1) The freeness test is based on Tappi Method T 227.
2) The freeness tester (see figure 11) must be cleaned and calibrated before use. The chamber and funnel are gently brushed with soap and rinsed with distilled water thoroughly. Check that all surfaces and orifices are free of fibres to prevent any disruption of flow. Check inside the funnel to ensure the spreader cone is firmly in position. Let tester drip for thirty seconds then close the bottom lid. Two freeness beakers are also cleaned and one is placed right below the bottom orifice of the funnel. The other one is tared on top of the Mettler balance.
Figure 11: On the left, freeness tester and accessories; in the middle, the valve is shut after water or pulp suspension is poured in; on the right, the valve is open to release the suspension.
3) Prepare ~3 L of 20 °C distilled water in a 4 L plastic beaker. Weigh exactly 1000 g of 20 °C distilled water to the tared 2 L freeness beaker. Pour the water to the chamber and place the beaker in position to receive the water from side orifice. Close the top lid and shut the vent valve (or cover hole with finger) and open the bottom lid gently (see figure 12). Open the vent valve quickly (or lift finger from hole) to release the water. Wait until dripping stops. Weigh the beaker that receives water from side orifice. It must fall between 880-890 g to verify the accuracy of the tester. Repeat procedure to carry out second blank check. If it is off from the required range, check the water temperature, adjust it if necessary, and check the tare of the beaker. Repeat again until it hits the targeted range. If it is still off, contact the Lab Technician for troubleshooting because the entire freeness tester may need to be taken down and thoroughly inspected.
4) If you wish, you may use a 1000 ml plastic graduated cylinder to replace the 2 L beaker (see figure 13). The accuracy of the graduated cylinder can be verified by checking whether 1000 g of 20 °C water matches the 1000 mL mark or not. You then fill up the cylinder with 20 °C distilled water to the 1000 mL mark and pour it to the tester. The other steps are identical except using graduated cylinder instead of beaker.
Note: Always use weighing to verify amounts for calibration regardless of whether a beaker or graduated cylinder is used.
Figure 12: Sequence of using Freeness beakers to carry out calibration. 1: Weigh 1000 g of 20 °C distilled water; 2: Pour the water into the chamber; 3: Open the valve and bottom lid to release water; 4: Receiving water from side orifice.
Figure 13: Sequence of using 1000 mL graduated cylinder to carry out calibration.
5) Once the calibration of the freeness tester is verified, carry out the tests for the pulp suspension immediately. Check the temperature of suspension again; normally it remains the same within 10 minutes after it is prepared. Or, calibrate the freeness tester before you prepare the pulp suspension.
6) The testing procedure (see figure 14) of pulp suspension is identical to the calibration procedure. Tare the freeness beaker. Agitate the pulp suspension vigorously with a plastic 500 mL beaker and take samples to the freeness beaker. Weigh to 1000 g precisely. Pour the suspension to the tester as soon as 1000 g is reached, close the lid and shut the valve. Place the freeness beaker to the side orifice and open the bottom lid carefully, open the valve to release the suspension. When dripping stops, check the weight of water received from side orifice (beaker remains tared). Record the weight and temperature to the filter paper and your lab book.
Figure 14: Sequence of freeness test of pulp suspension. 1: Pulp suspension is poured to chamber; 2: Collection of filtrates; 3: Weigh the filtrate collected from the side orifice.
7) Pour the remaining pulp suspension in the chamber to the side orifice beaker. Use filtrate from the bottom orifice beaker and distilled water to flush the chamber and pour it to the side orifice beaker. Filter the whole suspension in the Buchner funnel using the same procedure as for consistency determination. The filtrate must be re-circulated three times or until it turns clear (see figure 3). Dry the pad in speed dryer for at least an hour or until constant weight is reached (see figure 15). Subtract the filter paper weight and you have the OD pulp weight from 1000 g of suspension. For example, 4.834 – 1.895 g/1000g × 100% = 0.294%. Repeat the same test for the second sample to get the average. In this way you also get your consistency value. Use the Freeness Correction Tables to calculate the final freeness.
Figure 15: On the left, suspension is being poured to Buchner funnel with a tared filter paper; on the middle, pad is dried in a speed dryer; on the right, check the OD weight of pad.
