How to Calculate Paper Stock Consistency

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Consistency is the term used within the pulping and paper making Industries to describe the dry solid content of a wood pulp slurry in water. Pulp consistency is roughly divided in to three ranges. Each piece of equipment and unit operation in the pulp and paper-making processes are designed to operate optimally within a specific consistency range.

In the USA the Technical Association of the Pulp and Paper Industries--TAPPI--issues standard procedures for testing commonly used in the pulp and paper-making processes. A simplified version of the TAPPI procedure for determining paper-stock consistency for slurries up to 4 percent is presented here.

Dry the filter paper in the drying oven for one hour and then record the weight.

Obtain a sample of the pulp slurry. Shake the sample well. If it is expected that the consistency will be 1 percent or less, pour 500 milliliters from the sample container into the graduated cylinder. If it is expected that the consistency will be between 1 and 4 percent, pour 250 milliliters from the sample container into the graduated cylinder.

Place the weighed filter paper into the Buchner funnel and wet with distilled water. Apply suction to the funnel. Pour the pulp slurry sample through the filter paper and allow all water to drain from the resulting pad.

Remove the filter paper from the Buchner funnel, taking care to retain all pulp fibers. Place the filter paper in the drying oven and dry until a constant weight is obtained. Weigh the sample, and determine the weight of pulp fibers by subtracting the weight of the filter paper from the total sample weight.

Calculate the consistency using the formula Consistency (in percent) equals the fiber weight (in grams) divided by the sample volume used (in milliliters) times 100.

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