Fristaden Lab 85-2AN Magnetic Stirrer Hot Plate Mixer, 1,000mL, 0-2000RPM, 200W Heating Power 250°C Max Independently Controls Temperature and Speed 1 Year Warranty

met all my expectations, and works great

I first purchased this hot-plate stirrer on 19 November 2017, and with Prime it arrived two days later. The unit looks pretty good, it is sturdy and looks well-made. I tested it the next day with a beaker of water, and the stirrer and temperature sensor worked, but the hot plate never heated up, even when set to the maximum (99.9 C). I contacted Fristaden, and their customer service is excellent, I received a reply the next day. After a responsive few emails back and forth, during which I was invited to open the unit and send in some pictures (which I did), they recommended I return it to Amazon for a replacement. I did that, the return process was easy and free, but the Amazon return page did not offer the option to receive a replacement unit.So I simply ordered another one, by then it was November 28. Again, I received it within two days, and I even got a 10% discount on the unit. This time I tested it a little more carefully, again with a beaker of water with the stirrer bar and sensor in it. I turned on the stirrer, and it worked. I then turned the heater on to about 50, and the hot plate got warm, as I established with my fingers. The water temperature slowly started rising, and reached about 24 C (from about 21 C) but never got above that. I found that the hot plate was cooling down and then returned to being cold, after which it never heated up again, despite many more tests (power off/on, check all the wires inside, etc.). I opened it up again, the unit looks fine inside, and I measured some voltages (when on) and resistances (when off). Everything seemed okay, except that the resistance between the two braided leads going into the ceramic bottom of the hot plate seemed high to me (27 MOhm, maybe because of some component on the circuit board - I did not disconnect the braided leads), but then again I don't have the schematics. There is no obvious relay, I think the unit is controlled using a power transistor, but I don't know for sure. So at this point my guess is that the the heater coil inside the hot plate malfunctioned, although it could have been the transistor.To make a long story short, the unit seems well-constructed, and there seems to be no obvious reason that it shouldn't have worked. It may just be a manufacturing flaw in a single (but essential) component, or it may be bad quality control, or just a bad batch of units.. The electronics seems fine, and there is a setup menu that allows you to fine tune settings. The one-sheet manual is written in an English-like language, but understandable. The one thing not obvious is that if you want to change any of the parameters in the setup menu, you have to first set the first parameter to allow changes.I wanted to like this unit, and I again emailed back and forth with Fristaden, and they again suggested I return the second unit for a replacement, but since the two-day shipping now is 5-day shipping, and there is only one unit left (same batch?), I don't think I want to go through this again.Too bad, really.