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F.A.Q.

 

How can i get out viscosity in Pa*s and yield stress in Pa from the Viskomat?

For the viskomat NT there are several probes available. All probes may divided in 3 groups.

1. The classical geometries like the: basket probe, cone-plate probe, cylinder probe.

2. The special formed group: vane probe, sphere or ball probe.

3. The stirrer formed probes like the: mortar probe, cement paste probe, modified cement paste probe.

You may see pictures of all probes here.

With the first group you get out data as with all classical rheometers.
For the cylinder probe, which is according to DIN EN ISO 3219 dependent from the geometry, the size of the inner and outer cylinder, the height, the gap size etc. you get three calibration factors. Etha_rep, thau_rep and gamma_rep.
These factors are written in the calibration sheet you get with the probe.
The number etha you get out of the viskomat software (in the graphic) multiplied with etha_rep give the viscosity in Pa*s. The thau you get in the graphic for the yield value multiplied with thau_rep gives the yield value in Pa. The speed in rpm multiplied with gamma_rep gives the shear rate in 1/s.

The same factors are written in the calibration sheet for the cone - plate system and for the basket-probe.

The second types of probes are a little bit more difficult to handle. The vane probe is a kind of cylinder probe with the cylinder diameter of the vane, and the gap size is given by the diameter of the vessel. This kind of geometry tries to avoid the wall slippage effects, you get with the smooth walls of the cylinder geometry with Bingham materials like cement paste and mortars. If you are interested in the details please read:

Howard Anthony Barnes, Quoc Dzuy Nguyen
"Rotating vane rheometry — a review"
Journal of Non-Newtonian Fluid Mechanics, Volume 98, Issue 1, 15 March 2001, Pages 1–14

The ball or sphere probe is another very special probe. For Newtonian fluids (and ONLY for Newtonian fluids) Stokes law is existing.

F_d = 6 \pi\,\mu\,R\,v_s\, (source Wikipedia)
mu is the dynamic viscosity. So if you measure the force and the speed you can determine the viscosity. Unfortunately this doesn't work for a Bingham fluid. And most of all building materials are non Newtonian!
The third group of probes are formed like a stirrer.

WHY? Most of the building materials like cement paste, mortars and fresh concrete have, beneath some others, one big disadvantage: if you start to shear the material (over the yield value) you get a sedimentation of the coarser particles, because their density is higher then the density of the fluid around. So the water is on the top and the sand on the bottom. There are 3 options to avoid this:
1. You stay below the yield value. In this case you get only informations about the elastic properties.
2. You are measuring in an extreme short time. Our materials are time dependent, and this is an important information we like to get out.
3. You take a kind of stirrer which avoids sedimentation.

But there is one disadvantage of this stirrer formed probes. This probes have no well formed geometry compared to a cylinder or a cone. There is no analytical information about the gap size and the shear rate.

What you can do is to take a fluid with a well known viscosity and test it for example with the mortar probe of the Viskomat. You may do this you will find out that the calibration factor etha_rep for the mortar probe is about 25.0 at 20°C. So multiplying the viscosity factor of the Viskomat by 25.0 gives the viscosity of the specimen with the mortar probe at 20°C. For the cement paste probe this factor is about 2.5. More details you may find here.

But this is ONLY valid for Newtonian fluids, and we have no calibration factor thau_rep for Bingham fluids.

There have been some suggestion for a Bingham calibration fluid in the literature, but up to now this concept is not proven. 

For example in:


Hendrickx R, Rezeau M, VanBalen K, VanGemert D:
"Mortar and paste rheology: concentration, polydispersity and air entrapment at high solid fraction"
Appl. Rheol. 19 (2009) 52550.

the authors described an attenuation for the Schleibinger Paste Probe.

In:

P.F.G. Banfill and F. Hornung,
"Zweipunktmessung im ViscCorder"
Beton 2/1992

the authors gave a calibration factor for the Schleibinger Mortar Probe.

