A paint QC lab reports 95 KU. A customer's incoming inspection reports 2,100 cP. Both are correct, both describe the same drum, and neither can be converted into the other with any confidence.
Krebs Units and centipoise are not two scales for one quantity, and treating them as interchangeable is the most expensive habit in architectural and trade sales paint QC. ASTM D562 is the method that defines the KU side of that wall. This guide covers what it measures, where the ToronVisc STM-2T fits, and the one specification that decides the purchase.
The Short Version
- ASTM D562 measures the load needed to turn a paddle rotor at 200 RPM in a paint specimen, and reports it as Krebs Unit viscosity.
- The method is a consistency test, not an absolute viscosity test. It answers whether this batch matches the last one, not how many centipoise the paint is.
- The ToronVisc STM-2T covers 40 to 141 KU at a fixed 200 RPM, which is exactly the speed the method specifies.
- The 40 KU floor, not the 141 KU ceiling, is what rules models in or out. Thin coatings sit below it and need a rotational instrument instead.
- The KU, gram, and centipoise readouts are one range expressed three ways, not three independent measurements. A customer specification written in centipoise still needs a rotational instrument.
What ASTM D562 Actually Measures
ASTM D562 is titled Standard Test Method for Consistency of Paints Measuring Krebs Unit (KU) Viscosity Using a Stormer-Type Viscometer, and the edition cited here is D562-10 reapproved in 2023. The title says consistency rather than viscosity, and that distinction is the whole method.
The measurement works by determining the load required to rotate a paddle-type rotor at 200 revolutions per minute while immersed in the paint specimen. A weight is applied, the rotor turns, and the load that produces exactly 200 RPM is recorded in grams.
That gram figure converts to a Krebs Unit value through the scale the method defines. Nothing in the procedure attempts to isolate a shear rate or a shear stress, which is why the result is a consistency index rather than an absolute viscosity.
In our view this is the point most buyers miss, and it is worth being direct about. D562 exists because paint formulators and QC labs need one number that correlates with how a paint feels to brush, roll, and pour, reproducibly, across shifts and across sites.
It does not exist to characterize flow behavior. A method that fixes speed at 200 RPM has deliberately given up the ability to see shear thinning.
That design choice is also what makes the KU scale difficult to translate.
Why Krebs Units Are Not Interchangeable With Centipoise
A rotational viscometer reports viscosity in mPa.s or centipoise, derived from torque at a known spindle and speed. A Stormer-type instrument reports the load at one fixed speed with one paddle geometry. Converting between them requires assuming the paint behaves the same way at every shear rate, and architectural paints emphatically do not.
Published conversion tables exist and they are useful for orientation, but we strongly suggest treating them as approximate. Two paints with identical 95 KU readings can differ substantially in centipoise, because the paddle at 200 RPM samples a narrow slice of their flow behavior and the rotational instrument samples a different one.
If your specification is written in KU, measure in KU. If it is written in centipoise, measure with a rotational instrument.
The STM-2T does not sidestep this, and the specification sheet invites the wrong reading, so we want to be explicit.
Its range appears as a single line, 40 to 141 KU / 32 to 1,099 g / 27 to 5,274 cP, which is one range expressed three ways. The centipoise figure on screen is that same D562 scale converted for you, not a rotational measurement, so it saves a table lookup while carrying exactly the same assumption.
A customer specification written in centipoise still needs a rotational instrument.
With the method understood, the specifications become easy to read.
The ToronVisc STM-2T
| Specification | ToronVisc STM-2T |
|---|---|
| Range | 40 to 141 KU |
| Equivalent load | 32 to 1,099 g |
| Equivalent viscosity | 27 to 5,274 cP |
| Rotational speed | 200 RPM, fixed |
| Accuracy | 1.0% FSR |
| Repeatability | 0.5% FSR |
| Standards cited on the product page | ASTM D562, GB9269 |
Three things in that table are worth reading closely.
First, the fixed 200 RPM is not a limitation here, it is compliance: the method specifies that speed, so a variable-speed instrument would offer nothing extra for this test.
Second, repeatability at plus or minus 0.5 percent of full scale is the figure that matters for batch confirmation, and it is twice as tight as the plus or minus 1.0 percent accuracy.
Third, the three-unit range is one range expressed three ways, not three independent capabilities.
The standard configuration ships with the main Stormer viscometer, a KU-10 paddle spindle, an upright rod and base, and an RTD temperature probe.
