Simple Circuit for Adapting Thermocouple Recorders to Measure Voltage in High-Resistance Circuits. Frank T. Gucker, Jr., and Axel H. Peterson (present address, hfellon Institute, Pittsburgh 13, Pa.), Department of Chemistry, Indiana University, Bloomington, Ind.
Voltage Gain and Linearity. Over the input range from 0 to 6 volts, the average gain is 0.8; the incremental gain varies smoothly by about lOy0,while the maximum deviation in the linearity is only about 0.5%. Drift. The hourly drift in the zero point of an aged tube is about 0.05 to 0.1 mv. referred to the input. A new tube should be operated continuously for several days before being used for measurements. Heating Time. After the current is turned on, it takes about 3 minutes for the tube t o come within 5 mv. of final equilibrium, and 5 hours for the drift to be reduced t o 0.1 mv. per hour. Temperature. A change of 1’ C. in the ambient temperature causes an unbalance of 0.2-mv. equivalent input. Plate and Filament Voltage. A 1% change in plate voltage is approximately equivalent t o 0.5-mv. input change, while the same variation in filament voltage causes a transient of about 1 mv. which disappears in a few minutes. Filament Current and Noise. The peak-to-peak noise, observed on a General Electric recorder with a balance-to-balance response time of about 1 second, is 0.02 mv. when the filament is heated with a storage cell, but may he 10 times as great when regulated alternating current ii used
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OLTAGE and current measurements are very important in analytical chemistry, as most physical and chemical quantities norv may be converted into electrical outputs. However, not all can be recorded easily. Chopper-type thermocouple recorders are satisfactory only nith devices of low internal resistance, while a resistance as large as 0.1 megohm would lengthen the response time from seconds to minutes and introduce a broad dead zone and other difficulties. This paper describes a onetube circuit which adapts a thermocouple recorder to an input impedance ria high as 100,000 megohms without appreciable change of its voltage sensitivity or response characteristics.
The circuit is a balanced difference amplifier shown in Figure 1, employing a shock-mounted 6SN7 or 5692 twin-triode tube. T h r unknown and reference voltages are fed respectively to the grids, GI and GP,the potentials of the two cathodes follow thosc of the grids, and any difference appears between -4and C, whrrc it can be applied to a low-impedance recorder. Such a differential cathode follower is considerably more stable for direct current measurements than a single cathode folloner h similar circuit has been dcscribed (Elmore, W. C., and Sands, M., “Electronics, Eupriimental Techniques,” p. 55, Xew York, McGraw-Hill Book Co., 1049), but operating details have not been published. Thi- rirruit gave excellent pet formance in this laboratory.
Zero
Except for the 1-megohm carbon voltage divider, all resistors are wire wound. The referenee voltage is supplied by a large 6volt dry “ignition battery,” \\hich is very stable when feeding a 100,000-ohm 10-turn voltage P d e r , and has a temperature coefficient of about 0.027’0 per C. If the reference voltage is zero, the instrument operates as a vacuum tube voltmeter; if it approximately equals the measured voltage, the &iff erence niav be read on a recorder and the esact voltage determined with great acruracy; if it is adjusted to match the measured voltage, the instrument operates with its highest accuracy as a null detector, provided its zero is checked nith sufficient frequency. This adjuitmcJnt ‘is made by putting the snitch in the “zero” position and moving the 1-megohm voltage divider until a bias of a fev tenths of a volt is introduced and the recorder reads zero. A Rronm thermocouple recorder, after the cold-junction coinpensation is removed, can be operated across Rz as indicated ( r dircctly between A and C, replacing RI and Rz. The increasr froin 25 to 40 seconds in full-scale travel time is reduced to normal by changing the input capacitors to the recorder, while the dead zonr is not affected. A General Electric photoelectric recorder also is easily adapted to this circuit. A4Brown, Leeds and Northrup, or Foxboro electronic recorder is attached directly between -4and C for maximum sensitivity. TVhen a recorder with a fullscnlr deflection of 10 mv. is used t o measure a 5-volt signal, RI ma? be 20,000 ohms and RP a 100-ohm variable resistor. Th(5 refcrencr voltage serves convmiently to calibrate thv instrument. OPERATING CH 4R iCTERISTICS
Input and Output Impedance. For a grid bias between 0 aiid 6 volts, the measured input impedance, de,/dz,, of tubes from four different manufacturers averages between lo4 and lo6 inepohniq. The output resistance between A and C is about 1500 ohms 11 ithout the voltage-dividing resistors, R, and RP, used t o control the sensitivity of the system. To avoid interaction between the t x o triode sections, the total external resistance between A and C should be several times this output resistance, increasing n ith the voltage measured. Grid Current. This is generally leas than ampeie.
90V
Recorder I
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Figure 1. Differential Cathode Follower Microphonics. The 68x7 was much more microphonic than two 5682 tubes tried. A solid flick with the finger produces an erratic 20-mv. unbalance with t,he former, and only about 2 mv. with t,he latter. At long intervals, perhaps once or twice a day, the 6SN7 gives sudden changes of a few millivolts which n-ere not traced to any known external disturbance. Thc 5692 never gavcs this kind of trouble. Rapid Change of Grid Voltage. If t h r system is in equilibrium with both grids grounded, and both are changed to 6 volts, final equilibrium is not reached for about 3 minutes, during ahich the total drift is about 2 mv. This seems t o he a thermal effect, but the exact cause is not known. Cathode Resistor. -110; change in one cathodc resistor produces a 2-mv. unbalance. COXTRIBLTION 582, Chemical Laboratory, Indiana University.
Universal Apparatus for Continuous Extraction of an Aqueous Solution with Solvents More Dense Than Water. Don C. Iffland, Department of Chemistry, West Virginia University. Morgantown, W.Va. references describe versatile equipment for continuM ously extracting a water solution with solvents less dense than water, particularly ethvl ether, but no versatile equipment ASY
has been described for continuous extractions of aqueous solutions using solvents more dense than water; consequently thik operation is used only infrequently. The apparatus described here is a Wehrli extractor [IYehrli, 1577