But again:

For the

Basket Pobe
Cone Plate Probe
Cylinder Probe

Shear stress and strain rate are well defined. The calibration factors are delivered with the probes, and they are defined in the System Setup menu of the Viskomat.

The mortar, cement paste and modified cement paste probes are delivered with a calibration factor for Newtonian fluids. To calculate the viscosity in Pa*s from the gradient of the torque over speed (dT/ dV). For Bingham fluids this factor is not defined.
So you can compare the flow curve of cement mortar A and B but you have to be carfeully if you like to read out the yield stress in Pa and the viscosity in Pa*s.
If this is necessary, you have to use the Basket, Cone-Plate or Cylinder probe. But this values are only necessary for computer flow simulation or similar task.
For product development of binders, additives etc. the relative comparison between two materials, measured for example with the mortar probe,  is sufficient!
Some rheometer manufacturers claim that they can calculate viscosity (Pa*s) and yield stress (Pa) from a stirrer formed probe, but this calibration is  NEVER true for non Newtonian fluids!

Is the rheometer operator dependent? will different operators come up with different results?

As you know, building materials are very sensitive to the specimen preparation. So in my answer I will assume that :
- the cement, the aggregates, the distribution of grain sizes, the temperature and the admixtures are the same.
- the mixing process is the same
- the time between start of mixing and start of measurement is the same
- the amount of concrete / mortar is the same
For the Viskomat NT the results will absolute the same, independent from the operator, if all operators are carefully with topics above

The same you will get for the BT2 as long as you have a fresh concrete nearly following the Bingham model. If you have a concrete with a extreme non-linear behavior, and two operators working at absolute different speeds, you will get different results, if and only if you are only looking to the two numbers you will get (yield value and viscosity). But if you are also looking on the flow curve itself, you see the difference suddenly. The flow curve is displayed on the palm-computer as well as on your laptop (see also topic 6!)
So in this case you have to instruct the operators to use a certain speed range. This range is displayed with the test results as well. With the new eBT2 speed is always reproducible.

Is there a warranty for this devices?

Standard is one year for both devices. Ask Schleibinger for special service and warranty contracts.

If the rheometer needs maintenance in the future, what should we do ?

Call your local dealer or send it to Schleibinger.


Does it need any calibration after it is arrived?
No!


Is there any limitation for the rheometer? i.e slump values

With the BT2 you can also measure quite stiff concrete, like highway concrete. But due to the more statistical behavior of 20l of sand compared to 20l SCC the statistical deviation of the single measurement values (about 500 per convolution) will increase. So we recommend the BT2 for concretes with a spread table value of more than 45cm.

For the Viskomat NT a limit is hard to define. The mortar should have a shock table value that is larger then 100 mm.

For the Viskomat XL the maximum grain size is 8 mm. There is no technical limit, but if the material is to stiff you will get a kind of plug flow, meaning the material is standig in the vessel and the vessel is slipping around the material.

For SCC, a negative yield stress value is resulted if Bingham model is used, can we use a HB model instead?

The Viskomat supports several models like Newton, Bingham, quadratic and cubic equations, and also the Bulkley-Herschel model.

The BT2 calculates at the palm-computer the Bingham values. In the Excel program, thats delivered with the BT2 you can use every function that is supported by Excel. We are planning a Bulkley Herschel calculation for the Palm too. Up to now the power of this small computers was to weak for this kind of number-crunching. (see also topic 1)


If two rotations are made by the BT2 operator, will there be more data points showing on the graph which lead to a more accurate result?

In principle yes, but with the second revolution you will get much lower results, because you are digging a gap during the first revolution. The trick of the BT2 is to measure only in untouched concrete, which is the greatest advantage over the other known systems.
A single turn measurement is the better way to get more accurate results. For the second measurements remix the specimen (in an optimal way not by hand, but with a mixer) and measure again. For each measurement you will need less than one minute.


If concrete behaves as a shear thinning material (non-linear), how can we ensure that the yield stress and plastic viscosity are consistent when different BT2 operators are testing the material? Otherwise, what is the speed range should we specify for this operation in order to give reliable results (i.e. yield and viscosity)?

