Nuts and Volts - June 2017

As I’ve explained to some of my clients in the past, you can’t just mix together two datasets recorded by two different dataloggers and sensors under different conditions and expect valid results. In the case described above, there’s a delay between the time of a position report and the time the flight computer collected the environmental data. In some cases, this probably isn’t much of a problem, but it’s hard to know for certain.

I can simplify and shorten the work I do after recovery if the same datalogger records both the environmental data and the GPS position data at the same time. This has the added benefit of increasing the precision of my environmental data. Sounds like a winner of an idea all around.

I next realized that a datalogger that records altitude can also record the speed and direction of the balloon. Since the balloon is a captive of the wind, this datalogger would be recording wind speed and direction as a function of altitude.

Therefore, by adding a temperature, pressure, and relative humidity sensor to the datalogger, I could turn the datalogger into a complete near space weather station. As you’ll see later, it can expand its functionality to include ground weather stations, a hiking data computer, and a UAV weather station.

An Introduction to the Sensors

The first sensor is the LM335: a three-pin temperature sensor that produces a voltage in proportion to its temperature. The LM335 is a TO-

92 device that only requires two of its pins be connected (the third is an adjustment lead for output calibration, but can be compensated for in a spreadsheet). The LM335 is essentially a temperature-controlled zener diode with a linear output.

Ideally, it produces zero volts at a temperature of Absolute Zero (don’t we all?) and five volts at a temperature of 500 Kelvins. So, if one multiplies the output voltage of the LM335 by a factor of 100, the result would equal the temperature in Kelvins. It’s also easy to convert the temperature into Celsius by subtracting 273. For those who are metrically challenged, the temperature can be converted into units of Fahrenheit following one of two popular methods.

The second sensor is the HIH- 4000: a three-pin relative humidity sensor. This Honeywell produced device generates a voltage proportional to the relative humidity.

My near space missions are tracked using amateur radio. After recovery, the position reports from the balloon are saved into a file and then imported into a spreadsheet where I can make charts of the data. That data is not very useful unless I link the data with the balloon’s altitude. The process is not particularly onerous, but it does take some time and does lack some accuracy since the time the data was collected doesn’t correspond exactly with the time the balloon’s position data was transmitted. So, I decided to create a weather station that records position and environmental data at the same time. The result is the recording weather station I’m describing this month. You’ll see that with minor modifications, it can turn into other things like a weather station for ground use, for UAV (unmanned aerial vehicle) use, and hikers.