in-line viscometer 3

[CARF]

Hi,

Does anybody know good in-line viscometers for toamto paste?
My collegues say Nametre is a good one?
There must be more??

Thanks !
C.

 

[jmw]

Nametre is indeed a good viscometer.
Others to consider of similar principal (vibrational) include Hydramotion, Sofraser, Solartron, TDCollaborative, CJC Associates and a few other vibrating element types.

Some vibrating element types are analogue i.e. they use the change in amplitude at resonant frequency or the decay time to interpolate the viscosity. VAF and Solartron produce digital viscometers i.e. they measure the frequency at the half power points. All measure dynamic viscosity as a primary measurement. The Solartron displaces the fluid so it also measures density which is valuable in its own right but also allows the true kinematic viscosity to be determined.

Note that most vibrational instruments are considered, by the purists, only to be suitable for Newtonian fluids, which tomato paste is not, but many will provide a practical and process friendly solution (i.e. will not require lots of maintenance and calcibration).

However, if the use of a vibrational instrument is not backed up by calibrating the measurement against a suitable laboratory instrument or if the process conditions are not well controlled then the value of these instruments will be lost.

For non-Newtonian fluids rotational viscometers may prove an alternative as they can operate at constant shear or constant stress. I would sugesst Brookfield, Solartron, or Katronics for rotational visocmeters of the Couette or Searle types but there are many others available.

However, modern process objectives being what they are I would suggest that if you can find a solution with a vibrational instrument that would be the optimum approach.

Note that there are some new technologies entering the market including ultrasonic and acoustic wave types. Beware of some that claim to be ultrasonic, this term is as much imprecisely used as "digital" I have used digital in the sense implicit in the ASTM Standard for digital density meters where some just use it to refer to the display). Some instruments use th propagation of ultrsound and its effects to derive the viscosit while others just provide a vibrational instrument that happens to vibrate at "u;ltrsonic" frequencies, there is a difference but in the end, all you want is omething that works.

 

[CARF]

Thank you JMW !

Good to read that Nametre is not the only one in this world and there are more measurement principles.

Another question we have is: tomato paste is quite sticky stuff. Would fouling of the vibrating element cause a misreading? Or do we need to place it at high flows, so it can clean itself?

Thanks,
CARF

 

[jmw]

Vibrating element viscometers:
the damping effect of viscosity is very much a surface affect, unlike density which is mass centred; hence with the Solartron fork viscometer, which also measures density, coating will have a more pronounced affect on the viscosity than the density.

As you suggest, flow rate is important to help keep the senors clean. If this were a laboratory measurement then there would be no problem since it would be a fixed sample but in a process measurement it is important to refresh the fluid and in particular to flush the surface of the sensor with new fluid.

Flow can be a valuable cleaning aid but care must be taken to maintain the flow constant and at rates and in directions recommended by the manufacturer.

Note also that some displacement type sensors e.g. the Solartron, may also need a flow correction while others that dn't dispalce the fluid e.g. Nametre, are not directly affected by the flow.

Some torsional types, though not directly affected by flow can be sensitive to force loading due to high lateral flows.

It is also important to note that care is needed with shear sensitive fluids.
Firstly, for non-Newtonian fluids the measured viscosity will be both temperature and shear rate dependent. Maintaining a constant flow allows the process instrument signal to be correlated with a controlle shear rate laboratory measurement.
Secondly, some fluids properties are permanently affected by high shear rates.

It is now a case of recomending careful discussion with the manufacturers.
Be prepared that they may not be able to give an absolute answer as some things are very process and application dependent and that you may need to evaluate the instrument on a sale or return basis.
Viscosity is not the easiest of process measurements to make, but done well, one of the most valuable.