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Magnetic Stir Plate

The system shall achieve a top plate surface temperature of at least 400°C, with a target maximum of 500°C. The heating system shall have a power rating between 1.0 kW and 1.5 kW.

The system uses multiple, parallel heating elements to ensure continued operation in the event of a single element failure.

The stirring mechanism shall be of a solid-state design, utilizing electromagnets (coils) with no moving mechanical parts. A 6-pole (three-phase) electromagnetic design is specified to provide sufficient torque and rotational control.

The stirrer shall achieve a maximum rotational speed of 2000 RPM.

The electromagnetic coil configuration shall generate a centering force to maintain the stir bar's position and prevent decoupling ("spin-out") during operation.

The system shall incorporate a stall detection function to identify when the magnetic stir bar decouples from the rotating field. Stall detection shall be implemented by monitoring the phase angle and/or current draw of the electromagnets for significant variations. Upon stall detection, the system shall execute an automatic recovery sequence, such as reducing speed or temporarily halting rotation, to re-engage the stir bar.

The system shall include a standard input for a user-replaceable external temperature probe (e.g., Type-K thermocouple or PT100 RTD) to monitor the solution temperature directly.

The device shall feature a data communication interface for remote monitoring and control. The interface must allow remote access to read and write setpoints and process values for both temperature and stirring speed, as well as provide the device's fault status. The communication protocol shall be a standard, parallelizable industrial protocol, such as Modbus over RS-485 or a similar robust standard. Need to consult an instrumentation or electrical engineer

The main enclosure shall be chemically sealed to protect internal electronics and components from spills and corrosive vapors.

The design shall be robust and serviceable, avoiding common single points of failure such as inaccessible fuses or non-spillproof controls.

The heating plate shall have a surface area of 20 cm x 20 cm.

The plate shall be constructed from aluminum, with the design accommodating for thermal expansion at maximum operating temperature. The plate surface may feature an optional spray-on alumina enamel coating for enhanced chemical resistance.

The aluminum top plate shall be a user-replaceable component to facilitate low-cost repair after chemical damage.

The physical user interface shall be usable by an eighty year old, or a layperson, or my parents, whoever is more annoying to teach. Be minimalist to maximize durability and resistance to chemical exposure.

100$ for a single unit, or as low as reasonable, for BOM costs

All design files, including hardware schematics, PCB layouts, mechanical drawings, and source code, shall be released under an open-source license.

Heating surface

Source: https://mightyohm.com/blog/2009/01/diy-pid-controlled-soldering-hotplate/ https://mightyohm.com/blog/2009/01/improvements-to-the-diy-pid-controlled-soldering-hotplate/ http://www.neufeld.newton.ks.us/electronics/?p=537

The hot plate is a milled aluminum heater block that acts as a heat spreader, which contains multiple high-power ceramic heater cartridges, perhaps those used in 3D printer hotends. Temperature is monitored by precise, high-temperature sensor (perhaps two for reliability), and the entire system is managed by a microcontroller running a PID control loop for temperature stability. This should create a redundant, powerful and easily serviceable heating assembly.

24V, 50W or 60W heater cartridges (4 to 6) are inexpensive, widely available commodity parts. Uing multiple cartridges in parallel provides redundancy until replacement; if one fails, the hot plate can still operate at reduced power, and replacement is a simple low-cost procedure.

A block of 6061 aluminum, approximately 18x18 cm and 1-2 cm thick, with milled slots and holes to accomodate the heater cartridges and temperature sensors. The aluminum will act as the heat spreader and thermal mass. Will need to design around its thermal expansion tho.

Any operations for milling the aluminum need to be limited to 2.5D so that the cost of machinging is low.

Cartridge heaters are cylindrical in shape with two wires sticking out, the cylinder usually has a diameter of 6mm and a length of 20mm, and the wires stick out for 100cm.

The lifespan of cartridge heaters is about 3000 to 10000 hours of operation, and it depends on the maximum recommended sheath temperature (about 650 C), and the highest ambient temperature of the connections. The connections in particular are the first to fail when improperly used.