In a theoretical way you are right here (as we discussed above).The BT2 is driven by a hand wheel and a gearbox. Therefore, the maximum speed is limited by the individual hand speed (try to turn a wheel as fast as possible - there is a certain maximum limit you can reach).The minimum speed is given by the electronic. The BT2 normally stops after one rotation. If you are to slow, there is a timeout and the BT2 stops and cancel the measurement. Schleibinger is actally (2011) developing a motor driven expansion unit for the BT2.


I have attempted to carry out a Viskomat measurement on one of our standard mixtures (a suspension of 43 wt% metakaolin, max particle size 60 micron, in alkali silicate solution). This is not a high viscosity mixture by our standards, for inorganic polymer (geopolymer) binders. It is thixotropic, but pours easily. Measured in a conventional rheometer (concentric cylinder), the dynamic viscosity was measured to be 7.2 Pa.s. Unfortunately, the measurement in the Viskomat failed to be completed - it seems that the mixture is too viscous.

This comes as a surprise. I used a profile which involved increasing the speed to 120 rpm in 2 min as the first ramp. Using the cement paste probe, the maximum torque (200 Nmm) was reached almost immediately, at very low speed, whilst when I used the mortar probe, the maximum torque was reached at about 70 rpm. I note that the manual suggests that a maximum torque in excess of 200 Nmm risks damaging the Viskomat. I'm assuming that the torque calibration is correct, but I have no way of checking this.

Thats ok i think, we calibrate the Viskomat with the mortar probe and a silicon oil of 12.5 Pa s. Here the Torque at 120 rpm is about 60 Nmm
With the cement paste probe the Torque is about 10 times higher.

The results seem to indicate that the Viskomat will be of limited utility for our work, (especially using the cement paste probe) unless we can legitimately use a different procedure, e.g. a much lower speed range. Many of our mixtures have a lower water content than the one I tested, and are therefore more viscous. Also, when we work on mortars based on these binders, we can expect the viscosity to be higher.

So, a few specific questions:

Is it reasonable to obtain torque-speed curves over a more limited speed range; e.g. 0-50 rpm ?

Yes. You only need the higher speeds if you have strong sedimentation effects. The "stirrer" then mixes the specimen better.

 

Is it reasonable to use the mortar probe for a homogeneous suspension of fine clay in alkali silicate, rather than the cement probe ?

Yes

Can we be sure that the torque calibration is OK ?

Take a oil with a known viscosity or crosscheck it with your standard equipment. With the mortar probe the calibration constant is about: 24.8 Pa s / (N mm s).

Has anyone else dealt with this kind of problem ?

I don't know. Most of our customers are working with standard mortars or cement pastes. Maybe we have to choose another design of the probe for your application.


I'm very concerned that the Viskomat is suitable for the work we need to do. I'll try a Portland cement sample with w/c=0.5, just to make sure that the results are as expected for a well-known mixture.

This must work.

Why don't you offer a ball formed probe ?

We can deliver you also several ball probes
These ball formed probes go back to the publications years ago. In Germany there was a PhD thesis some years ago at the Univ. Erlangen, and XXXX is selling a ball measuring systems with its rheometers.

But this ball system doesn't work with Bingham fluids. Have a look at the following link:

http://www.math.unifi.it/%7Erosso/RICERCA/Sedimentation-in-CWS/Sedimentation-in-CWS.html

In the second chapter you will see a factor alpha between 1.5 and 10.
And this is the problem. You have to know tau_0 to determin etha. With the Bingham Fluids it doesn't work. By using Newtonian you can calibrate our mortar ore paste probe as well.

What is the difference between the cement paste probe and the modified cement paste probe?

The cement paste probe is used as the name says for cement paste. The gap between the probe and the vessel is only 1.5 mm. The active area as about ten times higher as for the mortar probe, therefore the signal is about ten times higher. In recent years, many very fluid mortars having a high content of SP have been developed. This mortars contains a lot of fine particles showing less segregation. For this purpose the modified cement past probe was developed. It has almost the same shape and is a little smaller. The gap between the vessel and the probe is here 4.5 mm, which makes it possible to measure fluid mortar with a max grain size of 2 mm.

