Poorman's Space Program

Designing Near Space Experiments (Part 1)

Paul Verhage

In this and the next two columns we will learn how to design a few experiments for a BalloonSat, for without an experiment onboard, there's very little reason to launch a BalloonSat. Your challenge in the last two columns ("Designing and Constructing BalloonSat Airframes," Part 1 and Part 2) was to assemble a BalloonSat airframe. Now it's time to design an experiment for the airframe capable of flying into near space and returning useful data.

3.1 Data Loggers

Data loggers are stand-alone devices that record data from one or more sensors. Instructions for which sensor is selected and how often it is sampled are programmed into the data logger through a PC. Since a data logger stores data and doesn't transmit it over a radio signal, the data logger must be recovered after its mission and connected to a PC for downloading. The software that programs the data logger also downloads the data stored within it.

3.1.1 Thermochron Data Logger

Maxim IC sells the former Dallas Semiconductor's line of one-wire devices. One of these, the Thermochron, is a stainless steel can about the size of five stacked dimes that samples temperature and records the time of each sample. Inside of the can are a lithium battery (good for ten years) and a silicon chip containing a clock, temperature sensor, and memory circuitry. The software that programs the Thermochron is iButton-TMEX and is available at no cost.

Figure 3.1. A Thermochron is a very tough data logger installed in a hermetically sealed, stainless steel case. Programming and downloading takes place through its body.

Since each Thermochron has a unique ID, they're ideal for comparing temperatures between multiple objects during the same near space flight. Each Thermochron's ID is printed on its case, written into its memory, and included with its downloaded data. Therefore, it is important to document the ID of each Thermochron in an experiment before the BalloonSat's launch.

Purchase a plastic fob for each Thermochron. Fobs hold Thermochrons and increase their visibility. They also allow Thermochrons to attach firmly to the inside of the BalloonSat. More Thermochron information is available at the Dallas-Maxim web site, www.maxim-ic.com.

3.1.2 Programming the Thermochron

To program a Thermochron, follow these steps:

Plug the iButton reader into the PC.

Plug a Thermochron into the reader.

Select the iButton Viewer under the iButton-TMEX group.

Ignore the Format Window and go straight to the Thermochron Viewer.

Click the Wizard Tab.

Click NEXT (set the time in the Thermochron).

Select to set the Thermochron to the PC's clock. First, set your PC time to the time displayed on a GPS receiver.

Click NEXT (skip setting an alarm).

Click NEXT (set the Mission Start Delay). You must do a little math, as the delay is set in days, hours, and minutes from this moment. Don't set the delay time to the beginning of the mission; instead, begin recording data before the expected launch time.

Click NEXT (set the sample rate). The shortest sample rate is once a minute and adequate for most missions.

Click NEXT (do not set Temperature Alarms).

Click NEXT ( do not Enable Roll-Over).

Click NEXT.

Click FINISH .

After the PC finishes downloading instructions to the Thermochron, remove it from its reader and load it onboard the BalloonSat. Then after recovery, use the iButton software to download data from the Thermochron by following these steps.

Click Read Data.

After downloading the data, there are two choices.

Click Quick Graph to generate a graph.

Or click Export Result to export the results into a text file. Use a meaningful name for the file name so you can find the file again.

Stop the mission after downloading the Thermochron's data. There's no need to use the Thermochron's internal battery collecting more data.when you don't need the data.

3.1.3 HOBO Data Loggers

Hobos, a diverse family of data loggers manufactured by Onset, are perfect as BalloonSat flight data loggers. The four families of Hobos discussed in the BalloonSat Principia are the H08, U10, U12, and UA. The major difference between the first three families is their number of bits of resolution and amount of memory. The last family, UA, has a substantially different shape and is called the Pendant.

Figure 3.2. A two channel Hobo data logger with temperature and external channel. Its temperature sensor is internal to the unit and not visible. The external channel allows a sensor to be plugged into the black jack at the top.

Hobos, like other data loggers, do not actually record the voltage of the sensor attached to them. That's because data loggers are digital devices and voltage levels are analog values. Therefore, a data logger must first convert an analog voltage into a digital value before it can be stored in memory. A circuit called an analog-to-digital converter (ADC) performs the process of converting an analog voltage into a digital word.

