Marina Caporaloni

GPS and timing: didactic applications on how to choose the best clock

In spite of its low cost, a modern global positioning system (GPS) module can be used to generate an extremely accurate time scale, starting from the one peak per second (1 PPS) signal delivered to the user. We guided our students, following the first experimental physics course, to compare the GPS pulse with the 1 PPS signal obtained by division from a quartz oscillator built by themselves. They were able to show that, over short time scales, the stability of the GPS signal is significantly poorer than over long time scales. As a conclusion, they recognized the need to combine different oscillators in order to obtain the best performing clock over all time scales. In making the measurements, students had to refine their analytical skills in order to understand the limitations of modern automatic data acquisition systems.

A versatile implementation of the psychrometer technique as a learning opportunity in atmospheric physics courses

Even today the psychrometer technique, if properly implemented, is used as a calibration standard for humidity measurements. In order to simplify the cumbersome use of the classical instrument, we recently proposed an original configuration characterized by the unattended operation and real-time readout of temperature sensors. More recently, we have upgraded that system by applying the online data acquisition controlled by a LabVIEW code which also displays the final observable of relative humidity. The program implements the psychrometer algorithm, usually available only in the form of tables, and also the data recording on disk. We describe here how to properly build the instrument and how to guarantee its intrinsic accuracy (typical uncertainty within a few per cent) as well as all the details of the formulae used. The psychrometer, proposed as a project work to university students following new courses on meteorological instrumentation, was found to be a powerful source of learning opportunities. Although at first, the working principle of the instrument looks easy and offers an intuitive interpretation of the concept of humidity, later students become aware of how difficult requirements have to be satisfied in order to realize a reference standard. As a final verification, they are asked to guarantee that this psychrometer implementation strictly conforms to the official recommendations of the World Meteorological Organization (WMO). We present here a few examples of their activities in planning autonomously a series of checks and measurements.

Humidity Measurements: A Psychrometer Suitable for On-Line Data Acquisition.

A psychrometer suitable for on-line data acquisition is described for making humidity measurements.