July 29, Wednesday 

As the electrical components were settled, we were left with finalising the method of sealing the balloon and deciding the appropriate dimensions that could lift the circuitry. After the struggle with heat sealing, we were not enthusiastic on settling with that. We looked up different ways to seal Mylar and contact adhesive was a popular option that stood out.

 

 

We made an ellipsoidal stencil for the balloon that is 130cm by 50cm, which was able to contain an approximate volume of 0.136 cubic metres of helium. We then placed the stencil on the Mylar to draw out the balloon and gave an additional 5cm buffer to account for the sealing. When drawing the stencil and cutting the Mylar out, we had to ensure that the Mylar was taut at all times. As such, we always have weights placed on the stencil.

 

After using the stencil to cut out 2 identical sheets, we placed them on top of each other and folded the top piece into half along the minor axis. The stencil was then positioned in the centre on top of the 2 layers and contact adhesive was applied along the 5cm margin. The stencil was just used as a ‘cover’ to prevent contact adhesive from spattering out of the margin. After lathering the top side, the same was done for the bottom side with the addition of the valve to one of the corners on the longer axis of the balloon. The valve was rescued from a commercial, refillable birthday balloon. We waited 10 to 15 minutes for the adhesive to dry before carefully sticking the folded part of the top sheet to the bottom sheet.

 

We then repeated this whole process for the other half of the balloon.

To reduce the possibility of micro leaks, we applied adhesive along the 5cm margin and folded it in half towards the centre of the balloon (i.e. The margin is now 2.5cm). The second seal was done with adhesive as well instead of ironing since heat may cause the adhesive in the 1st seal to melt. 

 

 

 

Before inflating the balloon with helium, we had to test for micro leaks. To do so, we filled the balloon with nitrogen before spraying the perimeter of the balloon with soap water (50% soap, 50% water). The rationale behind nitrogen is that it is abundant and makes the balloon easier to handle. If a bubble forms at a spot, it indicates that a micro tear is present at that spot and there is nitrogen leaking out. As no bubbles were forming along the edges, it is indicative that the double seal is sufficiently strong. However, there was a bubble forming at the opening of the valve and thus, we had to remember to tape it up after filling it with helium.

 

We also drew a centre line along the major axis of the balloon where we intend to line up our circuitry and payload before inflating the balloon with helium. We wanted to see how much the line would shift after inflation to gauge the shift in centre of gravity and unsurprisingly, the line deviated a little from its original placement.

 

 

By tying the balloon to a weighing machine, we were able to determine its lift. Depressingly, it was only 0.206 N (21g). It was definitely not enough to carry our circuitry and attachment pieces. We had no choice but to make a larger balloon.

 

One of our deductions was that the pointy ends and thin profile severely limited how much the Mylar could expand as the seams were all very close to one another. We chose to abandon our marquise balloon and instead opt for a rounder shape.

Aerodynamics of the balloon was still a concern for us as we were unsure of whether drag would have significant effects at low speeds. Having a rounder, wider balloon would raise its drag coefficient.

 

 

Flow simulations of 3m/s over a long cylinder (drag coefficient = 0.83) and a streamline body (drag coefficient = 0.04) were done on ANSYS Student. Both bodies were fixed to have a volume of approximately 0.029m3 such that its shape was the only varying factor.

 

 

The simulations show a very small and negligible difference in velocity streamline, and this difference is likely to decrease as the velocity of flow decreases. Thus, we were more comfortable in overlooking drag coefficient and choosing a rounded shape that is capable of increased inflation.