Operation runs through a five-inch touchscreen, and a built-in printer interface is available for labs that attach a hard copy to the batch record.
What that range does not cover is where most purchase mistakes happen.
Where the 40 KU Floor Decides the Purchase
The ceiling gets the attention and the floor causes the problems. The STM-2T reads from 40 KU, equivalent to 27 cP, and a paint below that simply does not register.
In practice 40 KU is comfortable for architectural latex, trade sales paints, thick industrial coatings, printing inks, and most putties and fillers. It is not enough for thin solvent-borne coatings, wood stains, sealers, or anything approaching a varnish.
We have seen labs buy a Stormer instrument for a product line that turned out to sit at 25 to 35 KU, and no amount of technique recovers a reading that is off the bottom of the scale.
The check is simple: if any product in your line reads below 40 KU on a reference instrument, or if you do not know, a rotational instrument covering 1 mPa.s upward is the safer primary purchase.
Which leads to the question of whether one instrument is enough.
Krebs Against Rotational, and When You Need Both
| Criterion | ToronVisc STM-2T, Stormer | ToronVisc 79 Series, rotational |
|---|---|---|
| Reports | KU, grams, cP | mPa.s |
| Range | 40 to 141 KU, 27 to 5,274 cP | 1 to 1,000,000 mPa.s on the 79 |
| Speed | 200 RPM, fixed by method | 7.5 / 75 / 750 on the 79 |
| Accuracy | 1.0% FSR | 1.0% FS |
| Repeatability | 0.5% FSR | 0.5% FS |
| Sees shear thinning | No, single fixed speed | Yes, multiple speeds |
The accuracy and repeatability figures are identical across the two, which means instrument quality is not the variable. What differs is the question each one answers.
A Stormer instrument answers whether this batch matches the specification the customer buys against. A rotational instrument answers why it does not.
In our experience a paint manufacturer running a KU specification needs the Stormer first and the rotational second. A coatings developer formulating new products needs the opposite order.
Labs doing both, which is most medium-sized paint operations, end up with both instruments, and we believe that is the correct outcome rather than an upsell: a single-speed consistency test and a multi-speed flow curve are not substitutes.
Documentation is the part that turns either reading into something an auditor accepts.
Calibration and Documentation
The STM-2T product page cites ASTM D562 and GB9269 as the standards its testing procedures follow, and it lists a calibration certificate as available on request.
That citation is more than the rotational product pages carry, and we would still ask for the certificate explicitly in the quotation rather than assume it ships in the box.
For a D562 program we consider the following the working minimum:
- Record KU and gram load: Record the KU value and the gram load together, because the gram figure is the primary measurement and the KU value is derived from it.
- Control sample temperature: Condition the specimen to the 25 degrees Celsius plus or minus 0.2 degrees the method calls for, then record the temperature the RTD probe confirms, since a KU reading without a temperature is not comparable to anything.
- Document the paddle: Record the paddle used, which will normally be the KU-10 supplied as standard.
- Standardize sample preparation: Condition and stir the specimen the same way every time, written down, because latex paints are shear sensitive and a different stirring history produces a different number from the same drum.
Verification against a reference material on a defined interval closes the loop, and we suggest keeping those records alongside the certificate rather than in a separate file.
Price and Ownership Considerations
Pricing here is simpler than in the rotational lines because the STM-2T is a single model rather than a series. There is no range tier to select and no variable-speed option to weigh, which removes most of the negotiation surface.
The recurring costs are the paddle and the reference materials. A spare KU-10 paddle is worth specifying up front, since a bent or coated paddle changes readings before anyone notices.
Standard dispatch is generally within one week, though high demand items extend to two or three weeks.
Pricing is not published on the product page. We recommend asking for the instrument, a spare paddle, the RTD probe, and the calibration certificate as separate quotation lines.
What to Check Before You Choose
Four checks settle this purchase:
- Confirm the minimum KU value: Check the lowest KU value in your product line against the 40 KU floor, and if anything sits below it plan for a rotational instrument as well.
- Check customer specifications: Confirm whether your customers specify in KU, centipoise, or both, since the three-unit readout only pays off if you need more than one.
- Determine the measurement objective: Confirm you need consistency rather than flow behavior, because a fixed 200 RPM instrument cannot describe shear thinning no matter how good it is.
- Verify documentation: Confirm the calibration certificate and the verification interval in writing.
