Magnetic hotplate stirrer - Page 1
Forum section: electronics / general

author: cs.dk
author_rank: Unknown
timestamp: October 25, 2020, 08:57:07 am
content: I'm looking for a magnetic hotplate stirrer, any experiences with the "cheap" ones from ebay, amazon, etc.?

author: coppercone2
author_rank: Super Contributor
timestamp: October 26, 2020, 03:34:20 am
content: yea don't go cheap because its a electronic device that will have highly conductive stuff on top. You get a cheap plate and a cheap beaker and suddenly the plating bath can kill you (they are extremely useful for making your own plated contacts, having a heated stir plate will help out the quality you get).All I can recommend is refurbish and check the ground properly with a high current to make sure the ground actually works well.Also, I do not recommend getting cheap beakers to put on top of a hot plate even moreso then the cheap hot plate.You can replace the thermostat switch and refinish the little variac or rheostat (not sure what it is) that is inside of the old ones with some deoxit grease and scotch brite dremel attachments.

author: dirtcooker
author_rank: Regular Contributor
timestamp: November 12, 2020, 10:43:44 pm
content: Quote from: cs.dk on October 25, 2020, 08:57:07 amI just bought this one off ebay used for $100:Microyn SH-II-4C Magnetic Stirrer HotplateThere are other very similar variations of this, SH-4C-T and MT-SH-4C-T appear to be identical except stirring speed is 0-1600 rpm and are $240https://www.ebay.com/itm/Digital-Laboratory-Magnetic-Stirrer-Hotplate-w-timer-7x7-inch-600W-0-1600rpm-/153500887526https://www.amazon.com/Laboratory-Magnetic-Hotplate-Function-7x7inch-Capacity/dp/B0725BS5XJ/MT-SH-4C is similar but with a speed control knob▲Specifications1.Voltage: 100~120, 60 Hertz2.Heating power: 600 watts3.Stirring power: 20 watts4.Maximum heating plate temperature: 350°C5.Stirring speed: 200～2000rpm stepless adjustable6.Maximum Stirring capacity: 5000ml liquid (H2O) volume7.Square Heating plate size: 190mm x 190mm (7.5 inch)8.Heating plate material: Ceramic9.Overall dimensions (LxWxH): 320mm x 190mm x 125mm10. Weight with packaging: 5 kg▲Packing list1 x Main unit1 x Stirring bar (Diameter 9mm x 28mm length)1 x Power Cable with US Plug1 x Rack with Rods1 x Fuse1 x User ManualThis is a capable, well made, sturdy all metal instrument, but the user interface is arcane, and the user manual is poorly written and/or translated. I bought one used off ebay for $100. Upon receipt, the device appeared to be brand new. My guess is that someone bought and then returned it because it was too difficult to figure out how to use. I had to read the manual and experiment for an hour or so to get it to work. For example, the button at the lower right with the international power on/off symbol on it (a circle with a short vertical line) and the text “STIRRING” below it, must be pressed and held 3 seconds to start or stop magnetic stirring. Who would have thought?Be aware that whenever the rear mounted power switch is on, the heater may be operating, even when “OFF” is shown on the control panel. This only indicates that the stirrer is off. There is no way to turn the heater off manually from the front panel, so far as I can determine. The heater is controlled by the PID control loop, which lights the “HEAT” indicator when powered. At times the heater will be turned off, as for example at the end of a timing cycle, when the alarm sounds, and the stirrer is switched “OFF.” An overtemperature condition (user setable, by default 20c above the setpoint) lights the overtemperature alarm triangle, and shuts the heater off.The 3 second time delay of the “STIRRING” button press, as well as dozens of other parameters, are user changeable. So the first thing I did was reset everything to factory default values. To accomplish this, press and hold the “TEMP” button for 3 seconds until the prompt “Lc” appears. Set the value to “67” using the up/down arrows, then press “TEMP” once again. The prompt “rST” will be shown. set this to “1” then press “TEMP” again. A relay will click, rpm will show “500”, and the parameters are reset. Totally intuitive, right? Just don't lose the manual. I would really like to download a pdf version of it for safekeeping, but I haven't found out where to do that.To calibrate the PID controller, place a flask of water on the platform filled to a volume typical of the experiment that you wish to do, 1L for example. Mount the probe so that it is immersed in the water, toss in the stirrer and set it to, say, 700 rpm. Set the temperature to, say, 70c. Press and hold “AT” for 6 seconds. The “AT” prompt will be shown. Change the flashing “0” to a flashing “1” and then press “TEMP.” The “” indicator will start flashing, indicating calibration mode, which entails turning the heater on and off as it measures time constants for the controller. It may take a long time to finish this proceedure. I waited 40 min for it to finish for the above setup. You can press “AT” for 6 seconds to exit calibration, but the manual does not state what happens then to the PID parameters. Perhaps they are unchanged, perhaps they are reset to factory values, or something else. I would expect that the larger the mass of the water in the flask, the longer the time constants, and the longer that calibration will require. During calibration, the controller overheats the water by about 10C. Then it takes a while to cool down below the setpoint and to start exercising the control loop to measure the PID parameters. Apparently, you can also set these values directly, but I have not experimented with that.There are two temperature sensors. “A” is the PT100 thermal probe that you immerse in the solution. “B” is a thermocouple embedded in the underside of the hot plate. Press and hold the “SHIFT” button for 3 seconds to switch between the two. The sensor currently selected is shown on the display next to the speed button. I guess there wasn't room to place it somewhere more intuitive, as for example, near the “TEMP” button. PT100 “A” readings were within 1C of the readings on my multimeter thermocouple during the time I experimented.I did not take it apart, but I did measure resistance at 200 ma from the ground prong to various points on the chassis. Most of the values were in the tens of milliohms, except points near the motor case on the bottom were around 1 ohm. Still, this meets the IEC 2 ohm spec. Hi pot testing at 500v from ground to the ac inputs was >550 MOhms. On power up, my unit displayed version “Fsv1”

