Document Rjqq2LEX6Ja5Ez7OY7DQaogE7

SUBJECT: Resistivity of Liquid Aroolars IISTKOD: Resistance Measurement 8 between Conductors Apparatus / Megohm Bridge: General Radio Corapany Type 544B, AC operated - 500 volts, serial No* 171* This is a direct-current "heatstone bridge Tor 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 eq. cm. The outer electrode has inside diameter of 3" and like height of 3.25Vwith area of 198 sq. cm. The digtanee 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 practiee the cell constant supplied by General Electric Company is used. Por the electrodes now in use, the cell constant was given as 815. Glass Plate: Ityrex, about 3-1/2" diameter with concentric grooves to assist in spacing the electrodes. Obtained from General Electric Company. Oven: Cenco-DeKhotan sky drying oven with two holes in back wall for por celain tubes for lead wires. Boles must avoid oven heating elements which occupy 2" paths vertically and horizontally intersecting at the center of the circular wall. . _ . Lead Wires: Provide 300 ohm Amphenol twin conductor cable to bonne at the electrodes in oven to the posts of the bridge. The portions exposed to oven temperature are bared and then covered with porcelain beads. Provide also a flexible lead to ground the bridge to a water pipe. Glean the leads, beads, and or celain 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 plate with the connector posts directly opposite in order to give the widest possible spacing to reduce surface conductivity. The Unders 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*0. on,a hot plate and pour, into the electrode assembly until the level, of the liquid is 1/4 to 1 l/2" above the electrodes. Care must be used to make sure that the DSW 330894 STLCOPCB4077538 ! SUBJECT: Resistivity of Liquid Aroclors.'- Page 2. ME1S0D; Resistance Measurements between Conductors lip of the container, from which the liquid is poured, is clean* Tilt the beaker in such manner that all air bubbles in the Aroclor will rise to the surface. . ' Air cool to: 103-104*0. then without removing thermometer, transfer to the oven which is maintained at 10.0*C. and connect the electrodes to the two flexible wires which lead to the megohm bridge. 'Then the Aroclor oornes to temperature (100*0.), remove thermometer, check cen-. tering of electrodes, and procede to measurements. Measurement with Megohm Bridge Connect the two lead xvires to the megohm bridge with the lead from the outer electrode to the LOH unknown post of the bridge; the inner elec trode to the unknown post. Connect the GROUND post on the left side to a water pipe. Swing the spring oonneotor, pivoted on the "G" post, to the LQVJ post. Connect the attachment cord to the 110 V power supply. . V/ith the Aroclor at 10C*C., turn the control knob (CHECK-OFEILATE-CIIARGE) to the CHECK position. Throw all 3 switches at the rear of the Panel to ON. After 2 minutes bring the galvanometer pointer to zero by means of the ZERO ADJUST knob. Then turn the control knob to CHARGE for one minute. Turn the knob to OPERATE and return the galvanometer pointer to zero by adjustment of the MJLTIFLr B5T switch and the megohm dial. Read after one minute. Re peat the CHARGE end OPERATE 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. j Example: ( . . "Multiply by" reading: 100 1 ' Megohm dial reading 5.4 meg. ohms Electrode Constant 815 cm Resistivity 100 X 5.4 X 815 4.4 X 105 meg. ohm cm " 440 X 10 ohm cm . Values for resistivity are qialified by designation of temperature and voltage. These are 100*C. and 500 volts for the test above. Like con ditions are used at Plant B. 0S\N 330895 STLCOPCB4077539 SUBJECT: Resistivity of Liquid Aroolors - Page 5 METHOD: Reststance I.Teaaurements between Conductors Caro of Apparatus The "Operating Instructions", General Radio Form 458-B, which accompany . the megohm bridge, recite details of installation, measurements, uses, and construction, illustrated by figures and circuit and wiring diagrams. No attempt is made to reproduce such information here. The operator should acquaint himself with the contents of the O.R. manual and refer to it for maintenance of tubes and parts. ".Tien not in use the megohm bridge should be closed and stored in cabinet. Cleaning Electrodes . . After the measurements are made the electrodes ere removed from the tested liquid, end allowed to drain until the liquid stops running from the electrodes. V/hile still hot they are placed in a beaker filled with benzene under a well vanilated hood and allowed to cool. 'Then cool, the electrodes'are removed from the benzene and scrubbed with powdered trlsodium phosphate and a benzene carbontetrachloride mixture (50-50 by volume). This scrubbing can be done with either a brush or the handsc The electrodes are then rinsed vdth acetone, followed by tap water and then 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 120*C. for one hour or until they are used again. The glass spacer plates are cleaned and handled in the same manner as tlie electrodes except that they are wrapped in lense 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. . re 8/3/48 . DSW 330896 \ STLCOPCB4077540