ADCs are designed with a maximum input voltage and a maximum binary output. For instance, many ADCs are limited to an input voltage of 5.0 volts while the Hobo is limited to half that, or 2.5 volts. The output from an ADC is a binary number, a series of ones and zeros, with a fixed number of binary digits (or bits). The ADC inside the Hobo 08H family provides an eight-bit number. Therefore, H08 Hobos convert an input of 0 to 2.5 volts into a binary value containing eight bits spanning from 0000 0000 (0 in base 10) to 1111 1111 (255 in base 10). An ADC that produces an output of eight bits is said to have eight bits of resolution. Some of the Hobos record voltages with 10 or even 12 bits of resolution. These Hobos also have a maximum input voltage of 2.5 volts, but their increased number of bits of resolution allows them to detect smaller changes in the voltage of their sensor.

Because the air temperature in near space drops below the lowest temperature a Hobo can record, a Hobo is best used to record temperatures inside the BalloonSat. However, you can still determine the altitude of the troposphere/stratosphere boundary with the Hobo inside the BalloonSat because the interior temperature of the BalloonSat tracks changes in the outside air temperature.

The most affordable Hobo data loggers are the H08, U10, U12, and UA families and described below. Hobos in the H08, U10, and U12 families are a little larger than a box of matches and weigh nearly 28 grams (one ounce). The UA Pendants are slightly smaller and weigh 15 grams (0.5 ounce). The H08 family is an 8-bit data logger that can record nearly 8,000 measurements. The U10 is a 10-bit data logger that records 52,000 measurements. And the U12 is a 12 bit data logger capable of recording up to 43,000 measurements.

One-Channel Hobos

The least expensive Hobo data loggers are the single channel units record only temperature. Onset sells two single channel loggers, the H08-001-02 and U10-001. The temperature sensors in these Hobos are part of their internal circuitry; making them ideal Hobos for recording the BalloonSat's internal temperature.

Two-Channel Hobos

Onset sells two Hobo data loggers with two channels, the H08-003-02 and U10-003. Both units employ internal sensors that record temperature and relative humidity. Since it makes more sense to record the relative humidity of the air outside of the BalloonSat than then inside of it, a BalloonSat containing one of these Hobos needs an opening so the Hobo's relative humidity sensor can sample the outside air.

Four-Channel Hobos

The H08-007-IS, H08-007-02, and U12-013 data loggers record temperature, relative humidity, and two external channels. The H08-004-02 and U12-012 record temperature, light intensity, relative humidity, and one external channel. And the H08-006-04 and U12-006 are designed only for external sensors.

External channels require external sensors that plug into jacks on the side of the Hobo. Therefore, BalloonSats that carry Hobos with external channels therefore, are larger than other BalloonSats because of all the sensor wires.

A BalloonSat carrying a Hobo with light sensor needs an opening over the sensor. A useful addition to the BalloonSat for the light sensor is half ping pong ball over the opening. A ping pong ball diffuses sunlight and makes the light sensor less sensitive to pointing direction.

Hobo Pendant Loggers

Pendant loggers measure 23 by 13 by 10 mm (0.9 by 0.5 by 0.4 inches). They're complete data loggers that do not accept external inputs. Pendant style data loggers are programmed and downloaded through their Base Station via a USB port.

Figure 3.3. A G Force Logger Pendant.

One-Channel Pendants

The UA-001-64 records temperature. With 64k bytes of memory and ten bits of resolution, it can record up to 28,000 measurements. The UA-001-08 records light intensity. With 8k bytes of memory, it can record 3,500 measurements.

Two-Channel Pendants

The UA-002-08 and UA-002-64 record temperature and light intensity. The -08 version records 3,500 measurements and the -64 version records 28,000 measurements.

Three-Channel Pendants

The UA-004-64, which is called the Pendant G Logger, records three axes of gravity, force and tilt. It's limited to a maximum of three g's acceleration. Unlike the other pendant style loggers, this data logger has only eight bits of resolution but can record up to 22,000 measurements. However, with those eight bits of resolution, it can detect changes in acceleration as small as 7% of a g or tilts as small as 1.4 degrees. The Pendant G Force can record acceleration values 100 times per second. However, to have enough memory for an entire flight, the sampling rate must be no more than five samples per second, or 5 Hz.

There's more Hobo information at the Onset Computing web site, www.onsetcomp.com.

