Week 5 Progress

Revisions were made to the design. PVC would be used for the design in this term. Bamboo is very aesthetically pleasing but there are many disadvantages. One problem with bamboo is that bamboo will rot when it is exposed to water for long time. This is exceptionally inconvenient for the design since the hydroponic system design has water running through it constantly. An idea was brought up to coat the inside of the bamboo with a deck sealant to make the inside of bamboo waterproof, but this idea was not very plausible. Bamboo is also quite flexible, so when the planter is exposed to strong winds as well as intense weather, a structure made of bamboo poles will not be ideal. Also, bamboo is not grown commercially on the East Coast, so the best place to get it is from California or from overseas, but the cost for getting bamboo poles will be rather high (over $30 per pole plus extra money for removing nodes and high shipping fees). The biggest problem about using bamboo is that big hole cuts are needed on the side bamboo poles to connect the horizontal poles. The cuts need to be about the diameter of the horizontal poles, so these cuts will remove much fiber support which can increase local stress of the joint regions and cause fracture.  On the other hand, PVC pipes are much sturdier, so they can withstand the intense weather. PVC can also withstand constant exposure of water and UV. PVC pipes are much cheaper and easier to obtain. Connecting PVC tubes is much easier with the help of connectors as shown in Figure 3. After the comparison, PVC, as the material that is more available and is easier to wok with, replaces bamboo as the construction material, but constructing with bamboo or other more sustainable materials will be one of the future research focuses of this project.



Figure 3. Example T-joint connector [14]



Using T-joints instead of cutting holes on the vertical pipes not only prevent strength loss of the vertical support, but also increase the tensile strength at the connection region by increasing stress concentration factor, which is the ratio of maximum local stress to overall background stress. Based on the relationships shown in formula (1) and (2), increasing the fillet radius r or the major shaft diameter D can increase SCF. D, r, and d are labeled in Figure 4 [15]. T-joint connectors increase both the fillet radius and the major shaft diameter, so it can increase SCF. With higher SCF, the connection joint can tolerate a higher local stress σ * SCF.

SCF ∝ r/d       (1)

SCF ∝ D/d      (2)

Figure 4. Dimensions used for SCF calculation [15]

Another design revision is that the planter would lean against a wall or hook onto a wall to increase the sturdiness and allow the planter to withstand more weight.

The hydroponic design was finalized so construction of the planter can start soon. A pump was obtained from the ECE department. For pressure control, a valve can be set up to limit the pressure of the water going through the system if the pump is too strong, and the reservoir at the base of the planter needs another valve to limit the amount of water the will be circulating through the system at once to minimize the amount of water that is needed to keep the system running.

A wick system was discovered that could be incorporated into the final design for the hydroponic system and make the system simpler. Figure 5 shows an example of the wick system which at this point is in its initial phases in our design. The idea is that the water moves its way up the wick and keeps the plants watered without over watering the plants or wasting water [16]. This hydroponic wick system will be taken into consideration during construction.

Figure 5. Wick System [16]

Finally, the PVC pipes were obtained through a generous donation, so the group will begin physical construction work in Week 6.