Week 7

Week 7

As we narrowed the critical pressure threshold for the glass VacuStor tube, we noticed a slight complication. Depending on the speed at which we pierced the cap of the tube to transfer the liquid, we got differing amounts of sample transfer. Piercing the tube faster resulted in more sample transfer. We realized that this is due to pressure leak during the puncture, but before the bevel has entered the tube. The sample only begins to be drawn into the tube when the bevel fully enters the tube. However, if the bevel is longer than the cap thickness, there is pressure leakage until the bevel enters wholly into the tube. Because there is such a huge pressure difference between the internal pressure and the external pressure, we have to find a needle that has a bevel shorter than the cap thickness. Thus, we slightly modified the experiment by manufacturing new needles with capillaries that have a shorter bevel.

Modified Needles with Capillary Tubes

We used the same method to make these needles as the previous needles. However, we used 30 Gauge 1/2 inch needles instead of 21 Gauge 1 inch needles. Using the UV epoxy, we attached the capillaries to the needles. This was the result:

Modified Capillary Tube with Needle

This needle has a much shorter bevel as compared to the previous needles we used, and is also much thinner and shorter. The following pics show a comparison of the two needles (the green needle is the old needle):

Comparison of Bevel Length

Comparison of Length of Needle

This should aid in the sample transfer to provide more accurate results. Hence, this week I made these needles, which will be the needles that we will use for the remainder of experimentation.