author: coppercone2
author_rank: Super Contributor
timestamp: November 13, 2020, 12:12:39 am
content: thats a very advanced unitalot of them just have a electric stove controller (mechanical)

author: coppercone2
author_rank: Super Contributor
timestamp: November 14, 2020, 02:09:36 pm
content: by the way, I am not sure I like such a advanced instrument for chemistry, because its is very complicated and it could make a danger when it malfunctions.I would recommend you get a backup unit of the simple kind to be able to finish what you are doing on that beast if it breaks down. Bit more liability then a old radio not working or whatever we are used to.I imagined 5 liters of nitric acid doing a steel corrosion study deciding to boil over. I was appalled at what kind of construction I saw in a laboratory grade expensive peristaltic pump, it looked like it was put together by a over stimulated 3rd grader. Point to point wiring on most of the things with no consideration to wire length, strain relief or solder consistency. Literary looked like a really bad 'i don't like electronics, I like this major for the money' students (tumble)breadboard (before the 2nd year major switch to business) inside of the thing when I took it apart. On the other hand some people like VWR have extremely good quality PCBs and stuff. I fixed up that pump (transfer pump for PCB etching) with a bit of thought. I almost called the company to complain but it was 10 years old. It also had a very creative system for pump speed control.For the 1000$ original price tag, I give it a 1/10 for good quality motor. It looks like you can make something similar out of the little plastic thing that supports a pizza box. Also had top side switches that were unsealed.

author: cs.dk
author_rank: Unknown
timestamp: November 15, 2020, 03:18:26 pm
content: Got my self one of these, seem to work fine.https://www.amazon.co.uk/Steinberg-Systems-Magnetic-Stirrer/dp/B088HFT66G

author: dirtcooker
author_rank: Regular Contributor
timestamp: November 18, 2020, 05:22:16 pm
content: Quote from: cs.dk on November 15, 2020, 03:18:26 pmThat looks decent, but it's only 180W.

author: dirtcooker
author_rank: Regular Contributor
timestamp: November 18, 2020, 05:32:55 pm
content: Quote from: coppercone2 on November 14, 2020, 02:09:36 pmYes I would not like to find out what happens if it overheats a nitric acid distillation. I have tested it well with water, and so far, so good. The name brand units are really expensive, so I decided to roll the dice with this. The interface is overly complex, but inside it is just a plain old PID controller. The PT100 sensor is stainless steel. At low temperatures (e.g. 30-40c) it seems to be ok with concentrated acid. I would prefer glass, but can't find one. Any ideas?It works OK as a pc board preheater, although the surface area is small.

author: coppercone2
author_rank: Super Contributor
timestamp: November 18, 2020, 08:14:55 pm
content: don't leave it unattended I guess and you can keep an eye on it with a external or IR thermometer