Inexperienced buyers of orbital shakers often neglect to take into account orbit diameter when selecting an instrument, not realizing how important it is. The orbit diameter of an instrument determines what applications and vessels it is more suitable for.
For example, look at the 19 mm (3/4") diameter orbital motion of the Orbi-Shaker™:
This is a fairly standard orbit for many benchtop orbital shakers. It has the smooth, fluid motion that many people associate with orbital shakers. Contrast that with the 3 mm diameter (about 1/8") orbital motion of the Orbi-Shaker™ MP, which is designed to shake microplates:
If you think a 3 mm orbit is similar to the motion of a vortexer, that's because it is. Most vortexers have a 3 mm orbital motion.
As a rule of thumb, consider the following when determining which orbit is right for you:
- 3 mm (about 1/8"): Best for microplates, microcentrifuge tubes, and other very small vessels.
- 15 mm - 25 mm (about 1/2" - 1"): Best for cell culture dishes, and flasks and beakers up to about 2 liters.
- 30+ mm: Best for large vessels above 2 liters or especially wide vessels such as Fernbach flasks.
What if your vessel is on the cusp? For instance, say you want to shake 50 ml centrifuge tubes in a vertical position? The good thing about orbital shakers is that many vessels aren't overly sensitive to the orbit diameter. In the case of 50 ml tubes, you could use a shaker with a 3 mm diameter or an ~20 mm diameter. Likewise, if you have a 2 L Erlenmeyer flask, you could use a shaker with an orbit diameter anywhere from 15 to 50 mm. Unless your application is extremely finicky, in which case it will probably specify a specific orbit diameter (and even then you usually have some slack), you don't need to be overly concerned about it.
That said, for applications that aren't simple mixing, such as cell / bacterial culture, things get a little more complicated and involve things like aeration. Let's say you have a protocol that says you should shake at 25 mm but your orbital shaker has a 15 mm diameter. What can you do?
You can usually compensate for a somewhat different orbit diameter (say, +/- 50% of what a protocol calls for) by changing the speed. If your orbit diameter is smaller than what your protocol calls for, simply turn up the RPM a bit to compensate. Conversely, if the orbit diameter was too large you would decrease the speed somewhat.
So don't worry about orbit diameter - it doesn't need to be perfect - but do be aware of orbit diameter, as it's more important than a lot of people realize.