3.1.4 Programming Hobo Data Loggers

A program called Box Car programs Hobos in the H08 family while Hobo Lite programs the U10, U12, and UA families. Both programs are available separately from Onset. The H08 Hobo is programmed through a serial cable that connects to a PC's serial port, and the U10 and U12 Hobos are programmed via a USB cable. The UA pendants are also programmed though a USB cable, but the cable connects to the base station and a light link transfers data and instructions.

Before programming a Hobo, set the PC's clock to the same time displayed on a GPS receiver. By aligning the PC clock to a GPS clock, data from the Hobo can be correlated to the time and altitude of the near space flight. Program the Hobo the night before launch as there's not much free time during a launch. A Hobo is programmed though the following steps:

Start Boxcar.

Connect the Hobo to the PC.

Click Logger.

Click Launch . If the Battery Level gas gauge on the right side appears to be nearly empty, replace the Hobo or its battery

Type a meaningful name into the Description window.

Select an interval, or time between measurements. Be sure the interval allows the Hobo to record for at least three hours.

Click Advanced Options.

Make sure the Wrap Around option is not clicked. This is to ensure your flight data is not overwritten if the BalloonSat cannot be recovered for a day or more.

Click Delayed Start and then enter a start time about an hour before the launch.

Select to Enable or Disable channels. By disabling unused channels, the Hobo will have more memory space.

Click the Start button.

The Hobo begins digitizing and recording voltages at the time programmed in the DELAYED START field and will continue recording data until either it runs out of memory or until stopped by downloading data.

After completing the mission, remove the Hobo from the BalloonSat, connect it to a PC with Boxcar, and follow these steps to retrieve its data.

Start Boxcar.

Click Logger.

Click Readout.

The Hobo's data is stored as a file on your PC. Therefore give it a meaningful name so you can identify it later. After downloading and saving data, Boxcar creates a chart using the first channel of data. The basic Boxcar program can only display one channel of data at a time. To change the displayed channel, follow the steps below.

Click View.

Click Display Options.

Click Channel then select the channel you want to look at. The results are displayed in a new graph.

If you're happy with the data, then export the data into a file for later processing in an Excel spreadsheet. To do this, follow these steps.

Click File.

Click Export.

Select Custom.

Custom allows you to edit the data as a text file before moving it to a spreadsheet.

3.1.5 Available Hobo Data Logger Sensors

At least three sensors are available for Hobos with external channels. The TMC1-HD is a temperature sensor for the H08 Hobo family. The TEL-7001 a carbon monoxide sensor for the U12 Hobo family. And Cable-4-20MA is a current converting cable for the H08 Hobo family. The TMC1-HD is limited to a minimum temperature of -40 degrees C (-40 degrees F), and so cannot measure the lowest outside air temperatures in near space. The Cable-4-20MA is useful for measuring the output of current sensors like solar cells.

3.1.6 Making Sensors for Hobo Data Loggers

An alternative to purchasing external sensors is to make them. The easiest to make are those containing a fixed resistor and a variable resistor that changes its resistance in response to environmental conditions. A sensor like this, with two resistors wired in series with a battery and ground (typically, ground is the negative terminal of a battery) forms a voltage divider circuit as illustrated below.

Figure 3.4. A typical voltage divider circuit. As the sensor's resistance changes, so does the voltage measured by the Hobo.

In the voltage divider circuit, the voltage measured by the data logger (which is the same as the voltage dropped across the bottom resistor) is proportional to the resistance of the second resistor and the supply voltage. This voltage can be calculated with the following equation:

Vi = V applied X ( Ri / ( Ri + Ro))

In this formula, Ri is the resistance of the resistor of interest (the resistance of the bottom resistor or sensor) and Ro is the resistance of the top or fixed value resistor.

By itself, the voltage divider circuit is not very interesting. However, things get interesting when one element becomes variable and changes its resistance due to changes in some environmental condition. Then by measuring the voltage drop across the variable resistor, we can measure the environmental variable of interest.

When designing a sensor for the Hobo, keep in mind that the voltage and current limits of the Hobo are 2.5 volts and 20 mA respectively. Sensors that produce more than 2.5 volts are still useable if a second voltage divider circuit reduces the maximum voltage of the sensor circuit to a safer 2.5 volt limit.

Part 2 of "Designing Near Space Experiments" will be published in the August 2008 installment of The Citizen Scientist.