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Monsanto Chemical Company , . Anniston Method No. 14-35-48 SUBJECT; Resistivity of Liquid Aroclorb _______ METHOD; Resistance Measurements between Conductors Apparatus ,,^ Megohm 3ridge: General Radio Company Type 514B, AC operated - 500 volts, serial No. 171. iVis is a direct-current lheat stone bridge for measuring high resistances, bridge balance being obtained thru use of a vacuum tube voltmeter. Range: 0.1 megohm to 10,000 megohms, covered by a dial and 5-position multiplier switch. A resistance of 1,000,000 megohms can be detected. . Test Electrodes* Two concentric nickel cylinders (or brass, nickel plated) with feet. Obtained from General Electric Company. The inner electrode has outside diameter of 2.8" and height of 3.25", with area of 184 sq. cm. The outer electrode has inside diameter of*3" and like height of 3.25", with area of 198 sq. cm. The distance between electrodes is therefore 0.1 inch or 0.254 cm. By theory the electrode constant, area/ length, is 191/0.254 or 752 where 191 is the average area. In practice the cell constant supplied by General Electric Company is used. For the electrodes now in use, the cell constant was given a9 815. Glass Plate: Pyrex, about 3-1/2" diameter with concentric grooves to assist in spacing tije electrodes. Obtained from General Electric Company. Oven: Cenco-DcKhotensky drying oven with two holes in back wall for por celain tubes for lead wires. Holes must avoid oven heating, elements which occupy 2" paths vertically and horizontally intersecting at the center of the circular trail. Lead '/ires: Provide 300 ohm Amphenol twin conductor cable to connect the electrodes in oven to the posts of the bridge. The portions exposed to oven temperature are bared and then covered with norcelsin beads. Provide also a flexible lead to ground the bridge to a -rater pipe. Clean the leads, beads, and porcelain tubes Periodically. Assembly of Test Cell The grooved glass plate is placed in an 800 cc,, beaker. The clean elec trodes are placed in the grooves of the glass olate with the connector posts directly opposite in order to give the widest possible spacing to reduce surface conductivity. The cylinders should be very close to equidistant. The apparatus should be dried for 2 hours at 100*C. before use. Heat the sample to be tested to about 110*C. on a hot plate and pour into the electrode assembly until the level of the liquid is 1/4 to l/2" above the electrodes. Care trust be used to make sure that the . DSW 330874 STLCOPCB4077518 3UBTEGT: Resistivity of Liquid 'Aroc3.org ~ Page 2*__ METHOD: Rggi stance Measurements between Conduct ors lip of the container* from which the liquid is poured, is clean., Tilt the beaker in such rranner that all air bubbles in the Arcelor will rise to the surface* - . . Air cool to 103104*C. then without removin'?: thermometer, transfer to the oven which is maintained at 1Q06C. and connect the electrodes to the two flexible wires which lead to the megohm bridge. 'Then the Aroclor comes to temperature (100C.), remove thermometer, check cen tering of electrodes* and procede to measurements* , Measurement with Megohm Bridge .. Connect the two lead wires to the megohm bridge with the lead, from the outer electrode to the LO'T unknown post of the bridge, the inner elec trode to the "+r unknown post. Connect the GROUND past on the left side to a water pipe. Swing the spring connector,, pivoted on the "G" post, to the LOT post. Connect the attachment cord to the 110 T power supply,, i- j ra With the Aroclor at 10CC.{1 turn the control knob (CTEGK-QPGRETE-OIIARG to the CHECK pcs it! on* Throw, ell 3 switches at- the rear of the Panel ON* After 2 minutes bring the galvanometer pointer to zero by means c the ZERO ADJUST knob* Then turn the control knob to CHARGE for one minute. Turn the '-mob to OPERATE and return the galvanometer pointer to zero by adjustment of the IjULTIPLY B? switch and the megohm dial.* Read after one minute* Re peat the CHARGE and 0P31ATE positions for verification* The resistivity of the Aroclor is the product of the "multiply by" reading times the dial reading times the electrode constant* Report in units of 10 ohm cm, rounded off to two significant figures* Example: "Multiply by" reeding: Megohm dial reading Electrode Constant 100 5.4 meg. ohms 815 cm Resistivity 100 X 5*4 X 615 4,,4 X 105 meg* ohm cm' 440 X 10^ ohm cm 4 Values for resistivity are qualified by designation of temperature and voltage* .Thess are 100#C. and 500 volts for the test above* Like con ditions are used at Plant B, DSW 330875 STLCOPCB4077519 SUBJECT; Resistivity of LiQuid Aroclors ~ Page 3 r.E'I'HOD; Resistance I.Tea but extents between Conductors Care of Apparatus The "Operating Instructions", General Radio Form 458-B, which accompany the megohm bridge, recite details of installations, measurements,, uses,., and construction, illustrated by figures and circuit and firing diagrams. No attempt is made to reproduce such information here. The operator should acquaint himself with the contents of the G.R. manual and refer to it for maintenance of tubes and parts. Then not in use the megohm bridge should be closed ana stored in cabinet. . Cleaning Electrodes ' After the measurements are made tlie electrodes are removed from the tested liquid, and allowed to drain until the liquid stops running from ' the electrodes. Vihile still hot they are placed in a beaker filled with benzene under a well ven listed hood end allowed to cool. Then cool,, the electrodes are removed from the benzene and scrubbed with powdered tri sodium phosphate and a benzene carbontetrachloride mixture (50-50 by volume). This scrubbing can be done with either s brush or the hands. The electrodes are then rinsed v&tli acetone,, folia?ed by tap water ahu than distilled water. The electrodes should not be touched by the hands after the acetone wash. After final rinsing the electrodes are placed in an oven at liJO^C. for one hour or until tbay are used again, . The glass spacer plates are cleaned and handled in the same manner as the electrodes except that they are wrapped in dense paper before being placed in the oven. Calibration For a rough check on the condition of the bridge, one or more cartridgetype resistances of known approximate value should be kept on hand for periodic or emergency verification of the meter itself. rs 3/3/48 DSW 330876 STLCOPCB4077520