Savannah State dock to measure some of these properties in Country Club Creek.
Place is very important in science, so we attempted to compare the estimates of our location given by three identical (somewhat old) Garmin 12 GPS handheld receivers. Despite the fact that I tested them out before class, two of the three had low batteries and quit on us before getting a reading. After class, I changed the batteries and completed the comparison in the parking lot outside of my lab building. All three gave measurements within about 3 m of each other or within one parking space (see photo below in which each yellow pin indicates the position given by one GPS unit). The image was created using Google Earth.
One thing that is immediately obvious when you visit the Georgia coast is that our water isn't very clear. We used a Secchi disk to measure turbidity (how cloudy the water is due to suspended particles) and were only able to see it to a depth of just over 1 ft. In clear water, a Secchi disk can be seen at depths of over 100 ft! We also measured salinity, temperature, pH and dissolved oxygen using a variety of instruments from simple pH strips and handheld refractometers to the workhorse YSI 85 and somewhat more capable YSI 556. Not all of these instruments agreed on every measurement, highlighting my point during lecture that it is essential to make sure an instrument is properly calibrated before using it to collect data.
Stay tuned for more pictures and future posts from the students...
Morning, I just wanted to add a few salinity-measurement specific comments, things to think about when you are measuring salinity in the field (or the laboratory!)
ReplyDeleteRefractometry and conductivity are strongly temperature sensistive. Calibration at normal laboratory temparatures will affect readings in the field. Conductivity instruments normally account for this with co-measurment of temperature and mathematical correction, but this introduces error in the measurement. High-precision, high-accuracy field conductivity meters make sure that the sample and electrodes are in a temperature controlled water bath to maintain constant temperature and calibration.
Another less common measure, but extremely helpful with widely-varying temperature samples, vapor pressure osmometry is not temperature sensitive. While less precise than conductivity, it could be argued that osmometry is more accuracte than some conductivity measures. Further osmometry requires taking a larger instrument in the field, but the sample size is the smallest of any salinity measure at 10uL.
Lastly, titration with silver nitrate is by far the most accurate and prcise method to measure chlorinity (salinity) if you are a trained and practiced technician/researcher. However, this is the most time consuming and sample size specific salinity measure.