Ideally, your mold’s diameter must be over 0.1 mm bigger than the diameter for optimal and swift heat dissipation. Oversized holes will reduce the lifespan of the heater.

Excessive cycling will cause the premature failure, and is an issue when higher watt density cartridges are turned on and off rapidly to keep temperature. Consider getting a variety of watt densities? Also ensure that the temperature lag between heater and sensor does not cause cycling.

Given that cartridge heaters are basicallly low resistors, you can check if they work by measuring their resistance with a multimeter. If the resistance is exceptionally high (like 1MOhm or larger), your cartridge heater is likely fried. Might be worth adding a resistance checker circuit to identify which cartridge heater is fried, would certainly help non-technical staff.

On-off control is frequently used for cartridges but it might be worthwhile to use thryistor power controls. REX-C100, I have some of these, for 12$ inclusive shiping a bargain and they do a pretty good job on standard setting already.

I got the kind with relais though, just removed this and drive a SSR with the 12V for the relais - this works flawless.

Some thermocouples are just not good I found out, if there is a problem its always on contacts or thermocouple. The PTC has also a smaller risk of misreadings for any reasons, more robust and professional and they got pretty cheap too nowdays, I prefer them.

A other variation which makes even more sense is to have two heating coils, one to heat up by relais and one to hold and fine-regulate by SSR - the second much less powerful then the first of course. In this configuration a heatsink on the SSR gets virtually obsolete, no need to stress the part too much and even if it should burn out - having 150W full on is a big difference to having 1500W blasting, what can get unpleasant fast.

Suggestions from a repair engineer

As a repair enginner, the first things I saw were;

. electrolytic capacitors in a hot unventilated enclosure, at least use solid/polymer capacitors

. no way to replace the capacitors - could you use lands that protrude beyond the capacitor leads ... then they will be easier to replace. (to get old ones off, grip with pliers etc. and twist cw/ccw/cw/ccw... starting gently, the body will come away without damaging the tracks, allowing de-soldering of the remaining bits of leads)

the same applies to the lands for the indutors.

Stirbar spinout

Stirbar spinout

if you use magnetic stirbars they must rotate synchronously with the rotating magnetic field, as torque load on the spinbar increases, it will lag the applied field by an ever increasing angle, theoretical maximum 90 degrees, then fly away.

to get it spinning again you need some random chaos to get the spinbar moving close enough to synchronous with the rotating field that it can be 'captured' or slow down the rotating field to a near stop then ramp up again, but not so far or fast this time :)

that's my understanding so far.

This does raise the potential for high speed rotating magnetic fields coupling to non-ferrous conductive disks (ptfe coated aluminium, pure platinum, catalytic disks ...)

EDIT: it just occured to me, when you do experiments again, put LEDs (e.g. red/green in one package for +/- current) near the respective electromagnets, which indicate electromagnet voltage, or preferably current (magnetic field proportional to current). That way a video could be analysed frame-by-frame, or in slo-mo to see if the above is true. It would look 'cool' with LED magnetic field rotation indicators too :cool:

Inspiration

Magnetic stirrer with electromagnet coils

Magstir bars dying/being lost

Temperature can cause demagnetization

Keep loosing them down the sink? Keep two small but strong magnets bound near the flask/beaker/container hole/neck, so that the stir bar is grabbed

According to most manufactures stir bars should be stored as pairs:

There are several measures that should be noticed to increase the life span of magnetic stir bars. First, stir bars should be stored in pairs to maintain magnetic strength. Stir bars should not be stored in a random mass or dropped on a hard surface especially steel.

from http://www.labdepotinc.com/c-336-magnetic-stir-bar.php

Stir retreivers

Stir retrievers can be done by simply adding a small magnet inside a test tube and dip the tube in the stirring solution to retrieve the stir bar. There's no need to seal the tube. However, a more simple way of retrieving the stir bar is to use a magnet from outside the flask to draw the stir bar through the walls of the flask and drag it to the vessel opening. Do not use strong magnets, as if they slip from your hand, they might crack the glass.