Final Recommendation
For a paint, ink, or coatings manufacturer running QC against a Krebs Unit specification, we believe the ToronVisc STM-2T is the straightforward choice.
Its 40 to 141 KU range covers architectural latex and trade sales products comfortably, its fixed 200 RPM matches the method rather than compromising on it, and the simultaneous KU and gram readout keeps the primary measurement and its derived value on one record.
Labs whose products fall partly below 40 KU, or who need to understand flow behavior rather than confirm consistency, should treat the Digital Rotational Viscometer ToronVisc 79 Series as the primary instrument and add the STM-2T when a KU specification requires it.
Formulation groups developing new coatings will get more from the Advanced Touchscreen Viscometer ToronVisc P Series and its wider range.
Related Products
- Krebs Stormer Viscometer, ToronVisc STM-2T
- Digital Rotational Viscometer, ToronVisc 79 Series
- Advanced Touchscreen Viscometers, ToronVisc P Series
- Viscosity Tips and Conversion Guide for ToronVisc Users
- Optional Accessories for ToronVisc Viscometers
Talk With Our Team About Krebs Stormer Selection
The 40 KU floor is the detail that decides this purchase, and only your product line can answer it. Our application specialists can review your KU range and your customer specifications before you commit.
Request a quote for the ToronVisc STM-2T Krebs Stormer viscometer.
Contact our application specialists about your KU specification.
What Buyers Usually Ask
Can I convert Krebs Units to centipoise reliably?
Not reliably enough to satisfy a specification. Published tables give a usable orientation, but the conversion assumes the paint behaves identically at every shear rate, and architectural paints do not.
Two products reading the same 95 KU can differ substantially in centipoise because a paddle at a fixed 200 RPM samples a narrow slice of flow behavior. The practical answer is to measure in whichever unit your specification is written in.
The ToronVisc STM-2T displays all three, but its centipoise figure is that same D562 scale converted rather than an independent rotational reading, so it does not remove the limitation.
Why is the speed fixed at 200 RPM instead of adjustable?
Because the method fixes it. ASTM D562 determines the load required to rotate a paddle rotor at 200 revolutions per minute, so 200 RPM is the test condition rather than a setting.
A variable-speed Stormer instrument would add cost without adding capability for this method, and readings taken at any other speed would not be D562 results. If you need viscosity at several speeds, that is a rotational viscometer question, not a Stormer one.
What happens if my paint reads below 40 KU?
Nothing usable. The ToronVisc STM-2T reads from 40 KU, equivalent to 27 cP, and a specimen below that falls off the bottom of the scale rather than producing a low but valid number.
This is the most common selection mistake we see, because buyers check the 141 KU ceiling and assume the floor is near zero.
Thin solvent-borne coatings, stains, and sealers usually need a rotational instrument reading from 1 mPa.s, such as the Digital Rotational Viscometer ToronVisc 79 Series.
Does the instrument need a temperature-controlled bath?
In practice yes. ASTM D562 calls for the specimen to be brought to 25 degrees Celsius plus or minus 0.2 degrees, a band tight enough that conditioning in a water bath is the normal way to reach it and an instrument probe alone cannot hold it.
The STM-2T includes an RTD temperature probe in its standard configuration, so it confirms the specimen reached that temperature and records it with the reading, but the conditioning step sits upstream of the instrument rather than inside it.
Paint consistency moves with temperature, so a KU value without a recorded temperature is not comparable to another lab's figure.
Is one Krebs Stormer instrument enough for a paint QC lab?
It depends on whether you only confirm or also investigate. For confirming that each batch meets a KU specification, one STM-2T handles the whole job, and its plus or minus 0.5 percent of full scale repeatability is what makes batch to batch comparison meaningful.
The moment a batch fails and someone has to explain why, a single fixed speed cannot help, because the explanation usually lives in shear thinning behavior.
Most medium-sized paint operations end up running a Stormer instrument for release and a rotational viscometer for troubleshooting.
References
- ASTM International. (2023). ASTM D562-10(2023): Standard Test Method for Consistency of Paints Measuring Krebs Unit (KU) Viscosity Using a Stormer-Type Viscometer. ASTM International, West Conshohocken, PA. https://store.astm.org/d0562-10r23.html
- ASTM International. (2020). ASTM D2196-20: Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer. ASTM International, West Conshohocken, PA. https://store.astm.org/d2196-20.html