Overhead Stirrers: Sizing Torque and Viscosity for Your Application
- 3 hours ago
- 9 min read
Summary
You size an overhead stirrer from torque, not from speed — and you need one the moment a magnetic stirrer starts slipping. That happens at roughly 5 litres of volume or a few thousand mPas of viscosity, whichever comes first. A magnetic follower couples to the drive magnet by field strength alone, and once the fluid resists more than that coupling can transmit, the bar decouples and spins uselessly while the fluid stands still.
An overhead stirrer transmits torque mechanically through a shaft, so it cannot decouple. Pick the model by the volume and viscosity you must handle, then choose the impeller by what you are trying to do to the fluid. Expect roughly £880 for a digital unit, £1,075 for a complete stand-and-impeller package, and £2,100 for a torque-measuring control model.
What is an overhead stirrer?
An overhead stirrer is a motor mounted above the vessel that drives a shaft and impeller directly into the fluid, transmitting torque mechanically rather than magnetically. That single difference is what lets it handle volumes and viscosities a magnetic stirrer cannot touch.
The instrument is a system, not a box: motor head, stirring shaft, impeller, and a stand and clamp to hold the whole thing over the vessel. Buying the head alone and discovering you have nothing to mount it on is a common and irritating mistake, which is why the packaged solutions below exist.
You will see the same equipment sold as an overhead mixer or a laboratory stirrer. For this class of instrument those are the same thing.
When do I need an overhead stirrer instead of a magnetic stirrer?
When the fluid resists more than the magnetic coupling can transmit. Three thresholds, any one of which is enough:
Volume above roughly 5 litres. Below that a magnetic stirrer generally copes; above it the follower is trying to move a mass it cannot reach.
Viscosity above roughly 2,000–5,000 mPas. Thick syrups, gels, polymer solutions, slurries and creams all sit above this. A magnetic bar in a viscous fluid cuts a hole and spins in it.
Solids in suspension. Anything that settles, cakes or forms a bed will jam a follower. An impeller keeps it moving.
The tell that you have crossed the line is decoupling: the bar stops tracking the drive, rattles, and the fluid goes still. Turning the speed up makes it worse, not better.
For everything below those thresholds, a magnetic stirrer is the better instrument — quieter, cheaper, easier to clean and with no shaft to align. The hotplate stirrers guide covers that side of the decision, including when you need heat with the stirring.
How do I size torque and speed?
Torque is what does the work; speed is what you tune afterwards. Buyers habitually compare rpm figures, which are nearly identical across the range, and ignore torque, which is where the models actually differ.
The practical route is to size by the model's rated maximum stirring volume in water, then check the viscosity rating against your worst-case fluid rather than your typical one. The IKA model numbers approximate that volume rating — closely enough to navigate by, not closely enough to specify from, since the EUROSTAR 20 is actually rated to 25 litres.
Model | Max stirring volume (H₂O) | Max viscosity | Speed range |
RW 20 digital | 20 L | — | 60–2,000 rpm |
EUROSTAR 20 digital | 25 L | 10,000 mPas | 30–2,000 rpm |
EUROSTAR 40 | 40 L | 30,000 mPas | 30–2,000 rpm |
EUROSTAR 60 control | 60 L | 50,000 mPas | 30–2,000 rpm |
EUROSTAR 100 control | 100 L | 70,000 mPas | 30–2,000 rpm |
The RW 20's viscosity rating is not published on its product page; ask us before committing it to anything thick.
Three rules that save money and regret:
Size on the worst case, then leave headroom. Viscosity climbs as a reaction proceeds, as a polymer builds, and as temperature falls. A stirrer sized to the starting fluid stalls halfway through.
Low speed is harder than high speed. Holding 30 rpm steady against a viscous load demands far more of a motor than spinning at 2,000 rpm in water. If your process runs slow and thick, that is the specification that matters.
Ignore the rpm comparison. Every model in this range does 30–2,000 rpm. It is not a differentiator.
What does torque measurement give me?
A live viscosity reading, and an early warning that something is going wrong. This is the real difference between the digital and control tiers, and it is worth understanding before you decide which to buy.