DIY magnetic stirrer

A simple magnetic stirrer can be made by simply gluing two or more magnets (depending on the choice and size) on the rotating part of an electric motor. A computer fan can be used as electric motor. To control the speed of the fan, a voltage regulator should be added to the circuit. A good tutorial can be found here.

If you want to make a hot-plate magnetic stirrer, you will first need to properly separate the stirring part from the heating element, otherwise the heat will slowly degrade the magnet over time. For the hotplate, a heating element, either home made from Nichrome or an already existing hot-plate heating element is added inside a metal disk, which will act as the hotplate. To insulate it from the stirring section, an insulator such as mineral wool is added on the bottom of the heating plate. Make a rotating stirrer as explained below, and place it below the heating element, but make sure the insulation does not interfere with the rotating motor. The temperature and rotation speed are controlled through a thermostat and a voltage regulator respectively. Keep in mind that this type of stirrer is not easy to make and if you don't know what you're doing it's best to appeal to the help of an electrician, though it would be simpler to just buy a hotplate magnetic stirrer. A good tutorial for a hotplate stirrer can be found here (though it's in Italian). Magnetic stir bars, of various sizes.

A simple way to make a stir bar is to take a piece of ferromagnetic material, such as a piece of iron, steel or a small magnet and coat it with a protective layer, such as Plasti Dip or Teflon paint. This however, does not offer total inertness and may not work at high temperatures. Unfortunately, PTFE, being a thermoplastic polymer, cannot be properly melted, so coating a stir bar in Teflon at home is not possible or may not hold. Sealing the bar in glass is another option, but you will need to leave a bit of space inside, as metals tend to dilate more than glass during heating and there's a risk it may crack the glass coating if not properly done.

Stir bar

General

Stir bars are small items made containing two magnets, coated in PTFE or glass, and come in various forms, like cylindrical, cylindrical with a pivot ring, oval (almond or egg-shaped), triangular, cross, dumbbell (circulus), spherical, saturn, etc. PTFE is more commonly used as it doesn't break when dropped on a hard surface and doesn't crack if exposed to thermal shock, while glass-coated stir bars are used for working with molten alkali metals. Stir bars cannot be used above 225-270 °C (437-518 °F), as PTFE breaks down above that temperature range, while the magnets will begin to lose their strength if heated too long at high temperatures. Types and use

Each type of stir bar is designed for a specific task:[1][2]

Cylindrical stir bar: general purpose stirrer for smooth stirring, used for all flat bottom glassware. Plain stir bar: similar in role to cylindrical, but capable of generating more turbulence at low speed. Octahedral stir bar: similar to the pivot ring type, but with more turbulence at low speed. Oval stir bar: used in round-bottom flasks Pivot ring stir bar: used in containers with slightly curved or uneven base, but not curved like round bottom flasks, the pivot allowing the stir bar to adopt the optimum position for stirring. Spherical stir bar: used for stirring in test tubes or for eccentric stirring. Triangular stir bar: used for dissolving solids or mixing sediments, the flat base gives a scraping ability for disturbing solids. Cross stir bar: used for general purpose stirring but stable to reduce “jumping”. Crosshead stir bar: used in tube shaped containers like test tubes, beakers or flasks. Drumbell stir bar: used for flasks with concave bottom, like jars or reagent bottles, capable of generating strong turbulence. Saturn stir bar: similar to pivot stir bars in aspect, they are used when stirring powders in liquids without getting stalled. Spinring stir bar: allows maximum stabilization of the magnetic stirring bar with the addition of a “hoop” around a standard octagonal bar, best suited for larger open-neck vessels, such as buckets and beakers.

Availability

Magnetic stir bars can be purchased from most lab suppliers. They can also be found on eBay and Amazon DIY stir bar

A simple stir bar can be made by gluing two magnets on a small cylinder, then coat the item in a chemical resistant coating. This type of stir bar is not however, compatible with many reagents.

