DON MCCABE

CONSISTENCY MEASURING DEVICES

OPTICAL

PRINCIPAL OF OPERATION: Use of a light source and measure the backscattered light or transmitted light. Light sources can vary; from infrared through to polarized light.

APPLICATION: Second most common device. Best suited for small variation and at lower consistencies

STRENGTHS: No moving parts. Easy to maintain and relatively easy to install. Insensitive to flow changes

WEAKNESSES: Not generally well suited for higher consistencies. Signal can be heavily influenced by filler materials, entrained air and black liquor. Newer "Peak Method" can compensate for fillers. Species changes can pose a major problem.

PRINCIPAL OF OPERATION: Use of a microwave transmitter and receiver to measure the microwave velocity. The speed of the microwave is measured. Microwaves pass through water and fibers at different speeds thus the measured speed is proportional to the consistency.

APPLICATION: Suited for applications in which fiber type can change and where simple calibration is required coupled with low maintenance requirements.

STRENTHGS: Insensitive to species changes and can accurately operate over a wide consistency range ( as much as 0 – 75% ). Not overly sensitive to fillers.

WEAKNESSES: Can be sensitive to air entrainment. Not suited for high conductivity pulp. Both receiver and antenna must be in contact with the pulp and can be prone to residue build up.

PRINCIPAL OF OPERATION: Use of a radioactive source and detector. A pulp slurry’s density is proportional to its consistency with a denser slurry absorbing more radiation.

APPLICATION: Suited for applications in which fiber type can change and where simple calibration is required.

STRENGTHS: Relatively wide range ( 1.5 – 15% ) can be measured accuratley. No contact with pulp required and can be mounted on the outside of a pipe and is not prone to residue build up.

WEAKNESSES: Not suited for impurities whether they are black liquor, fillers, or entrained air. Since this is essentially a density meter temperature fluctations have to be accounted for. Must deal with a radiation source.

PRINCIPAL OF OPERATION: A target ( blade ) is inserted into the flow of pulp slurry. The force on the target is measured, by various methods, and is dependent flow and consistency ( shear )

APPLICATION: Best suited where cost is an issue and better suited for the higher end consistencies and precision is not a necessity. Range of consistencies 0.8 – 16.0%

STRENGTHS: Generally the cheapest of all meters. Simplicity of operation has led to its popularity. Maintenance is usually low and simplicity of design usually leads to low repair costs.

WEAKNESSES: Shear force on the blade can be greatly infuenced by flow although with different blade designs this can be minimised. Blade is fully or partially inserted directly into pulp flow and is subject to trauma and residue buildup. May need flow straightening ddevices or install in long length of pipe.

PRINCIPAL OF OPERATION: A target ( blade ) is inserted into the flow of pulp slurry behind a protective barier and is oscillated back and forth. The time it takes to complete an oscillation or its degree of motion is dependent consistency ( shear )

APPLICATION: Very similar to to the static blade type. Range of consistencies 1.2 – 9.0%

STRENGTHS: Although more expensive than the static type it is still relatively cheap. The influence of flow is greatly reduced and accuracy is improved.

WEAKNESSES: Similar to the static but are minimised with less of the blade being exposed to pulp flow. Blade is fully or partially inserted directly into pulp flow and is subject to trauma and residue buildup.

PRINCIPAL OF OPERATION: A rotating element ( of various shapes ) is located in a slipstream, or out of the main flow path ( a chamber off the pipe ), or in the flow path itself depending on the design. The tourque ,or shear, is measured by the power demand.

APPLICATION: 0 – 16.0%. Best suited where accuracy and precision is required

STRENGTHS: Highest accuracy of the shear measuring devices. Very versatile and low sensitivity to flow. Typically the device is larger than other two devices and therefore more representative of the pulp slurry. Most models are sheilded from main flow.

WEAKNESSES: Comparatively high in installation and unit cost. Can still be influenced by residue build up. Some sensitivity to furnish changes.

GENERAL COMMENTS

- No one consistency type of meter is better than others.

- Selection of type depends on balance of customers needs versus wants ( accuracy versus precision versus cost )

- Process variation ( temperature, consistency, flow solids level, brightness air entainment, etc) need to be evaluated.

- Mechanical limitations have to be considered ( pipe length, pipe geometry, transmitter location etc)

- Technology continues to advance to the point where several of the above weaknesses are minimised. Grade calibrations, smart transmitters and software packages can reduce some of the above concerns

- Technology is also being applied to more exotic methods of measurement including photo imaging and ultrasound.