Hi everyone. We’ve been working constantly at getting this Balloon off the ground. Until now, though, we’ve had no way to directly measure how far “off the ground” we’d get. Fortunately, Brad and Dan spent their Friday night building just such a measurement device. When you’re flying a constant altitude balloon like ours, the amount of helium it holds, combined with the weight of the robot pilot dangling off the balloon, directly determines the altitude that you will fly at. When your goal is to cross the Atlantic using the thin Jet Stream, you need to fly at a pretty precise altitude, or you’ll miss it entirely!
Unfortunately, our balloon manufacturer was unable to give us a precise volumetric measurement of the delivered envelopes. Unwilling to bet our sweat and tears on such a response means developing a method to measure the volume of air going into the balloon.
We came up with a few ideas that wouldn’t work:
The last idea inspired us, though. If you can measure the speed of the air you’re putting into the balloon, and measure the cross sectional area of the tube through which the air travels, you can measure the volume of air that’s moved through the tube over time! It’s a simple equation:
Air Speed (M/S) * Cross-Sectional Area (M^2) * Time (S) = Volume (M^3)
How can you measure air speed? Well anemometers work OK, but they’re big and bulky and would block our tube. Airplanes measure their airspeed using a device called a “Pitot Tube.” It looks like the image to the right.
The Pitot Tube works by measuring the difference between the ambient air pressure and the ram-air pressure. Air is forced into the opening of the pitot tube at high velocity, and the difference in pressure gives you the fluid velocity by the following relationship:
That seems pretty simple, doesn’t it? Well, we were able to pull it off with parts lying around the LVL1 hackerspace in one evening!
We happened to have a long plastic conical nozzle, about 3 inches long, sold at auto parts stores as part of a miscellaneous kit of vacuum hose adapters. Looked pretty much like a pitot tube already! It also happened to allow some tubing we had ( 1/6″ ID 1/8″OD) vinyl tubing to fit up right out the tip perfectly.
We hot glued the tubing into the nozzle. and mounted it on a thin sheet of aluminum with a cable tie and hot glue. This would allow us to position the pitot tube in the center of the airflow in the filling tube, with minimal disruption in the airflow itself. Disrupting the airflow too much is BAD for pitot tube accuracy. Streamline it, and the things around and behind it. Make sure it points directly into the airstream.
For measuring balloon volume, we don’t need helium, plain old air is fine. So, we called upon our in-house air source: the high-power vacuum cleaner blower head from a shop vac. It would make quick work of inflating the giant balloon.
We embedded the aluminum strip, with pitot tube, parallel to the airflow in the hard plastic extension tube of our shop vac. A simple slot was cut on each side using a dremel cutting wheel, and secured with hot glue. (Use safety glasses!)
The pitot gives us ram air pressure, which is only half of the answer – the other half comes from the static source. Where do you measure that from? Well, we think that it should be measured in the balloon, a ways away from the fast-rushing airstream coming in. To do that, we the open end of another tube of the same type about 4 feet into the balloon, away from the direction we’d be aiming the blowing the air in.
Now, with two tubes to give us air pressure from two different places, we just needed a differential pressure sensor to measure difference at the static location and the the pitot tube in the airstream. Fortunately for us, a few months ago we had designed and built a sensor module, complete with a differential pressure sensor that perfectly covered the pressure range we needed to measure today. We just pressed the static tube and the ram air tube onto the two ports of the sensor, and were ready to measure the pressure!
We connected the sensor board up to an Arduino microcontroller, and wrote a quick program to read the pressure in kPa every half-second. We connected the Arduino to a PC running a serial port data logger to save the numbers as a text file.
The procedure to get volume would be: record the pressure every half second, inflate the balloon to full, paste the recorded pressure list data into a spreadsheet to convert pressure to airspeed, and then each airspeed * time to get volume of air each time period, and integrate that M^3 volume over time. For our balloon, we ended up with 28.04 M^3, which is just about 990 cubic feet. That’s a reasonable answer. We think it’s probably right.
One thing we know for sure, is it was easy, cheap and fast. It should work with helium right on the launch pad, which *might* really help amateur ballooning launches, where it’s really hard to tell how much helium you’ve put in the balloon. We’ll publish some more details on how to precisely replicate this in the future.