What is the purpose of the "cone/plate shaped probe" I imagine that it is to obtain results in absolute terms.

Yes, you are right here. The disadvantage is that this only works for very pasty materials like a tile-glue ore similar things.

What are the measurement geometries?

A good question:
For the Viskomat generally we are using following stirrer: 

  • Mortar Probe
  • Cement-Paste Probe 
  • Modified Cement -Paste probe. 

Here you have well defined sizes in mm, but no well defined geometry in a rheological way, because the dimensions are clear, but you have no well defined areas, like with a cylinder-cylinder or cone-plate system. But this kind of stirrer formed probes are avoiding segregation of the material. You may calibrate it with an oil with a known viscosity, but this calibration is only valid for Newtonian fluids. Mortars and fresh concrete are no Newtonian fluids. 

We can also offer a

  1. cone/plate system
  2. a cylinder-cylinder system
  3. ball-probe
  4. vane-probe
  5. basket probe acc. to Prof. Vogel

but we don't recommend probe type 1 and 2 for paste or mortar (see questions above) . The ball probe is a special problem see above.
The vane probe is based on the idea that the material between the plates of the vane is forming a cylinder made of the material itself. 

For stable mortars without sedimentation we recommend the basket probe. You have a well defined geometry and so a well defined shear stress. 

The cylinder system has the following dimensions:
Inner cylinder diameter 50.0 mm, gap size 2.12 mm specimen volume 127.7ml 

Cone-Plate-Systen: diameter cone: 83mm, angle 16.5° 

Mortar probe: 4 impellers, diameter of the stirrer 67 mm

Cement-Paste-Probe: diameter of the stirrer 80mm 

Modified Cement Paste Probe diameter 72mm
(all sizes may be changed without notice)

When is speed-controlled mode inadequate and stress-controlled mode justified?

95% of all measurements are made with the speed controlled mode. The stress controlled mode is for example detecting the elastic versus the plastic effects of some materials. For example you say go up in 20 s to 20 Nmm and the back to 0 Nmm and look then to the angle versus torque diagram . Also fits more for glues and plasters ore if you have some cellulose ether ore similar in your material.


What is the difference between the Viskomat NT and Viskomat PC? Are they both current models?

The Viskomat PC was followed by the Viskomat NT. The PC model is no more produced since 1997. But they are compatible. That means all results you have got with the PC you will get with the NT. Additionally you have a lot more features with the NT.

What is the difference between the Viskomat NT and the Viskomat XL?

The measurement principle is the same for both instruments. The vessel size of the Viskomat NT is 365 ml for the Viskomat XL 3000 ml. The maximum speed of the Viskomat NT is (2011) 600 rpm the maximum speed for the Viskomat XL 100 rpm . 

 

Can you explain the term beachure and the difference between it and beaker?

My understanding is that a beaker is a deep glass or other vessel used by chemists, generally with a lip for pouring.? I can't find beachure in my dictionary but I presume you mean a a double-walled cylindrical container. 

In 1994 a native English speaker translated us a data-sheet for the Viskomat PC. Somebody copying this text make a typo and then this wrong word was running for years through our office. So you may google for beachure and you hit schleibinger.com. So beachure is absolute nothing then a beaker ore say vessel ore something.

Can you confirm that both the beaker for temperature control and the circulating cooling and heating unit are necessary for temperature-controlled work?

Yes. You can also use another cooling unit but this must have a pump which can push and pull. 99% of all this units have two pumps, ore one which only pulls.

 

Will be other rheometers for mortar and fresh concrete on the market?

Google for example BML, FCT, Two-Point and Btrheom. Look for its technical data and its price, and the you will see that you get best quality made in Germany for a very reasonable price from Schleibinger.


Will be continued..

 


 

Last edited: June 18, 2018, 16:55
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