Consistency Meters - Papermachine Automation

Consistency Meters

Rotor designed consistency meters

The most common consistency meter found, the rotor design CM (Consistency meter) sits just outside of the stock flow stream. A deflector rotor pulls stock into the recess where the measurement device rotates at a constant speed. The stock gets thicker the torque on the motor to maintain that speed increases. The consistency can then be measured against the motor torque. This is known as a strain gauge. These types of measurement devices require a certain flow to function correctly and can measure consistencies of 1% - 10%.

unlike fixed blade consistency meters the rotor design is not affected by the variations in stock flows because the device creates its own flow from the deflection rotor onto the measurment device.

Fixed blade Consistency meters

Fixed blade CM work in the same way as the rotor design except the paddle is placed within the fiber flow stream. The fixed paddles moves with the fiber flow. The consistency increases in the pip, this in turn increases the force against the blade. The force is measured by the meter and calculated to a consistency. This is another instance of a strain gauge. 

The limitation with this type of measurement device is the stationary aspect. As stock flows past the paddle, fiber and rejects can stick/ build up reducing the accuracy of the device.
Variable flow within the pipe will alter the consistency measurment. A higher flow will add a higher force onto the consistency meter resulting in a higher measured consistency.

The major advantage of this type of meaurment is cost - usually customers purchase a fixed blade/ dynamic blade consistency measurment will the intention of replaying it with a better model.

Microwave Consistency meters

Microwave transmitters work on the principle that sending microwaves through water the waves travel at a certain speed. When fiber is introduces the microwaves move faster through the stream. The consistency can therefore be measured depending on the speed of the microwaves being sent and received by the meter. 

Using the calculation;   Velocity = C / sqrt(e)   

Where; C = Speed of light in a vacuum

     E = Dielectric constant of liquid (water)

These devices are more accurate than the rotor/ blade design. The microwave Transmitter also has no moving parts for the pulp to affect/ build up on like the blade transmitter.

The CM is not affected by the flow rate, colour, and brightness, like traditional microwave ovens they are highly affected by metals. these consistency meters are used within very clean pulp systems like the aproach flow because the likleyhood of metals entering the stream are very low. The microwave transmitter needs to be the same size as the pipe being used. Due to the expensive nature of these devices microwave transmitters are typically used on smaller pipe work or substituted for cheaper models. 

For more Info on Instrumentation within paper-making check out my other blog posts!

Level transmitters;
https://www.papermakingbible.co.uk/2018/04/process-level-indicators-papermachine.html

PID controllers;
https://www.papermakingbible.co.uk/2018/02/pid-control-loop-parameters.htm