The EUROSTAR Digital offers torque trend measurement at ±6 Ncm — it shows you viscosity changing on the display, which is enough to see a reaction thicken or a suspension break. The EUROSTAR Control has an integrated torque sensor accurate to ±1 Ncm, which turns the stirrer into a crude but genuinely useful process viscometer, and logs it out over USB, RS232, Ethernet or Wi-Fi.
Who needs which:
Digital is right for routine mixing, dissolution and suspension work where you want to see that something changed but do not need to record what.
Control earns its price where the torque curve is the data — reaction endpoint detection, formulation development, scale-up work, or anything that has to be documented. Both tiers are rated for ISO Class 7/8 cleanrooms and IP 54.
Both hold set speed constant as torque varies, which matters more than it sounds: without it, a thickening fluid slows the shaft and changes your mixing regime halfway through the run.
Which impeller should I fit?
Match the impeller to the flow pattern you need, not to the vessel. This is the choice that most affects mixing quality and the one buyers think least about.
Impeller | Flow pattern | Best for | Avoid for |
Propeller, 3 or 4 blade | Axial, top-to-bottom circulation | General mixing, low to medium viscosity, keeping solids suspended | Very viscous fluids — it cavitates a hole |
Turbine / Rushton | Radial, outward then split | Gas dispersion, emulsions, high shear | Shear-sensitive material, cells, flocs |
Anchor | Close to the wall, scraping | Viscous fluids, heat transfer at the vessel wall | Bulk mixing — it moves the wall layer, not the core |
Paddle | Gentle radial | Low-shear blending, dissolving | Suspending anything that settles fast |
Dissolver disc | High shear at the disc | Wetting powders, breaking agglomerates | Anything shear-sensitive |
Two practical notes. Shear is a specification, not a side effect — if you are mixing cells, flocs or an emulsion whose droplet size matters, a turbine will destroy what a propeller would have preserved. And an anchor and a propeller are not alternatives; in genuinely viscous work you often need the anchor for wall heat transfer and something else for the core.
If your job is dispersion rather than stirring — breaking agglomerates, making an emulsion, homogenising — a stirrer is the wrong instrument entirely. See dispersing equipment.
What else do I need to buy?
A stand, a clamp and a shaft — and the answer is usually the packaged version. An overhead stirrer head on its own is not a working setup.
Plate stand. A weighted baseplate with an upright rod. Right for beakers and vessels that sit on the bench.
H-stand. A wider, more stable frame for larger or off-centre vessels, and for anything you would rather not have topple.
Boss head clamp. Holds the motor to the rod. Check it is rated for the head's weight — an undersized clamp creeps down the rod under vibration.
Shaft and impeller. Sized to the vessel: as a rule of thumb the impeller diameter runs about a third of the vessel diameter, positioned about one impeller diameter up from the base.
The RW 20 digital Package (SKU 0009011602) is the complete-setup route — stirrer, R 1826 plate stand, R 182 boss head clamp and an R 1342 four-bladed propeller in one order, currently at a 20% reduction. For the EUROSTAR range the equivalents are the plate stand Solution, the H-stand Solution and the Hub Solution.
Alignment matters more than it looks. A shaft that runs off-centre whips at speed, wears the bearing, and in a glass vessel eventually breaks it. Align before you power up, every time.
Overhead stirrers from LS Scientific
Current pricing in both markets. All IKA.
Stirrer heads
Model | Torque measurement | Speed | UK price | Nigeria price |
Trend, ±6 Ncm | 30–2,000 rpm | £876.85 | ₦1,611,081 | |
Integrated sensor, ±1 Ncm | 30–2,000 rpm | £2,106.15 | ₦3,869,737 |
Naira figures are converted at the rate shown on our store and exclude duty and clearance. Request a landed quote for Nigeria.
The Control models available are the EUROSTAR 60 control (60 L, 50,000 mPas) and the EUROSTAR 100 control (100 L, 70,000 mPas). Both are brushless, IP 54, ISO Class 7/8 cleanroom compliant, and offer reversible rotation and temperature monitoring. The Digital head covers the EUROSTAR 20 digital (25 L, 10,000 mPas) and the EUROSTAR 40 (40 L, 30,000 mPas); select the model on the product page.
