SCAN-C 62:00 - PFI

SCAN-C 62:00Page 4Annex ATest-pad-holding unitsThis annex describes the design of two different testpad-holdingunits.A.1 The test-pad-holding unit in Figure 1 has fourparts:(a) is the test-pad former with its wire (b). The wireis not soldered to the tube but is fixed in position bypushing the tube (a) into the vessel (c), which has aperforated bottom. The part (d) fits into the tube (a) andforms a cover. It has a small hole to allow air to pass. Allparts are machined to fit snugly to each other. Duringcentrifugation the unit hangs on the rim of the centrifugebucket, leaving enough space underneath toaccommodate drained water.The parts (a), (b) and (c) are assembled beforefiltration. The lid (d) is placed in position after filtrationand before centrifugation.A.2 The test-pad former (r) of the test-pad-holdingunit in Figure 2 has its wire-screen soldered to the tube.The former fits into the vessel (s) where it rests on aflange. A circular metal plate is pressed towards theflange from below by means of a spring. The plate actsas a non-return valve, which opens only when thecentrifuge runs at full speed, thus preventing anyrewetting of the pad when the centrifuge is stopped. Thelid (t) fits onto the vessel (s) and has a handle tofacilitate its removal.During centrifugation, the pad holding unit rests onthe bottom of the centrifuge bucket. If this has a semisphericalbottom, a semi-spherical metal block is used asa support for the former. The main function of the metalblock is to create a space under the test-pad-forming unitfor the removed water.Only the test-pad-former (r) is used in the filtrationstep.Figure 1. Test-pad-holding unit, to hang on the rim ofthe centrifuge bucket.Figure 2. Test-pad-holding unit, to rest on the bottomof the centrifuge bucket.

SCAN-C 62:00Page 5Annex BThe influence of test-pad-forming procedure on the resultsThe Water Retention Value of three different pulps,bleached hardwood, bleached softwood and unbleachedsoftwood pulps, have been measured for bothunbeaten and laboratory-beaten pulp. Results wherethe test pad was formed in a test-pad former(Alternative 2) were compared to those where the testpad was formed in a Büchner funnel (Alternative 1).The results showed that when the test pad was formedin a test-pad former, the WRV results were between5 % and 10 % higher than the results obtained whenthe test pad was formed in a Büchner funnel. Thisapplies to Water Retention Values in the interval0,8 g/g to 1,9 g/g, see Figure 3.Warning – The results also indicate that forsamples with a high WRV (>1,9 g/g), thedifference between the two procedures is muchlarger.Neither of the two alternative procedures is consideredto be more correct than the other.Figure 3. The increase in WRV, in %, when the test pad is formed in a test pad former (Alt. 2), comparedwith the results obtained when the test pad is formed in a Büchner funnel (Alt. 1).403530Relative increase in WRV, %,25201510501 1,2 1,4 1,6 1,8 2 2,2WRV, g/g, Test pad formed in a Büchner funnel (Alt.1).SCAN-test Methods are issued and recommended byKCL, PFI and STFI-Packforsk for the pulp, paper andboard industries in Finland, Norway and Sweden.Distribution: Secretariat, Scandinavian Pulp, Paperand Board Testing Committee, Box 5604,SE-114 86 Stockholm, Sweden.