THE BASIC PRINCIPLE
The maximum amount of vapor-phase water that can exist in air (saturation) is strictly a function of the air temperature. If the air is cooled, water vapor condenses to liquid, decreasing the amount of water vapor. A great example is the cold glass of iced tea on a hot summer's day that “beads up” with water droplets. The glass cools the surrounding air and water vapor condenses to liquid water on the glass. The dewpoint is the temperature at which water vapor in the air begins to condense; this depends on how much water vapor is in the air. Dewpoints below 0°C (32°F), where water vapor condenses to solid ice instead of liquid water, are instead known as frost points. This physical principle of frost points is the basis for our hygrometer that measures the amount of water vapor in air.
The FPH uses active cooling and heating to maintain a thin layer of frost on a small polished metal mirror. To achieve this, the mirror temperature must be changed as the amount of water vapor in air varies. The thickness of the frost layer is monitored by shining a small beam of light at the mirror and measuring how much of the light is scattered by the frost. Too much scattering means that the mirror is too cold (frost is too thick) and too little scattering means that the mirror is too warm (frost is too thin). The instrument's microprocessor-controlled feedback loop will then heat (cool) the mirror surface to reduce (increase) the frost to the desired thickness. This feedback occurs many times per second and produces a constant thickness of frost on the mirror. The controlled frost point temperature of the mirror is a direct measure of how much water vapor is in the air.
Once a balloon is launched there is no physical connection (cable, wire) between the instruments and the ground. Instead, data packets from instruments are transmitted at radio frequencies (near 404 MHz ) by a radiosonde. We receive these telemetered data on the ground with a directional Yagi antenna and high-resolution radio frequency receiver, then port the data to a PC running custom software that records and displays all the parameters measured during a flight. The software was written specifically for use with Vaisala RS80 radiosondes, but since Vaisala no longer manufactures the RS80, we are developing and testing new software for use with InterMet radiosondes.
Typical balloon soundings use helium to lift the payload at ~5 m/s (11 mph) until the balloon pops at ~30 km, at which point the payload falls rapidly (>30 m/s or 65 mph) despite the efforts of a small parachute. Our desire to obtain high quality water vapor measurements during both the ascent and descent phases of each flight led us to develop a simple valve mechanism that releases helium from the balloon before it pops. This effectively turns the balloon around, from ascending to descending, without having the payload rocket down. The mechanism consists of a small pressure sensor that, when reaching a pre-set low pressure (~10 mbar), sends current to a nichrome wire to cut the anchors on a tethered PVC “plug” in the neck of the balloon. This action opens the balloon to the ambient atmosphere and slowly releases the helium.
In some cases it is desirable to recover the payload after a flight for the purposes of instrument troubleshooting. We utilize Global Positioning System (GPS) data telemetered from the payload to track the balloon using a car-based GPS receiver, laptop computer, and an application that displays the current payload location on a road map display (includes an estimate of where the payload will land). This has proven highly effective in recovering payloads, even after they have descended from altitudes >30 km (98,400 ft).