Complete packages, stands and hubs
Item | What it includes | UK price | Nigeria price |
Stirrer, R 1826 plate stand, R 182 clamp, R 1342 propeller | £1,075.00 | ₦1,975,153 | |
Stirrer with plate stand | £1,447.80 | ₦2,660,117 | |
Stirrer with H-frame stand | £2,896.55 | ₦5,321,980 | |
Stirrer with connectivity hub | £3,950.10 | ₦7,257,721 |
Naira figures are converted at the rate shown on our store and exclude duty and clearance. Request a landed quote for Nigeria.
The RW 20 package is the cheapest complete working setup on this page, and for volumes to 20 litres it is the one most labs should start with.
If you are below the threshold
Item | UK price | Nigeria price |
£900.60 | ₦1,654,718 | |
£437.00 | ₦802,922 |
Naira figures are converted at the rate shown on our store and exclude duty and clearance. Request a landed quote for Nigeria.
Prices are current at the time of writing and include applicable sale reductions. Check the product page for live pricing.
Tell us your volume, your worst-case viscosity and what you are trying to achieve in the vessel, and our technical team will size the head and pick the impeller with you.
Frequently asked questions
When should I use an overhead stirrer instead of a magnetic stirrer? Above roughly 5 litres of volume, above roughly 2,000–5,000 mPas of viscosity, or whenever solids settle or cake. Any of the three will cause a magnetic follower to decouple and spin without moving the fluid.
What is torque on an overhead stirrer, and why does it matter more than speed? Torque is the rotational force the motor can deliver against the fluid's resistance. It is what determines whether the stirrer can turn your fluid at all. Speed ranges are nearly identical across the range — 30 to 2,000 rpm on every EUROSTAR — so speed is not a useful basis for comparison.
What does the model number mean on an IKA EUROSTAR? It approximates the maximum stirring volume in litres of water — a EUROSTAR 60 is rated to 60 litres, a EUROSTAR 100 to 100 litres. It is an approximation rather than a rule: the EUROSTAR 20 is rated to 25 litres. Check the viscosity rating separately, because that is what limits you with anything thicker than water.
What is the difference between the digital and control models? Torque measurement. Digital shows a torque trend at ±6 Ncm, enough to see viscosity changing. Control has an integrated sensor accurate to ±1 Ncm with data output over USB, RS232, Ethernet and Wi-Fi, which makes the torque curve recordable data rather than an indication.
Which impeller should I use? A propeller for general mixing and keeping solids suspended, a turbine for gas dispersion and high shear, an anchor for viscous fluids and wall heat transfer, a paddle for gentle low-shear blending, and a dissolver disc for wetting powders. Match the impeller to the flow pattern you need.
What size impeller do I need for my vessel? As a starting rule, impeller diameter about one third of the vessel diameter, positioned roughly one impeller diameter above the base. Adjust from there based on how the fluid actually moves.
Do I need a stand and clamp as well? Yes. The stirrer head alone is not a working setup. You need a plate stand or H-stand, a boss head clamp rated for the head's weight, and a shaft with an impeller. Packaged solutions bundle these and usually cost less than buying separately.
Can an overhead stirrer measure viscosity? Not to the standard of a rotational viscometer, but a control-tier model with an integrated torque sensor gives a usable relative measure — enough to track viscosity change through a process and detect a reaction endpoint. For an absolute number you need a viscometer.
Why does my stirrer shaft wobble? Almost always misalignment between the chuck and the vessel centre, or a bent shaft. A whipping shaft wears the bearing, adds vibration to the clamp and can break a glass vessel. Realign before every run.
References and standards
IEC 60529, Degrees of protection provided by enclosures (IP code). Defines the IP 54 rating quoted on the EUROSTAR range.
ISO 14644-1, Cleanrooms and associated controlled environments: classification of air cleanliness. Defines the Class 7/8 cleanroom compliance quoted on these units.
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