Centrifugation is an important separation technique for sample preparation. It allows you to separate substances from the liquid with different densities. For observing pond lives, you can find the organisms easier by increasing the concentration of organisms. An article described an inexpensive hand-held centrifuge, but its speed is depending on how fast you crank. There is also a danger of tube fly away from the centrifuge. Other homemade centrifuge has used to centrifuge samples in two 15 mL centrifuge tubes with VCR motor. The VCR centrifuge can achieve 250 rpm with 6 xg. I don’t have VCR to spare but I do have an outdated computer. I removed the computer cooling fan and used other very inexpensive and easily obtainable materials such as foam plate and bowl and white glue to make a centrifuge. The picture is shown below.
Materials you will needs:
- 12V Computer cooling fan (you can remove it from an old computer or purchased it from electronic store)
- Cell phone charger 12 V or AC adapter
- Foam plate and bowl
- Spacer: two rods-shaped and one circle-shaped
- An old CD
- A piece of board
- Four screws
- White glue
- Craft knife
- Protractor and ruler
- Drill (Optional)
- Centrifuge Tachometer (Optional, if you want to measure the speed)
Step by Step instruction
- Place the computer cooling fan on the center of a piece of board. Pre-drill four holes on the board through the four corner of the computer cooling fan (there should be some holes on the fan for the computer manufacturer to fasten the fan to the computer case).
- Put the two rod-shapped spacers on the board and below the cooling fan.
- Use four screws to fasten the cooling fan to the board. There should be some space for air to pass through from the bottom the fan since you have placed the spacer under he cooling fan.
- Glue the round-shaped spacer on the fan (make sure that it is centered)
- Glue the CD on the spacer (I concerned about the foam plate might sag over time. It could touch the cooling fan so I added a hard surface in between).
- Glue the foam plate to the CD
- Take a foam bowl and cut approximately 1.5 cm from the edge and glue it onto the plate (Optional).
- Take another foam bowl and draw three lines with pencil using the protractor and ruler. These lines should pass through the center of the bowl and equally divide the circle into 6 arcs. Mark 6 circles from about 0.4 cm from the edge using the centrifuge tube as guide. Cut out 6 holes on the bottom about 0.4 cm from the edge of the bottom using a craft knife. These holes will be served as the tube holder for your centrifuge tubes.
- Extend these three lines to the side of the bowl and mark 6 pairs of holes at the mid-point on the both side of the line. The distances between the each pair should be slightly larger than the diameter of the tube.
- Take six wires (I used ties from the bread package). Poke a hole through the outside of the bowl through the mark and come out from the other and tie it on outside.
- Glue the foam bowl to the plate.
- Place a book on the top of the foam bowl overnight or until the glue is completely dry.
- Connect the AC adapter or cell phone charge with the cooling fan.
Let’s put the centrifuge to test
The choice of the material for the rotor is obvious. Computer cooling fans have very little power to carry heavy rotor. The rotor has to be light so the foam plates and bowls are used. They are also resembled the shap of a real centrifuge rotor. I am wondering how fast the light-weight motor and light-weight rotor can go. The centrifuge tubes are also very light-weighted. They are clear acrylic vinyl tubes with a softer translucent cap stopper. The tubes are about 7 cm long and 1 cm interior diameter. The volume for those tubes are about 3.5 mL. For the speed test, I used a centrifuge tachometer that is typically used for centrifuge calibration. I place a tiny reflectory sticker slightly off-center of the centrifuge rotor. The tachometer emits a IR beam and count the IR beam reflected from the sticker on the top of the rotor. The rpm was calculated using the count of the IR reflection and the internal timer of the tachometer. The speed of the centrifuges was measured 15 seconds after I started the centrifuge. :
- no tube: 950 – 1100 rpm (70.6 xg – 94.7 xg)
- two tubes: 750 – 920 rpm (44.0 xg – 66.2 xg)
- four tubes: 500 – 650 rpm (19.6 xg – 33.1 xg)
- six tubes: 250 – 300 rpm (4.9 xg - 7.0 xg)
The speed of the centrifuge is not constant. It could be varied as much as 20 percent. The more tubes that I put on the rotor the slower it was since the tube took up significant weight of the spinning part. The centrifuge also took longer time to get the speed to be stabilized when it bared more weight. In addition to the speed of the centrifuge, the diameter of the centrifuge rotor also affect the efficiency of the centrifuge. The larger of the diameter that more efficient the centrifuges are. The efficiency of the centrifuge is measured by relative centrifugal force (RCF). It is calculated by the following formula: 0.0000118 x rpm ^2 * r. As the number of tubes increased from two to six, the relative centrifugal force dropped in a magnitude of 10. Significantly longer time to reach maximum speed is required for more tubes.
What can be improved? From this experiment, I demonstrated that a simple computer cooling fan centrifuge can achieve the the rpm suitable for concentrating pond life (too high will actually damage the structure of your specimen). How it is still not ideal:
- Low sample volume: one tube can only accommodate 3.5 mL with total volume of 21 mL
- Centrifugation speed are significantly lower if more samples are loaded. A more powerful motor is needed.
- The operation is somewhat unstable. From the video, you can see the centrifuge has some vibration even when I balance the tube. One side of the centrifuge stays down when I tilt the centrifuge.
- Centrifuge speed is not constant and it takes a little while to get up to speed.
although I have been operating this centrifuge for a little more than four hours and continuously for an hour, the reliability is still up to test. I hope that the white glue will hold up :-)
The real world test To put the centrifuge into real world application, I fill up two 3.5 mL centrifuge tubes with pond water (I sampled it from nearby pond two weeks ago. I store them in a small jar with aquarium pump to maintain sufficient oxygen.) and spinned them in the centrifuge for 2o minutes. Amazing! These are the partial collection of what I found in a drop of water (I can’t identify many of those. It looks like I have plenty of homework to do!):
The above photos were taken with Tucsen Microscope Camera in Zeiss AxioVision LE 4.8.1 software in best fit setting at 100X to 400X through the eyepiece of Nexcope CM701 biological microscope. See my other post about the free Zeiss AxioVision LE Software.
I’ve found many diatoms in this collection but I still could not identify many of them. Some useful diatom identification websites:
More projects using the microscope centrifuge:
What lives in the drainage pans of flow pots (a year later, it’s still working)