Which stir bars will fit through glass joints (like 24/40)

Stir bar bounce

Stirology 101: Stir Bar Bounce

Why is my stirring bar bouncing around?

If your stir bar has a little too much pep in its step (i.e., it’s wobbling around on the bottom or bouncing along the edges of your vessel), then it’s lost its coupling with the magnetic field originating from the stirrer.

This can be caused by a variety of reasons. Some of the most common reasons include one or a combination of the following:

The stirring bar is too weak. Try switching to a stronger stirring bar or even a rare earth stirring bar. The stirring bar size, type, shape is inappropriate for the application components. Try a different stirring bar. For example, if the length of the stirring bar magnet is longer than the length of the magnetic field, the stirring bar likely won’t work. The liquid is too viscous for the stirrer/stirring bar combination. The volume is too large for the stirrer/stirring bar combination. The bottom of the vessel is too convex and the stirring bar cannot find stability at the center of the stirring point. Your vessel is not directly on the center of the stirring point. Try repositioning your vessel. The rpm is set too high for the stirrer/stirring bar combination. Try decreasing the rpm or a soft reset (see below). In the case of electromagnetic stirrers, the power setting of the stirrer is too low—this means not enough power is going to the drive coils and the force of the magnetic field is too weak. Try increasing the power setting to make the magnetic field stronger. The stirring bar is too far from the stirring platform. Try reducing the vertical distance. Related to no. 9, the bottom of the vessel is too thick. Your vessel is too wide for the stirrer/stirring bar combination. Try a narrower vessel or a larger stirrer. A short interruption or reduction of power, causing the magnetic field to slow or become less intense. Remember it could be (and many times in fact is) a combination of the above. Or sometimes the stirring bar is simply thrown off based on a split second of the perfect storm of physics phenomena. Try a soft reset—i.e., press the power button off and back on. 2mag stirrers are equipped with SoftStart, so a soft reset will stop the stirring bar, recenter it, and slowly increase the rpm to the set speed to minimize/eliminate decoupling and stir bar bounce.

You can send us a video of your application and/or contact us to consult more on this issue. We can usually tell what the problem is and recommend solutions.

Happy Stirring!

Silicone Rubber Lab Mats

https://www.tedpella.com/histo_html/hotplate.aspx Silicone Rubber Lab Mats

Reusable, reliable, and reversible

Ideal for every type of laboratory and more specialized areas such as cold rooms, clean rooms, sterile suites and areas where high sterility is a requirement The lab mats can be used with items such as centrifuges, vortex mixers, hot plates, stirrers, pipettes, and so much more Reversible, with two color/design options available

Design includes retaining strip with internal metal inserts for retention of magnetic objects, such as stir bars to help keep items organized Lip around the edge can hold approximately 300mL of liquid Keeps benchtops clean and safe from stains, spills and wear Reusable, helps reduce waste making it a more environmentally friendly option over disposable mats Anti-skid surface helps keep the mat and items in place Noise dampening, reducing sound from the vibration of common bench top equipment Made from a durable FDA food grade silicone rubber material that creates a stain resistant washable working surface Chemical resistance: chemically inert and does not react with most chemicals Can be easily cleaned using any standard disinfectant by simply wiping it and is dishwasher safe Multiple colors available in each style for color coding options Protect counter tops from hot items up to 200°C (392°F) Mats can be rolled up for compact storage Large working surface, 35 x 60 x 0.2cm (14 x 23.5 x 1"), accommodates benchtop equipment, weigh boats, stir bars, scales, etc. Autoclavable at 121°F for 15 minutes

Inspiration

https://2magusa.com/collections/stirring-bars https://2magusa.com/collections/stirring-bar-retrievers https://www.tedpella.com/centrifuge_html/magnetic-stir-bars.aspx

Stirbar as pump in microfluidics https://www.researchgate.net/publication/289611734_An_on-board_microfluidic_pump_driven_by_magnetic_stir_bar