/u/sodakdave recently made a post regarding K&J Magnetic Stir Bars which use neodymium iron boron magnets and are just $4.16 for the 25mm version. We have Bel-Art's Scienceware samarium cobalt Spin-bars at the lab ($8-12 for the 25mm version) so I was curious as to how they compared, since they both technically use rare-earth magnets of roughly equivalent strength gram for gram.

###Testing Equipment

For the sake of this test, we used two different magnetic stir-plates: Corning PC-420D and PC-420. The 420D is a digital stir-plate with an RPM readout and a gentle acceleration, the 420 is an analog stir-plate that has a 0-10 dial and spins up as fast as it can to its indicated speed. Both have a max of roughly 1100 RPM.

We used 400ml of room temperature 100% VG in a 500ml Erlenmeyer flask as our liquid.

###Specs

• K&J: 8mm dia. x 25mm long. Listed weight as 4.03g
• Bel-Art: 9.5mm dia x 25.4mm long. Weight not listed

Note, just for the fun of it, I threw both on our gram scale. There's magnetic parts in the scale which reduces the accuracy obviously, but relatively speaking the K&J was measured at 5.46 grams and the Bel-Art was measured at 8.31 grams.

###Spinout Test

For the sake of the test, I turned power to maximum RPM to see if I could get either bar to spin-out. Once max RPM was reached, I stopped rotation, gave it a few minutes to settle, and repeated for a total of 3 times on each stir plate. I also did a single gradual increase of of RPM on the 420D to see if there was a spin-out RPM point.

• K&J: One spin-out (out of six) on the Max RPM test, no spinout on the gradual increase test.
• Bel-Art: No spin-outs (out of six) on the Max RPM test, no spinout on the gradual increase test

Since that was pretty inconclusive, I decided to try and push it and grabbed an old bottle of VG we had in our fridge. The temperature is unknown (no thermometer) but it had the consistency of molasses or honey; very thick and approximately 150 ml in a 250 ml beaker. Corning PC-420D was used to get the RPM estimate.

• K&J: Spin-out was at approximately 550 RPM
• Bel-Art: Spin-out was at approximately 600 RPM

###Final Opinion

Without cutting them open and verifying, in my opinion the green Bel-Art spin-bars might have slightly larger magnets in them (the serious weight difference) which could explain the marginally better performance. They have slightly more surface area which means the resistance should have been greater and, everything else being equal, should have spun-out faster in the cold VG. That said, at 2-3 times the price, for eliquid-mixing purposes, they aren't worth the extra money. Both of these spin-bars are strong enough that the weak point will be your stir-plate, not the bar itself.

The other positive with the K&J spin/stir-bars is the variety of sizes available. We picked up a few 1.5" bars to test (a size not available with the Bel-Art bars) and, while the sensitivity to spin-out is obviously greater than the 1" class (there's an unstable period below 500 RPM), with a gradual ramp-up I was able to get them as fast as the viscosity of room-temp VG would allow. At around 700 RPM the PC-420D would never reach full speed, instead indicating that it was still "ramping up" to 700 RPM continuously. A mildly unfair comparison but the 2" Bel-Air bar cannot reach a stable 500 RPM and spins out in the low 200s.

TL;DR: K&J Spin-bars are perfect for DIY and half the cost of Bel-Art bars ...Bitch